BODIPYDyesandTheirDerivatives: SynthesesandSpectroscopicPropertiesAuroreLoudetandKevinBurgess*DepartmentofChemistry,TexasA&MUniversity,P.O.Box30012,CollegeStation,Texas77842ReceivedMay7,2007Contents1. Introduction 48912. TheBODIPYCore 48922.1. Fundamental Properties 48922.2. SynthesesfromPyrrolesandAcidChloridesorAnhydrides48932.3. FromPyrrolesandAldehydes 48932.4. FromKetopyrroles 48953. Modificationstomeso-AromaticSubstituentsontheBODIPYCore48953.1. FluorescenceControl viaPhotoinducedElectronTransfer48964. BODIPYswithHeteroatomSubstituents 48984.1. FromElectrophilicSubstitutionReactions 48984.1.1. Sulfonation 48984.1.2. Nitration 48984.1.3. Halogenation 48994.1.4. Potential forOtherElectrophilicSubstitutionReactions48994.2. FromNucleophilicAttackonHalogenatedBODIPYs49004.3. FromPalladiumMediatedCHFunctionalization49014.4. FromNucleophilicAttackatthemeso-Position49015. Aryl-,Alkenyl-,andAlkynyl-SubstitutedBODIPYs 49025.1. Aryl-SubstitutedBODIPYsfromAryl-Pyrroles 49025.2. CondensationReactionsofthe3,5-DimethylDerivativeswithBenzaldehydeDerivativesToGiveAlkenyl Systems49035.3. FromPalladium-CatalyzedCouplingReactionsatthe3-and5-Positions49056. EnergyTransferCassettes 49066.1. Through-SpaceEnergyTransferCassettes 49066.2. Through-BondEnergyTransferCassettes 49086.2.1. Porphyrin-BasedSystemsasModelsofPhotosynthesis49086.2.2. PolypyridineComplexesContainingAccessoryBODIPYChromophores49116.2.3. RelativelyCompactSystemsasPotentialProbesinBiotechnology49127. SubstitutionofFluorideAtomsintheBF2-Group 49137.1. WithAlkyl Groups 49137.2. WithAryl Groups 49137.3. WithAlkyneGroups 49147.4. WithAlkoxideGroups 49168. UseofMetalsOtherthanBoron 49169. BODIPYAnalogueswithExtendedAromaticConjugation49189.1. RestrictedSystems 49189.2. ExtendedAromaticSystems 49199.2.1. Di(iso)indometheneDyes 49199.2.2. DyesBasedOnBenz[c,d]indole 49209.2.3. Porphyrin-FusedSystems 492010. Aza-BODIPYDyes 492010.1. Tetraaryl Systems 492010.2. ExtendedAza-BODIPYSystems 492310.2.1. DyesBasedonBenz[c,d]indole 492411. OtherAnaloguesoftheBODIPYs 492511.1. Biimidazol-2-ylBF2Complexes 492511.2. Pyridine-BasedSystems 492511.3. 2-KetopyrroleComplexes252492611.4. AzobenzeneDerivatives 492611.5. MiscellaneousN,N-BidentateDiphenyl BoronChelates492611.6. Boryl-SubstitutedThienylthiazoles 492712. Conclusion 492813. References 49291.IntroductionAdvances in imaging techniques now make it feasible todo things that were not previously possible. Experiments inwhichinteractingproteinsareobservedinsidelivingcellsare now common for dynamically averaged systems,1-3andthe field is close to observation of similar events on a singlemoleculelevel.4-8Labelscanbeattachedtoproteins, forexample, antibodies, whichaccumulateinspecificorgansforimaginginanimalsandhumansubjects.9Thetechno-logical advances in this area are remarkable. However, thereisagrowingrealizationthat imagingeventsincellsandwhole organisms by fluorescence is limited by the probesavailable. For instance, there are few that emit at 800 nm orabove, yet living tissues are most transparent to light at andabove this wavelength.4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (hereafter ab-breviated to BODIPY) dyes tend to be strongly UV-absorb-ing small molecules that emit relatively sharp fluorescencepeaks with high quantum yields. They are relatively insen-sitive tothe polarityandpHof their environment andare reasonablystable tophysiological conditions. Smallmodifications to their structures enable tuning of theirfluorescencecharacteristics; consequently, thesedyesarewidely used to label proteins10-14and DNA.15However, thesecompoundsalsohavesomeundesirablecharacteristicsformany applications in biotechnology. For instance, most emitat lessthan600nm, andonlyahandful ofwater-solublederivatives have been made. Thus, there is the potential that* To whom correspondence should be addressed. E-mail, burgess@tamu.edu;tel, 979-845-4345; fax, 979-845-8839.4891 Chem.Rev.2007,107,4891493210.1021/cr078381nCCC:$65.00 2007AmericanChemical SocietyPublishedonWeb10/09/2007modifications to the BODIPY framework will lead to probesthat can be used more effectively for imaging in living cellsand whole organisms, but that is largely unrealized.Our researchgroupis interestedinmakingBODIPY-relatedprobesthatwillrealizesomeoftheirpotentialforintracellularimaging. Thisreviewisintendedtofacilitatethis process by summarizing the basic chemistry andspectroscopicpropertiesofcommonBODIPY-derivatives,and highlighting ways in which other interesting probes couldbe prepared. Readers particularly interested in applicationsof BODIPY dyes as labeling reagents,11,13,16-43fluorescentswitches,44,45chemosensors,46-83and as laser dyes84may beinterested in this reviewas a guide to synthesis andspectroscopicproperties,butdetailsonuseofthedyesinthose ways are not included here. Readers interested in smallamounts of commercially available dyes for labeling shouldalsoconsult the Invitrogen(formerlyMolecular Probes)catalog.11,852.TheBODIPYCoreThe IUPACnumbering systemfor BODIPYdyes isdifferent tothat usedfordipyrromethenes,86andthiscanlead to confusion. However, the terms R-, -positions, andmeso- are used in just the same way for both systems.2.1.FundamentalPropertiesBODIPYdyeswerefirst discoveredin1968byTreibsand Kreuzer.87BODIPY 1, which has no substituents, hasnot been reported in the literature. This may be because ofsynthetic difficulties obtaining this compound related to thefact that none of the pyrrole-based carbons are blocked fromelectrophilicattack.Synthesisofthecorrespondingdipyr-romethene precursor has been reported, but this compoundisunstableanddecomposesabove -30to -40 C.88Thesymmetrical, dimethyl-substitutedcompound2has beenprepared89and could be considered as a reference to whichother simplealkylatedBODIPYs canbecompared. Thesymmetrically substituted systems 3 and 4 have apparentlynot been reported, reflecting synthetic limitations for evensome simple BODIPY systems. However, the unsymmetri-cally substituted BODIPYs 5 and 6 have been prepared.90There are relatively minor differences in the reported UV-absorption maxima, fluorescence emission maxima, andquantum yields of these compounds, and these should notbe over-interpretedbecause small calibrations errors arecommon in these types of experiments. However, when thesymmetrically-, tetra-, hexa-, andhepta-alkylatedsystems7,90,918, and9areincludedinthecomparison, thenanunambiguous trend toward red-shifted absorption and emis-sion maxima with increased substitution becomes apparent.AuroreLoudet wasborninFrance. Shereceivedher B.S. degreeinbiochemistry and M.S. degree in chemistry at UniversitePaul Sabatier inToulouse, France. In2000, shecametoTexasA&MUniversityandjoinedKevinBurgess group. Herresearchfocusedonthesynthesisofnear-IRaza-BODIPYdyes.KevinBurgessisaprofessoratTexasA&MUniversity,wherehehasbeensinceSeptember1992. Hisresearchinterest focusesonpeptido-mimeticsformimickingordisruptingproteinproteininteractions,asym-metricorganometalliccatalysis, andfluorescent dyesformultiplexinginbiotechnology. Thelatter project involvesmuchchemistrythat centersaround the BODIPY dyes. We choose to write this review to organize theliteratureinthatfieldforourselvesandothersinthefield.4892 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessAlkylation or arylation at the meso position has no specialeffect on the absorption and emission wavelengths (compare2with11, and7with12) eventhoughthissubstitutionposition is structurally unique. However, the quantum yieldofthemeso-phenylcompound 11isappreciablylessthanthe more substituted analogue 12. Such differences are widelyattributed to 1,7-substituents preventing free rotation of thephenyl group reducing loss of energy from the excited statesvia non-irradiative molecular motions. Consistent with this,introduction of ortho-substituents on the phenyl ring has beenobserved to increase quantum yields, and similar explanationshave been invoked.BODIPY derivatives in which aliphatic rings have beenfused to the pyrrole fragments are perhaps more constrainedthan ones bearing acyclic aliphatic substituents. Nevertheless,the effects on their emission maxima are not always easilyrationalized. Compound13hasashorteremissionwave-length maximumthan 9 even though both have threesubstituents on the pyrrole rings. On the other hand, 14 hasonly two such attachments, yet it has the longest wavelengthfluorescence emission.2.2.SynthesesfromPyrrolesandAcidChloridesorAnhydrides8-Substituted BODIPYdyes A(i.e., ones with sub-stituents in the meso position) tend to be relatively easy toprepareviacondensationof acyl chlorides withpyrroles(reaction 1).92,93These transformations involve unstabledipyrromethene hydrochloride salt intermediates. Theintermediatedipyrromethenehydrochloridesareeasier tohandle and purify as C-substitution increases, but even so,thesearenot generallyisolatedinsynthesesofBODIPYdyes.Other activated carboxylic acid derivatives could be usedin place of acid chlorides in reaction 1. In the particular caseof acid anhydrides, this concept has been reduced to practice.Reaction2shows howthe BODIPYderivative 15wasprepared from glutaric anhydride.37An attractive feature ofthis chemistry is that a free carboxylic acid is produced, andthismaylaterbeusedtoattachtheprobetotargetmole-cules.2.3.FromPyrrolesandAldehydesSyntheses similar to those depicted in reactions 1 and 2but whichuse aromatic aldehydes14(tothe best of ourknowledge,aliphaticaldehydeshavenotbeenreportedinthis reaction) require oxidation steps. The reagents for theseoxidations can introduce experimental complications. Thus,in reaction 3, the oxidant used was p-chloranil, and eventu-ally, the byproducts from this had to be removed (in fact,this was done after complexation with the boron).BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4893R,-Unsubstituted BODIPYs, for example, 17 can also beprepared from aldehydes using neat conditions.14The alde-hyde was dissolved in excess pyrrole at room temperature,and the dipyrromethane intermediate (the reduced form ofthe dipyrromethene) was formed and isolated. The BODIPYdye was obtained after oxidation with DDQ and complex-ation with boron (reaction 4). Acid chlorides probably wouldbe too reactive to use with pyrrole (since it is unsubstitutedand more reactive), so this aldehyde-based approach is themethod of choice.Use of, for instance, halogenated benzaldehydes (or halo-genated acid chlorides above) broadens the scope of the reac-tion by offering the potential for elaboration of these groupsin compounds like 16. In another example, systems 18 and19 were prepared from benzaldehyde derivatives that havelongchainacidsubstituents. Theseprobes wereusedtoinvestigate dynamic effects inmembranes.2Presumably,ortho-substituents were included on the meso-aromatic groupto restrict rotation of that ring and increase quantum yields.The fact that a diverse set of aldehydes could be used topreparemeso-substitutedBODIPYs provides ameans tointroduce more sophisticated functionalities for specializedpurposes. For instance, 8-hydroxyquinoline-2-carboxaldehydewas used to prepare the Hg2+-selective chromo- and fluor-oionophore20.60This probeis highlyfluorescent inthepresenceof transition-metal ions (Co2+, Ni2+, Cu2+, andZn2+)andheavy-metal ions(Pb2+, Cd2+), but 5equivofmercuric ions reduced its emission by more than 98% (thecolor of the solution also changed from light amber to red,enabling the progress of the complexation event to bevisualized). A 1:1 complex with Hg2+is formed in whichthe dipyrromethene core of 20 adopts a nearly orthogonalconformation with the 8-hydroxyquinoline moiety becauseof the methyl groups in positions 1- and 7- on the BODIPYcore. This particular arrangement displays the binding siteof 8-hydroxyquinoline for complexation of metal ions. Theshape of these BODIPY-based ligand is such that 2:1 L-Mcomplexes are sterically disfavored.Fluorescent pH probes 21-24 have been prepared fromphenolic benzaldehydes (a case where the acidchlorideapproach would have raised chemoselectivity issues).68Thesecompoundsareweaklyfluorescent inthephenolateformpresumably due to charge transfer from the phenolate donorto the excited-state indacene moiety. The pKa of the differentderivatives was tuned by varying the aromatic substituent.It is unusual to use any aldehydes that are not aromatic toprepare BODIPYderivatives. In this sense, the vinylicthioetherprobe 25(reaction5)isexceptional. Acatalyticamount of ytterbium(III) trifluoromethane sulfonamide wasused to mediate the condensation process, the intermediatedipyrromethane was oxidized with DDQ, and complexationwith boron trifluoride gave the product, though in very pooryield.94The fluorescence emission of compound 25 was red-shiftedrelativetodyeswithaphenyl groupat themesoposition (e.g., 11). Compound 26 has a sulfide in conjugationwith the BODIPY core, just as 25 does. The fluorescenceemission maximum of 25 is 26 nm red-shifted compared to26, indicating that electron donating groups in this positiontend to have that effect.4894 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgess2.4.FromKetopyrrolesCondensations of pyrroles withacidchlorides or withbenzaldehyde derivatives, as outlined above, are direct andconvenient methods to access symmetrically substitutedBODIPYdyes. However, anotherapproachisrequiredtoformunsymmetricallysubstitutedones. Generally, thisisachieved via preparations of ketopyrrole intermediates,followedbyaLewisacidmediatedcondensationofthesewith another pyrrole fragment.Scheme1a,bshowsreactionsof magnesiumanionsofpyrroles with activated carboxylic acid derivatives to givethecorresponding2-ketopyrroles.95,96Inpart c, onesuchketopyrrole is condensed with another pyrrole unit to give aBODIPYframework. That examplegives asymmetricalproduct, but the method is particularly valuable for unsym-metrical ones, as in Scheme 1d.3.Modificationstomeso-AromaticSubstituentsontheBODIPYCoreThe BODIPY core is robust enough to withstand a rangeof chemical transformations, and a variety of aromatic groupsFigure 1. Selected BODIPYs with meso-modifications to give (a) selective sensors of particular redox active molecules, (b) pH probes,(c) metal-chelators, and (d) biomolecule conjugating groups.BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4895canbeintroducedat themeso-positionfor appropriatelyfunctionalized BODIPYdyes. Alternatively, dyes withspecial meso-groups can be produced via in ViVo synthesis.These strategies have been used to produce several dyes formany different applications; just a few illustrative ones areshown in Figure 1. For instance, derivatives of this type havebeen formed as selective sensors of particular redox activemolecules (27,6228,6529,48and 30,97Figure 1a), pH probes(3155and 3251, Figure 1b), metal-chelators (33,7934,7935,7436,6737,6638,5339,8040,7841,82and 42,71Figure 1c), and asbiomolecule conjugating groups (43,4944,81and 45,98Figure1d).Here, we intend to restrict the discussion to the generalconcepts that influence the fluorescence properties of meso-modified BODIPY dyes; this review does not attempt to givea comprehensive list of all the compounds made. Withoutexception, the chemosensors operate by perturbing thereduction potential of the meso-substituent. The next sectiondiscusses theelectroniceffects of theseperturbations onfluorescence.3.1.FluorescenceControlviaPhotoinducedElectronTransferTransfer of electrons between nonplanar parts of fluores-cent molecules modifies their fluorescence intensities.99Thishas been known for sometime, but Nagano, Ueno, and co-workers haveskillfullyappliedcalculatedorbital energylevels and experimentally determined electrochemical datato rationalize quantumyields. Their initial work withfluorescein derivatives100,101was later expanded to encompassBODIPY systems as described here.102Somenonplanarfluorescent moleculescanberegardedas a highly fluorescent group with non- (or significantly less)fluorescent substituents (Figure 2a). Some such substituents,depending on their oxidation potentials relative to the excited-stateoftheBODIPYcore, canact aselectrondonorsoracceptors. If electron transfer occurs, then the fluorescenceisdiminishedwhenthefluorescent groupinitsexcited-stateisreduced.Inthissituation,thefluorescentgroupisactingas anacceptor; consequently, this maybecalledScheme 1aa(a and b) Two different methods for production of ketopyrrolesfrom magnesium derivatives of pyrrole, and application of these startingmaterialsintheproductionof (c) symmetrical, and(d) unsymmetricaldyes.Figure 2. The meso-substituent provides (a) no significantelectronic perturbation, (b) electrons to the excited state, and (c) alow lying LUMO to accept electrons from the excited state.4896 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessreductive-PeT or a-PeT (a for acceptor; Figure 2b).However, if the energy states are such that the excited-stateofthefluorescent groupcandonateelectronstothesub-stituent LUMO, then oxidative-PeT, d-PeT, occurs (d fordonor; Figure 2c). Indirectly, solvent polarity has an effecton this process. This is because photoexcitation and oxidationprocesses involve modification of ground state dipoles, andsolvents may stabilize or destabilize these changes accordingto polarity.The feasibility of electron transfer can be judged from thechange in free energy (GPeT), as described by the Rehm-Weller equation:103where E1/2 (D+/D) is the ground-state oxidation potential ofthe donor, E1/2 (A/A-) is the ground-state reduction potentialof the acceptor, E00 is the excitation energy, and C is anelectrostatic interaction term.Naganoandco-workersappliedtheseprinciplestothetriazole-BODIPY derivative 47 (Figure 3) to develop a nitricoxide probe.61The lowfluorescence of the diamine46was explained in terms of reductive PeT. When nitric oxideconverts the diamine into the benzotriazole 47, then reduc-tive PeTdoes not occur and fluorescence is observed.Scheme 2 outlines the synthesis of the probe and the oxidizedproduct.Arefinementoftheideaspresentedabovewasusedtoexplain the observation that fluorescence for the compounds47decreasedintheorderb>a>c(Figure4). Thisisbecause the nature of the substituent impacts the reductionpotential of the BODIPY core in that order (b is the mostnegative).FluorescenceoftheZn2+andNO2+chemosensors 4863and49104has beenexplainedusingreductive PeTcon-ceptsoutlinedabove. Chelationornitration, respectively,makes the reduction potential of the meso-substituent moreFigure 3. (a) Calculated HOMO energy levels of meso-substituentsfor BODIPY46and47; (b) reductive PeToccurs above thethreshold indicated.GPeT) E1/2(D+/D) - E1/2(A/A-) - E00- CScheme 2. Synthesis of Diaminobenzene- andTriazole-BODIPY Derivatives 46 and 47Figure 4. Reductive PeT increases in the order Et < H < CO2Et.BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4897negative, PeT is switched-off, and the probes becomefluorescent.Stronglyelectronwithdrawingsubstituentsonaromaticmeso-substituents lower the LUMO of the aromatic systemto the level where it might accept electrons from the orbitalcontaining the promoted electron in the BODIPYcore(Figure 2c). In such cases, oxidatiVe PeT comes intoplay. Evidencefor thisisseeninthequantumyieldsof50-52.104For dye 52, oxidative PeT is diminished becausethe ketones lower the energy of the BODIPYorbitalcontaining the promoted electron.There are at least two weaknesses associated with com-bining theoretical calculations and electrochemical data torationalize quantum yields. First, the reduction potentials forthe two components are calculated on isolated systems: thefactthattheyareattachedtoeachothermustperturbtheactual orbital levels. Second, energies of excited states arenotoriously difficult to calculate. Nevertheless, the approachemphasized by Nagano and co-workers helps dispel dogmasthat surround fluorescent probes. For instance, the assertionthat nitro groups or heavy atom substituents always quenchfluorescence is simply not true. The reality is that they tendto do so but only if their orbital energy levels interact withthe fluorescent chromophore in a way that facilitates PeT.4.BODIPYswithHeteroatomSubstituents4.1.FromElectrophilicSubstitutionReactionsSimple considerations of mesomeric structures reveal thatthe2-and6-positionsoftheBODIPYcorebeartheleastpositive charge, so they should be most susceptible toelectrophilic attack (Figure 5). However, there is no definitivestudy of regioselectivities in these reactions for BODIPYswithout pyrrole substituents; almost invariably, some of theother positions are blocked by substituents.4.1.1.SulfonationTothebest of our knowledge, less thanahandful ofsulfonatedBODIPYdyeshavebeenreported. Theywereobtained fromtetra-, or penta-substituted BODIPYs viatreatment with chlorosulfonic acid, then subsequent neutral-ization with a base (e.g., reaction 6). Monosulfonated systemscan be obtained when only one equivalent of chlorosulfonicacid is used. Introduction of sulfonate groups do not changetheabsorptionandfluorescenceemissionmaximasignifi-cantly relative to the unsulfonated dye. The sulfonatedBODIPY dyes 53-56 are strongly fluorescent in water and/or methanol, and are claimed to have even better stabilitythan the parent dyes.92,105-1104.1.2.NitrationThe 2,6-dinitroBODIPYdye57canbe obtainedvianitration with nitric acid at 0 C (reaction 7). Introductionof the nitro groups drastically reduces the fluorescencequantum yield.92,111-113To the best of our knowledge, thisis the only nitration of a BODIPY dye that has been reportedin the literature.2,6-Dinitro-BODIPY 58114and 59115-119) have also beenreported in the Japanese patent literature for applications asFigure 5. Electrophilic attack on tetramethyl-BODIPY.4898 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgesssensitizers and inks. We were unable to find procedures fortheir syntheses and spectroscopic data.4.1.3.HalogenationBromination of the 1,3,5,7,8-pentamethyl-substituted BO-DIPY PM 546 shown in reaction 8 gave the dibrominationproduct 60.92,120Predictably, introduction of bromo substitu-entontothedipyrromethenecorecausesasignificantredshift of both the UV-absorption and emission maxima, anditquenchesthefluorescencequantumyieldviatheheavyatom effect.2,6-Diiodo-tetramethyl BODIPY 61 was obtained via theroutedescribedinreaction9.121Just asforthedibromo-BODIPY 60, iodo-substituents cause significant red shift ofthe UV-absorption and fluorescence emission maxima, andquench the fluorescence quantum yield via the heavy atomeffect. Compound 61 is much more resistant to photobleach-ing than Rose Bengal; this is because the BODIPY core hasa more positive oxidation potential than the xanthone unitof Rose Bengal. Compound 61 is an efficient photosensitizer.2,6-Difluoro- and 2,6-dichloro- BODIPY derivatives 62114and 63115-119have found applications as electroluminescentdevices and sensitizers. Their synthesis has not been reported.Chlorination of the unsubstituted dipyrromethane shownin Scheme 3 occurs preferentially at the R-positions. Thus,the 3,5-dichloro BODIPY derivative 64 could be obtainedafter oxidation with p-chloranil and complexation with borontrifluoride etherate. Applications of such chlorinated materialsas SNAr electrophiles are described later in this review.4.1.4.Potential forOtherElectrophilicSubstitutionReactionsBODIPYdyes are intrinsically electron rich, and thereactionsshownaboveillustratethat theywill react withelectrophiles. It is thereforesurprisingthat thereactionsshown above represent the state-of-the-art in this area. Thereare very few sulfonation reactions despite the importance ofwater-soluble dyes; most of the sulfonated BODIPY com-pounds in the Invitrogen catalog feature carboxylic acids withsulfonated leaving groups. There are no reports of some com-mon electrophilic addition processes like Vilsmeyer-Haackreactions, but our group has investigated this type of reaction,and the mono-formylated BODIPY dye could successfullybe synthesized in excellent yield (unpublished results).Scheme 3. Preparation of 3,5-Dichloro-BODIPY viaChlorination of a Dipyrromethane IntermediateBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 48994.2.FromNucleophilicAttackonHalogenatedBODIPYsThe most common approach to introduce substituents onthe 3- and 5-positions of the BODIPY core involve de noVosyntheses withappropriatelysubstitutedpyrroles, but anexciting recent development reaches the same goal vianucleophilic substitution on the 3,5-dichloro-BODIPY 64.122,123Thenucleophiles usedsofar includealkoxides, amines,thioalkoxides, and the diethyl malonate anion. These reac-tions can be stopped at the monosubstitution stage or forcedtothedisubstitutionproduct (Scheme4); hence, theyareuseful for access to asymmetric 65 and symmetric 66, hetero-substituted BODIPY dyes, which would be difficult to obtainby other routes.The spectroscopic effects of electrondonatinggroupsattached to the BODIPY core for the compounds in Scheme4werestudied. The3-and5-substituentshadsignificanteffects, shifting both the absorption and/or emission spectra,and changing the fluorescence quantum yields. For example,introductionofanamino-orsulfur-centerednucleophilesresults in a significant bathochromic shift (red shift) of boththe absorption and emission.122Data were collected inmethanol (Figure 6); the absorption and emission maximawere red-shifted in cyclohexane, but otherwise, it was quitesimilar to that for methanol (data not shown). The quantumyieldsvariedwidely, but generallytendedtobereducedrelative to unsubstituted BODIPYs.8-(-Haloalkyl)-BODIPYdyes 67 and 68 are usefulstarting materials. These are easily synthesized by condensa-tion of a -haloacyl chloride with 3-ethyl-2,4-dimethylpyrrole (reaction 10).124Compounds 67 and 68 can be functionalized by nucleo-philic substitution reactions.58,124Thus, 67 and 68 areprecursors to a range of compounds B including fluorescentelectrophiles, disulfides for chemoselective labeling of cys-teine residues, fluorescent amino acids, and chemosensorsfor metal ions.58Scheme 4. BODIPY Dyes via SNAr ReactionsFigure 6. Spectroscopic data for some BODIPYs formed by SNAr reactions.4900 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgess4.3.FromPalladiumMediatedCHFunctionalizationPyrrolescanbefunctionalizedviapalladiumcatalyzedactivation reactions.125-127Our group applied the samestrategytosynthesizenovel BODIPYdyesviapalladiummediated C-H functionalization (reaction 11).128This routeprovides a direct waytoextendthe conjugationof theBODIPY core, without a halogenated or metalated interme-diate prior tothe couplingreaction. Highlyfluorescent,mono-(69)ordisubstituted(70)dyescanbeobtained. Awater-soluble,sulfonatedBODIPYdye 69dwasalsopre-pared via this route, but in low yield (2%).The C-H functionalization process was also applied to R,-unsaturated esters to form compounds 71 and 72. Mono- anddisubstituted products were obtained, but the alkene doublebond was shifted out of conjugation with the BODIPY core.4.4.FromNucleophilicAttackatthemeso-PositionBODIPYsystems withameso-cyanidesubstituent arespecial insofar as they fluoresce at significantly longer wave-lengths than the unsubstituted systems.129Further, they canbe accessed by direct addition of cyanide anion to the dipyr-romethene core, followed by oxidation. The first syntheticroute developed gave poor overall yields, mainly due to lossof material in forming the dipyrromethene,93but an improvedroute was developed by Boyer (Scheme 5).130In that work,Knorr condensation of 3-ethylhexan-2,4-dione with diethylScheme 5. Cyanopyrromethene-BF2 Complexes viaAddition of CyanideBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4901aminomalonate give ethyl 3,4-diethyl-5-methylpyrrole-2-carboxylate. Thispyrroleisthenconvertedtothedipyr-romethene hydrobromide via the presumed intermediacy of2-methyl-3,4-diethylpyrroleanditscondensationwiththeR-formyl derivative that is formed in situ.Comparison of compound 73 with other meso-cyanoBODIPYs (e.g., 74-77) shows they fluoresce and absorb atabout 60 nm longer wavelengths than simple alkyl-substitutedBODIPYs. Thislargebathochromicshift seemsattribut-abletoanet stabilizationoftheLUMOlevel duetothecyano group decreasing the energy gap. The presence of thecyanogroupalsosignificantlyreducesthemolecularex-tinction coefficient. Only in the case of 77 where the corebears two ester groups is this red shift attenuated. Presum-ably, in this case, the esters reduce the electron density ontheBODIPYcorethat isavailablefordelocalizationintothe nitrile, and this affects the fluorescence emissionwavelength.The red shift in fluorescence observed when a cyano groupis added to the meso-position is somewhat particular to thatsite. 2,6-DicyanoBODIPY 78hastwocyanogroups, butneither occupies the meso-position. Fluorescence from thiscompoundpeaksataboutthesamewavelengthassimilarBODIPYs without nitrile substituents, for example, 79 and8.130Similarly, substitutionatthe2-and6-positionswithcarboethoxy, acetamido, sulfonateanion, bromo, or nitrogroups do not produce any red shift.93,130,131Asynthesisof sulfur containingBODIPYsunder mildconditions was recently achieved via reaction of thiophos-genewithsubstitutedpyrroles togivethecorrespondingthioketone 80. The latter can then react with variouselectrophiles to formthe dipyrromethene intermediatesthat were combined with boron trifluoride in presence of basetoaffordthe thio-BODIPYderivatives 81. These meso-thioalkylgroupscanbedisplacedbynucleophilestogiveanilino-substituted BODIPYderivative 82 (Scheme 6).Amine adduct 82 was not fluorescent, presumably becauseof electron transfer from the amine group to the BODIPYcore in the excited state.5.Aryl-,Alkenyl-,andAlkynyl-SubstitutedBODIPYs5.1.Aryl-SubstitutedBODIPYsfromAryl-PyrrolesAryl-substitutedBODIPYdyescanbeformedviacon-densationof the correspondingpyrroles withacyl chlo-rides.132,133The 2-aryl pyrroles used were prepared viaSuzuki couplings134of N-tert-butoxycarbonyl-2-bromo-pyrrole (reaction 12); this was more convenient than start-ing with less accessible materials like 4-aryl-1-azido-butadienes135or N-tosylarylimine.136,137Removal of theN-BOCprotectinggroupunderbasicconditionsgavethedesired aryl pyrroles. These products were found to decom-pose on standing, rapidly under acidic conditions, and weretherefore best formed immediately before use.3,5-Diaryl BODIPY dyes were obtained from the pyrrolesshown above, via a one pot, two-step process featuring 4-iodobenzoyl chloride. The 3,5-aryl groups extend the con-jugationoftheBODIPYsystems.Comparedtothealkyl-substituted derivatives, for example, 83 and 84 which havegreen, fluorescein-like emissions (maxemis) 510 nm), bothScheme 6. Synthesis of Sulfur- and Anilino-BODIPYDerivatives4902 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessthe absorption and emission maxima of 3,5-diaryl substitutedBODIPYs85-89areshiftedtolongerwavelengths(maxemis ) 588-626 nm).132,133The para-electron-donating groupof 87 gives a larger bathochromic shift and increasedquantumyield, compared to the compound with thosesamesubstituentsintheortho-position89. Similarly, theextended aromatic substituent naphthalene of 86 red-shiftedtheemissionmaximumforthiscompound. However, theextinctioncoefficients for 85-89werenot markedlyin-creasedrelativetothealkylsystems 83and 84, andtheirfluorescencequantumyieldsweresignificantlylowerdueto nonradiative loss of energy via rotation around the C-Arbonds.138Fluorescence of BODIPYs in solution can be reduced atelevatedconcentrationsduetointermolecular -stacking.BODIPYdyes can stack in two geometrically distinctforms: H- and J-dimers (Figure 7). The former is most likelya result of intramolecular dimerization, with the stacking oftwo BODIPY planes with almost parallel S0fS1 transitiondipoles, and antiparallel electric dipole moments. This dimeris practically nonfluorescent, and exhibits blue-shifted ab-sorptionrelativetothat ofthemonomer. TheJ-dimer, inwhich the S0f S1 transition dipoles are oriented in planesat 55, isfluorescent andshowsared-shiftedabsorptionrelative to that of the monomer.139-141Compound90wasdesignedasaprobethat wasster-ically prevented from forming dimers in solution.142Further,the hindered internal rotation of the mesityl rings re-ducesnonradiativerelaxationofexcitedstates, enhancingfluorescencequantumyields. Dye90waspreparedfrommesityl aldehyde andpyrrole91(made via a Trofimovreaction).1375.2.CondensationReactionsofthe3,5-DimethylDerivativeswithBenzaldehydeDerivativesToGiveAlkenylSystemsInthepast fewyears, it has beenshownthat 3- and5-methyl BODIPY-substituents are acidic enough to partici-pate in Knoevenagel reactions. Thus, styryl-BODIPY deriva-tives can be obtained by condensation of 3,5-dimethyl-BODIPYs with aromatic aldehydes (reaction 13).143,144Withp-dialkylaminobenzaldehyde, the reaction can be restrictedto one condensation, but 4-alkoxybenzaldehydes tend to givemixtures corresponding to one and two condensations.145Di-(dimethylamino)styryl-substituted BODIPYdyes can beobtained with longer reaction time (7 days).146The condensation processes shown in reaction 13 providedirect entry into BODIPY derivatives that have red-shifted fluorescence properties, and functional groups thatcanbe usedinsensors andmolecular logic gates. UV-absorptionspectraof compounds fromonecondensation(e.g., 94 and 95) tend to reach a maximum around 594 nmFigure 7. Structures of H-dimer and J-dimer.BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4903inMeOH, andtheirfluorescenceemissionsaresimilarlyred-shifted. Both the absorption and, particularly, the fluo-rescence spectra are dependent upon solvent polarity.147,148The red shift is more pronounced in polar solvents indicat-ingexcitationofthedyesleadstomorepolarizedexcitedstates.Structures93-97illustratesomeeffectsofaminosub-stituents in these styryl systems. When the amine is directlyconjugatedwiththe styryl group, thena maximumredshift is obtained, but when it is part of the meso-substituent,then the bathochromic shift is less because this meso-substituent is nonplanar. Quantum yields for these materialsare only high when the amine is protonated disfavor-ingIntramolecular ChargeTransfer (ICT) intheexcitedstate.143,148Structures 95-97 were prepared as metal sensors wherecomplexation to ions alleviates ICT and increases thequantum yields. Comparison of the spectral data for 95 and97 reveals a second styryl group gives a red shift in the UVabsorbanceofalmost100nm, andinabout50nminthefluorescence spectrum.146There is no marked pHdependence for the alkoxy-substituted systems 98 and99.145However, the amino-phenol 100 has quenched fluorescence when the hydroxylgroup is deprotonated, but the UV absorbance and fluores-cence remain the same; fluorescence is enhanced andthereisablueshift whentheaminegroupisprotonated.This molecule has therefore been compared to a logicgate.754904 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessInvitrogens BODIPY630/650 and BODIPY650/665probes are the longest-wavelength amine-reactive BODIPYfluorophores reported to date.Biomolecules labeled at relatively large BODIPY/proteinratios can have diminished fluorescence due to interactionsbetween the probes; the fluorescence can also be red-shiftedfor the same reason.149Small molecules containingtwoBODIPYs have been produced to test these types of effects.The first prepared were somewhat flexible being based oncyclohexane.139,140More recently, rigid test molecules withcofacial BODIPY dyes have been made and studied. Onlyonetransitiondipolemoment ispossibleforthesestruc-tures. The xanthane unit was chosen as a scaffold, since itcan be easily functionalized on its 4- and 5-positions.Aldehydeorbis-aldehydefunctionalitiesonthexanthane(101 or 105, respectively) were used to construct theBODIPYunitsbycondensationwith2,4-dimethylpyrrole.A3-methyl substituent ontheBODIPYwasthenreactedwith p-dimethylaminobenzaldehyde to extend the conjugation(Scheme 7).150Compound 106 has two cofacial BODIPY groups, and theUV absorption maximum of the BODIPY part is blue-shifted(to 478 nm, with a shoulder at 504 nm) relative to 1,3,5,7-tetramethyl-BODIPY7 and the control compound 102.Compound 107 which has one styryl-extended BODIPY andone tetramethyl-BODIPYsubstituent has UVabsorptionpeaks corresponding to both these substituents (455 and 575nm). Uponexcitationat 480nm, themonochromophoricsystem 102 exhibits a very strong emission at 500 nm, whilethe fluorescence emission of the di-BODIPY system 106 issignificantly quenched; it shows two peaks, one at 505 nmand a broader excimer-type emission at 590 nm. Efficientenergytransferfromthedonor(tetra-methylBODIPY)tothe acceptor (extended BODIPY) was observed for thesystem107. Thecofacial chromophoresareseparatedbyapproximately4.5, allowingbothenergytransfer andformation of an excimer-like state. Incidentally, system 107actsasanenergytransfercassette; fluorescenceemissioncould be observed at 650 nm from the extended BODIPY(acceptor) upon excitation of the other BODIPY (donor) at480 nm. More discussion of energy transfer cassettes followsin section 6.5.3.FromPalladium-CatalyzedCouplingReactionsatthe3-and5-Positions3,5-Dichloro BODIPY derivatives such as 64 have similarreactivitiestoheterocyclicimidoylchlorides;thisopensanew window for derivatization using transition-metal-catalyzed reactions. Thus, new 3-, 5-aryl, ethenylaryl, andethynylaryl compounds 109 and 110 were obtained via Stille,Suzuki, Heck, and Sonogashira couplings (reaction 14).151The extended conjugation, and mono-/disubstitution patternsof these dyes give dispersed fluorescence emission maximawithin the series. Absorption maxima for the monosubstitutedcompounds 109areblue-shiftedby20-50nmrelativetothedisubstitutedones110. Thelargest redshift forbothabsorption and emission was observed for the styryl-substituted derivatives. The dyes with ethenyl- or ethynyl-aryl substituents gave the highest quantum yields (110c, 0.92;109d, 0.98;and 110d, 1.00).151ThemonosubstitutedBO-DIPYdyes can further be derivatized by nucleophilicsubstitution(seesectionabove) or byanother transition-metal-catalyzed coupling reaction to give unsymmetricallysubstituted probes.BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 49056.EnergyTransferCassettes6.1.Through-SpaceEnergyTransferCassettesTwo fluorescent entities may be joined in the samemolecule to give a cassette. One of these, the donor, maycollect radiation efficiently at the excitation wavelength andpass this energy to the second fluorescent moiety that emitsit at a longer wavelength. If the mechanism of energy transferis through space, then this system might be called a through-space energy transfer cassette. Through-space energy transfercassettes are typically used to artificially enhance the Stokesshiftofaprobe.OnesfeaturingBODIPYdyeshavebeensomewhat useful for DNA sequencing on a genomic scale152wherefour distinct fluorescent outputs arerequired, andusually only one excitation wavelength is used.Through-space energy transfer efficiencies depend onseveral factors, including (i) spectral overlap of the donoremission with the acceptor absorbance, (ii) distance betweenthe donor and the acceptor, (iii) the orientation factors, and(iv) theeffectiveness of alternativede-excitationmodes.Scheme 7. Syntheses of BODIPY Dyes Anchored on Xanthene Units To Test Self-Quenchingaa(a) A system with one BODIPY dye; (b) a similar compound with two, where only one has extended conjugation; and (c) another where both haveextended conjugation.4906 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessCassettes described in this section have no obvious way totransfer energy from donors to acceptors via bonds.The dendritic light harvesting system111 with fourBODIPYdonors and a perylenediimide (PDI) acceptorwas constructed via click chemistry.153Its UV spectrum isequal tothesumof thedonor andacceptor componentsindicating they are not electronically perturbing eachther.154The extinction coefficients of 111 at 526 nm(BODIPYmax) and582nm(PDI max) are240 000and45 000M-1cm-1, respectively; hence, thedonorabsorp-tionishugesimplybecausefour BODIPYunitsarein-volved. Nogreenfluorescence emissionfromBODIPYwas observeduponexcitationat 526nm, indicatingef-ficient energytransfer(99%). Onthebasisoftheenergytransfer efficiency, the authors of this work calculateda Forster critical radius of 47 . An antenna effect{emissionintensityat 618nmwhenexcitedat 526nm(BODIPY core) divided by that from excitation at 588 nm(at the PDI core)} gave approximately a 3.5-fold enhance-ment.Most studies on energy transfer between porphyrins andotherchromophoreshavefocusedonelectronandenergytransferintheplaneoftheporphyrin. Verticalelectronand energy transfer from axial ligands, on the other hand,havenot beenstudiedextensively, but systems112-115were synthesized to study these phenomena. An anti-monyporphyrinwaschosenastheacceptorbecausethiscentral metal can coordinate to ligands with oxygen, nitrogen,orsulfuratoms. Energytransferfromtheexcitedsingletstate of the BODIPY to the Sb(TPP) chromophore occursfor112, 113, and115. Thishappenswithefficienciesinthe13-40%range, decreasingasthelengthofthemeth-ylene bridge increases. Little or no evidence was seen forquenchingof the porphyrinexcitedsinglet state bytheBODIPY. Thiswastrueevenwhenpolar solventswereused and the donor and acceptor fragments would beexpected to pack against each other. However, for system114, which differs from112 only at the second axialligand(phenoxyvsmethoxy), theexcitedsinglet stateofthe Sb(TPP)- acceptor was quenched by the BODIPY andphenoxyligands. The quenchingbythe phenoxygroupoccurs, at least in part, via a nonradiative electron-transferprocess.155System 116isanotherartificiallightharvestingsystemwithBODIPYdyedonors, but heretheenergyistrans-ferred to a zinc porphyrin (ZnP), then electron transfer occursBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4907to a fullerene (C60-Im) unit; thus, the molecule was called asupramoleculartriad. TheZn-porphyrinconnectstothefullerene component via metal to axial-ligand coordination.Both steps in the process were efficient. Overall, the systemwas said to mimic the combined antenna-reaction centerevents in natural photosynthesis.156Like 112-116, complex 117 also features a donor and anacceptor systemlinkedvia axial coordinationtoa zincporphyrin.157Thisself-assemblingsystemisbasedontheexceptional affinity of phenanthroline-strapped zinc porphy-rins for N-unsubstituted imidazoles.158,159Efficient net energytransfer (80%) was observed for excitation of the BODIPYat 495 nm and emission at the free porphyrin. Energy transferfrom the excited ZnP-Im complex to the free porphyrin wascalculated to be 85%.Compound 118 was designed to be a molecular switch. Ithas two stable conformations governed by the bridgedresorcin[4]arenescaffold.160Intheabsenceofprotons(orperhaps other guest cations) the molecule exists in acontracted geometry maximizing FRET (fluorescence reso-nance energytransfer) betweenthe twoBODIPY-basedprobes. DecreasedpHvaluesswitchthemoleculetotheexpanded conformation, and this is evident by the reducedenergy transfer. An advantage of using BODIPY dyes in thisstudy is their low sensitivity to pH changes.The UV spectrum of 118 displays three strong absorptionbands assigned to the spacer (max 332 nm), the donor (max529 nm), and the acceptor dye (max 619 nm), respectively.Upon excitation at 490 nm, two emission bands at 542 nm(donor dye) and 630 nm (acceptor dye) are observed, in aratio of the donor/acceptor fluorescence intensity of 45:55,indicatinglowFRETefficiency(possiblyduetounfavor-able orientations of the transition dipole moments, and/or tothedynamicbehavior of thecavitandpart). Uponaddi-tion of TFA, the emission fromthe acceptor is nearlycompletelyquenched, whereasthedonorfluorescencein-tensity doubles.6.2.Through-BondEnergyTransferCassettes6.2.1.Porphyrin-BasedSystemsasModelsofPhotosynthesisThe prevalent mechanism of through-space energy transferis likely to be via dipolar couplings, that is, Forster energytransfer. Therateandefficiencyof this is governedby,among other things, the overlap integral between the donorfluorescence and the acceptor absorbance.4908 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessIf the donor and acceptor dyes are coupled to each otherviaaconjugatedbuttwisted-system, thentheprevalentmechanism of energy transfer is likely to be through bonds.Constraintsonthrough-bondenergytransferarenotwell-understood, but it appears there is no requirement for a goodoverlap integral. This is important in the design of fluorescentcassettes because, if this is true, there is no obvious limitationon the energy gap between the donor fluorescence and theacceptor absorbance; hence, cassettes with huge apparentStokes shifts could be produced.A pioneer of energy transfer systems featuring BODIPYdonorswasLindseywhostudiedtheminthecontext ofmodel porphyrin-basedsystemsfor photosynthesis.161Heproposedthat therates andefficiencies of through-bondenergy transfer are influenced by the following factors: steric interactions between the donor/acceptor whereinincreased torsional constraints decrease rates and efficien-cies of energy transfer; frontier orbital characteristics for the HOMO and LUMO;and related to that, the site of attachment of the donor/acceptor to the linkerand the nature of the linker.Lindsey has described the through-bond energy transfercassette 119 as a linear molecular photonic wire. It fea-tures a BODIPY donor and a free base porphyrin acceptor.BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4909Thisdonor-acceptorpairisseparatedby90. Efficientenergy transfer (76%) from the donor to the acceptor wasobserved upon excitation at 485 nm (i.e., the donor max abs).162Systems 120 and 121 were described as molecularoptoelectroniclinear-andT-gates, respectively. Inthesecassettes, the emission of the acceptor can be turned on oroff via reduction or oxidation f the attached magnesiopor-phyrin; the latter in its oxidized state quenches fluorescencevia ICT. For each cassette, more than 80% of energy transferwas observed upon excitation of the BODIPY donor part.163Compounds 122-125 are light-harvesting arrays featuringone, two, or eight BODIPYdonors and one porphyrinacceptor. IncreasingthenumberofBODIPYdonorsfromonetoeight onlyincreasestherelativeabsorptionat 516nmfrom68to94%(ofaBODIPYstandard)forthefreebase porphyrins 122-125, and from 91 to 99% for the Zn-4910 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessporphyrin (not shown). Near quantitative energy transfer wasobservedforthesystemscontainingoneortwoBODIPYunits, but 80-90%energytransfer wasobservedfor thesystem bearing eight BODIPY units.164Cassettes 126 and 127 were synthesized from the corre-sponding 21-thia and 21-oxoporphyrins by Sonogashiracouplings with 8-(4-iodophenyl)-BODIPY. Excitation of theBODIPYpart at 485nmgaveweakemissionfromtheBODIPY core and strong emissions from the porphyrin units,suggestingefficientenergytransfer. Directcomparisonoftheefficienciesofenergytransferisdifficult becausetheseparations and orientations of the donor and acceptorfragments aredifferent.165,166Interestingly, theanalogoussystemwithtwosulfurs, 128, gavepoor energytransfer(11%) upon excitation at 485 nm,167whereas 129 whichhas a normal porphyrin core gave 97% energy transfer.1686.2.2.PolypyridineComplexesContainingAccessoryBODIPYChromophoresThedual-dyesystems130-137featuringoneor moreBODIPY chromophores and a Ru(II) polypyridine complexhave been synthesized as models for solar energy conversionand storage. The BODIPY chromophore was chosen for itsstrong fluorescence, whereas the Ru(II) polypyridine complexwas chosen for its relatively intense and long-lived tripletmetal-to-ligand charge transfer (MLCT) emission. Thepyridine-based BODIPY dyes were synthesized by conden-sation of formylpyridine with 3-ethyl-2,4-dimethylpyr-role.169-172The new compounds exhibit intense absorptioninthevisibleregion(acetonitrilesolution), withamajorsharpbandat 523nmassignedtothe-*transitionforthedipyrromethenedyeandabroaderbandoflowerintensitybetween450and500nm. Thefreeligandsarestronglyfluorescent bothinsolutionat roomtemperatureand at 77 K in a rigid matrix, but no luminescence could beobservedinsolutionfortheRucomplexes,independentlyof the excitation wavelength. Nanosecond transient absorp-tion spectra, however, revealed that a relatively long-lived(ms time scale) excited-state was formed for all metalcomplexes. The latter was identified as the BODIPY-basedtriplet state, and is believed to be formed through a charge-separated level from the BODIPY-based1-* state. At 77K, all thecomplexesstudiedexcept for 130, exhibit theBODIPY-based fluorescence, although with a slightly short-enedlifetimecomparedtothefreeligands. However, thesurprisingfindingwasthat 133-135alsoexhibit aphos-phorescence assigned to the BODIPY subunits (emission at774 nmof 50 ms lifetime). This is the first report ofphosphorescence for BODIPY-based dyes. The authorspropose that the phosphorescence is due to the presence ofthe heavy ruthenium metal, which facilitated intensity to bediverted into the BODIPY 3-* state from the closely lyingmetal-based3MLCTlevel(forwhichluminescencedecayis highly efficient at 77 K).170,173,174Phosphorescence fromBODIPYunitsiscertainlyunusual. It onlywasobservedhere at 77 K for a system coupled with a ruthenium complex,and the efficiency of the process was not determined.Compounds 136 and 137 combine ruthenium polypyridineunits with BODIPY fragments. The BODIPY part ensuresahighmolar absorptioncoefficient of thesysteminthemetal-to-ligandcharge-transfer region(around500nm).The singlet excited states of 136 and137 are stronglyquenchedbythepresenceofruthenium: energytransfersBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4911totheRucenterwithhighefficiency(93%and73%for136 and 137, respectively), where it is then dissipated viaelectron transfer and/or singlet to triplet intersystem crossing.The net effect is that these complexes are not fluorescent.175Loss of fluorescence when BODIPY dyes are conjugatedwith metal complexes is common. For instance complexes138 and 139 have greatly reduced fluorescence relative tothe free ligands,171,172presumably due to intramolecularelectron transfer.171,176Other complexes, for example, of platinum,177containingBODIPY-basedligands havebeenprepared, but withoutcomment on their fluorescent properties. It is unlikely thatsuch materials will be useful as probes.6.2.3.RelativelyCompactSystemsasPotential ProbesinBiotechnologyPorphyrin-based cassettes tend to be larger than is idealfor applications relating to labeling of biomolecules, but thethrough-bondenergytransferaspect couldbeveryusefulfor applications whereinonesourceis usedandseveraldifferent outputsmust beobservedsimultaneously. Thus,there has beeninterest inmakingsmaller through-bondenergy transfer cassettes.BODIPY-based cassettes featuring simple aromatic donorscan be prepared from halogenated BODIPYs, for example,140 via palladium-mediated cross-coupling reactions.178Monoiodinated products can be quite useful for suchsyntheses. For instance, reaction 15 shows a typical stepwiseBODIPY synthesis that was used for this.178Fluorescence quantumyields of compounds 141-144excited at the donor max range from 0.02 to 0.75 (in chloro-form). In compound 145, the S1SS0 transition moments ofthe chromophores are mutually perpendicular in all confor-mations. Fast energy transfer, of the order of 0.45 ( 0.08ps, was observedfor this compound. For 141-144, thedonor and acceptor S1SS0 transition moments are mutuallycoaxial with the linker in all conformations. The transfer ratefor this set of compounds was eVen faster than for4912 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgess145, 200fs; infact, thiswastoofast tobemeasuredaccurately.179Thus, it appearsfromthisdatathat paralleland aligned transition moments are ideal for extremely fastenergytransfer. Thelengthofthelinkerinthisseriesofcompounds was also varied, but not enough derivatives weremade to arrive at conclusions relating this parameter withenergy-transfer rates.Morerecently, energytransfer cassetteslike145havebeen prepared; these are very similar except that themeso-donorgroupisoneortwopyreneunitslinkedwithalkynes. It was shown that the energy transfer efficiency isreducedwhenalkyneunitsareadded. Of course, useofconjugated pyrenes as the donor increases the UV absorptionat shorter wavelengths and the max for that donor-based band;thelatter effect decreases theapparent Stokes shift ob-served.180Energytransfer cassettes like146-148, inwhichtwoBODIPY chromophores are linked via an acetylenic linker,have been prepared.160As the size of the linker increases,energytransfer efficiencydecreases fromabout 98%to35%. These were described as FRET cassettes, but thereis a strong possibility that some of the energy transfer takesplace through bonds.7.SubstitutionofFluorideAtomsintheBF2-Group7.1.WithAlkylGroupsSome dialkyl-B BODIPY compounds have been preparedvia reactions of the corresponding dipyrromethenes with bro-modimethylborane, dibutylboron triflate, or 9-BBN triflate.Increasedstericbulkat theboronatomalongthisseriescorrelated with decreased fluorescence quantumyields.While the substituent at the meso-position has no effect ontheUVabsorptionandemissionspectra, thefluorescencequantumyieldsstronglydependonthenatureofthearylgroup. Introductionof stericconstraints onthearyl ringincreases the quantum yield (compare 149 and 152, 150 and153)).1817.2.WithArylGroupsAryl Grignards can displace fluoride fromthe borondifluoride entity of BODIPY dyes. In Scheme 8a, at 0 C,even with excess Grignard reagent, only the monosubstitutedproducts 154 and 155 were obtained, but the disubstitutedproduct 156 was formed by adding the Grignard reagent atroomtemperature. Reactionof aryl lithiumreagentswasmuch faster; these gave only the disubstituted products 157-159, evenwhenjust oneequivalent of anionwasadded(Scheme 8b).BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4913The new dyes 154-159 are highly fluorescent in solution.WhiletheabsorptionmaximumoftheBF2parent dyeisrelatively insensitive to the solvent polarity, the B-ArBODIPYstendtoundergosmallredshiftsinmorepolarsolvents. Theoriginof this solvent dependencemaybeminimization of interactions of the B-aryl groups withmorepolarmedia. Thefluorescenceemissionmaximaof154-159arealsored-shiftedrelativetotheparent BF2structures.UV absorbance by the aromatic substituents means thatthese compounds can be regarded as energy transfer cassettes,and when the B-Ar groups have good extinction coefficients,then there may be some value in this aspect of theirproperties. Thedipyrenesystem 159absorbsintherange230-317 nm corresponding to the S * transition of thepyrene units, and emits exclusively (100% energy transfer)from the BODIPY part.182However, there is no immediateapplication of this effect.7.3.WithAlkyneGroupsAcetylide anions are also good nucleophiles for thedisplacement of fluoride from the borondifluoride entity ofBODIPYdyes. Thepyrromethenedialkynyl boranecom-plexes160and161, for instance, weresynthesizedfrom4-lithioethynyltoluene and 1-lithioethynylpyrene, respec-tively.183-185Adding groups to the boron atoms does not bring theminto conjugation with the BODIPY core. Consistent with this,the absorption spectra of 160 and 161 have distinct BODIPYand ethynylaryl components. Upon excitation at 516 nm, both160 and 161 emit strongly, with high fluorescence quantumyield, in the region 535-540 nm. For 161, excitation at 371nm (pyrene absorption band) did not lead to emission fromthe pyrene but insteadtoemissioncharacteristic of theindacene core, indicating an efficient energy transfer fromthe pyrene to the indacene moiety.183,184,186It ispossibletoaddtrimethylsilylacetylidetodisplacefluoride from BODIPYs, then desilylate under basic condi-Scheme 8. Synthesis of C-BODIPYs Using (a) Aryl GrignardReagents; and (b) Aryl Lithium Reagents4914 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgesstions and Sonogashira couple with the terminal alkyne, forexample, 163(Scheme9).Whereasuseofexcesssodiumhydroxide in the desilylation reaction affords the bis(ethynyl)-BODIPY163, themonoprotectedderivative164canbeobtained using limited amounts of NaOH and shorter reac-tiontimes.187,188Access tothe monoprotectedderivativefacilitatesintroductionof different substituentsontheB-alkynes.184Compounds 165 and 166 can act as energy transfercassettes, just like the B-aryl systems mentionedabove.Irradiation of the donor (pyrene or anthracene) was observedto give only BODIPYemission, indicative of a nearquantitative energy transfer.Terminal alkyneslike164and170canbeoxidativelydimerized (Pd(II) and CuI under aerobic conditions) to givebutadiynes 168 and 171 (Scheme 10).187Deprotection of theproduct butadiyne 168 was very slow; a mixture of mono-deprotected 169a and bisdeprotected 169b was isolated after6 days.187A very similar concept was explored by the same groupbut using diisoindolodithienylpyrromethene-dialkynyl boranedyes.189These dyes have quite long wavelength emissions(over 750 nm); the parent BF2 dye 172 was known prior tothis work.190Intermediate 173 was obtained by reaction of172withtheappropriatealkynyl-Grignardreagent. Sono-gashira coupling with various ethynyl-arene derivativesafforded the systems 174-178. All the new dyes exhibit aUV absorption maximum at 708 nm with a molar absorp-tivity of 80 000 M-1cm-1. Higher energy absorption bandsScheme 9aa(a)Two-stepsynthesisofbis(ethynyl)BODIPY 163, andthemono-protected analogue 164; and (b) some examples of compounds that havebeen prepared via Sonogashira coupling of 163.Scheme 10. Synthesis of Homocoupled ProductsBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4915assigned to the pyrene, perylene, and bis- and tri-pyridinewere also observed near 350 and 450 nm. Upon excitationat 708 nm, a broad emission band at 750 nm, with quantumyields between 0.19 and 0.45 was observed in all cases. Fastenergy transfer to the central dipyrromethene core wasobserved upon irradiation of the pyrene and perylene unitsin177 and178, giving large virtual Stokesshifts. Theefficiency of the energy transfer for compounds 177 and 178was estimated to 58% and 38%, respectively. Formation ofaggregates was observed when the dyes were dissolved atconcentrations higher than 10-7M.7.4.WithAlkoxideGroupsThe first two B-methoxy BODIPYs prepared were 179-180.TheseweremadebyreactingthecorrespondingBF2compound with sodium methoxide in methanol (reaction 16).The shape of the absorption and emission spectra were thesamefor thestartingmaterial 12andthetwoproducts,suggesting the change in B-substituents had little effect onthe electronic states of the boroindacene core. Interestingly,the monomethoxy and dimethoxy products were more water-soluble than the corresponding BF2 compounds.83The onlyother report of B-ORBODIPYs systems Cdescribes their syntheses via treatment of the correspondingBODIPY with various alcohols in the presence of aluminumtrichloride (reaction 17).96This study was more expansivethan the first insofar as a more diverse set of alcohols werestudied. Thus, thealcohols usedincludedsimplealkoxy181-183, aryloxy 184-187, and several diol-derived sys-tems188-191. Acrossthisseriestherewereonlyminorchanges in the absorbance and emission spectra. Thedialkoxy-BODIPYs181-183appeartohavehigherfluo-rescence quantum yields than the diaryloxy-ones 184-187,and the binaphthol- and catechol-derived systems 188 and189 have extremely poor quantum yields.8.UseofMetalsOtherthanBoronThe BODIPYcore features a boroindacene unit, socompounds where the boron has been substituted with othermetals are, strictly speaking, beyond the scope of this review.However, there are a few dipyrromethene anion complexesofothermetalsthathavehighlyfluorescentspectroscopiccharacteristics. Thesearecoveredherebecausetheyareclearly relevant as probes.Free base dipyrrins react readily with a variety of metalssaltstoformthecorrespondingbis(dipyrrinato)-metal(II)or tris(dipyrrinato)-metal(III) complexes, but their fluores-cencepropertieshaverarelybeenstudied, andtheyhavegenerally been regarded as nonfluorescent.191-214The firstreport (2003)offluorescent propertieswasforzinccom-plexes of the boraindacene fragment.215,216The complexeswere prepared either from (i) treatment of a purified dipyrrinwithzincacetate, or (ii) atwo-step, one-flaskapproachinvolving oxidation of a dipyrromethane and complexationof the resulting crude dipyrrin with a Zn(OAc)2 in presenceof triethylamine. Complexes 192 and 193 are weaklyfluorescent, but194isastrongeremitter(it hasamulti-nanosecond excited-state lifetime); thus, free rotation of themeso-phenyl isagainimplicatedasamajor pathwayfornonradiative relaxation to the ground state for 192 and 193.4916 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessPorphyrin or phthalocyanine complexes of group 13 metalscan be fluorescent; this inspired syntheses of two newdipyrromethenes 195 and 196 from gallium(III) and indium-(III), respectively (reaction 18). In hexanes, the new com-pounds fluoresce green light, with quantum yields of 0.024and0.074forthegallium(III)andindium(III)complexes,respectively.217Nanoarchitectures 197a-d have been fabricated fromZn, Co, Ni, and Cu with dipyrromethene-based ligands, butare only strongly fluorescent with zinc.218The fluorescenceemission maxima of the zinc complexes in THF and in thesolid state are around 510-515 nmand 532-543 nm,respectively. In acetonitrile, no emission was observedBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4917possibly because of aggregation of the particles. Fluorescenceemission could be observed at submicron scale for particlesderived from Zn-bridged dipyrromethene oligomers; this issignificantbecausequantumdotsformedfromclustersofinorganic compounds are considerably larger.9.BODIPYAnalogueswithExtendedAromaticConjugation9.1.RestrictedSystemsAs mentioned previously, aryl-substituents on the BODIPYchromophore, for example, in 85 and 86, red-shift the ab-sorption and emission spectra, but decrease the fluorescenceintensity, presumably because of the free rotation of the arylgroups.132,133Consequently, more rigid systems like 198 and199 were prepared and studied.219These absorb and fluorescemoreintenselyat longerwavelengths, andtheirquantumyields are generally higher, with the exception of 198b.The new derivatives 198 and 199 were obtained by conden-sationof 4-iodobenzoyl chloridewiththecorrespondingpyrrole-based starting materials, which were prepared in sev-eral steps as described in Scheme 11. Most of the effort hereScheme 11. Syntheses of the Pyrrole-Based Starting Materials4918 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessis in the preparation of the pyrrole-based starting materials.Part a of Scheme 11 shows a route that featured intramolec-ular C-H activation by a nitrene, part b involves a Trofimovreaction,137and part c shows a classical condensation route.220Oxidation of 199a was attempted, but only the half-oxidized product 200 could be obtained; further treatmentwith excess DDQ at room temperature for overnight gaveno reaction (Scheme 12). Semiempirical calculations showedthat the activation energy required to form 201was excessivecompared with the half-oxidized form 200 due to steric rea-sons. The physical properties of the mono-oxidized productwere somewhat surprising. First, the maximum absorbancewas not shifted to the red relative to 199a, only the fluores-cenceemissionwasshiftedtotheredby21nm. Second,althoughtheconjugationofthesystemwasextended, theextinction coefficient was 3 times smaller than the reducedform of the dye 199a (41 000 vs 126 250, respectively); thequantum yield was also less than half that of 199a.2-MethoxygroupsonR-aryl groupsprovideaspecialopportunityfor syntheses of constrainedBODIPYdyes.Thus, 202, wherein rotation around a C-Ar bond wasprevented by B-Obond formation, was convenientlyobtained by demethylation and intramolecular cyclization of89 (Scheme 13).221Dye 202 has a red-shifted, sharperfluorescence emission than the BF2parent dye and itsquantum yield is 5.5-6.0 times larger.9.2.ExtendedAromaticSystems9.2.1.Di(iso)indometheneDyesAromatic ring-fused BODIPY derivatives, boron-di(iso)-indomethene dyes 204, have been prepared via retro Diels-Alder reactions222,223featuring a norbornane-derived pyrrole(Scheme 14).224The spectroscopic data for the di(iso)-indomethene derivatives 204 are shown below. The natureof the substituents at the meso position has no influence onthe absorption and emission properties of the new dyes. Thenew di(iso)indomethene dyes display a red-shifted absorptioncompared to the bicyclic-BODIPY precursors 203. The moreScheme 12. Synthesis of More Conjugated BODIPYDerivativesScheme 13. Synthesis of a New NIR BODIPY DyeScheme 14. Extended Aromatic Dyes from Retro-DielsAlder ReactionsBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4919expanded, conjugated, and rigid di(iso)indomethene BODIPYdyes204arealsocharacterizedbyasignificantlyhigherextinction coefficient relative to the bicyclic-BODIPY dyes203. Because of their rigid-fused system, which prevents anonradiative deactivation of the excited state, bicyclic-BODIPY203anddi(iso)indometheneBODIPYdyes204have small Stokes shifts. The Stokes shifts are, however,generally larger for bicyclic-BODIPY dyes 203 than for thedi(iso)indomethene derivatives 204. The absorption andemission spectra of bicyclic-BODIPY203 and di(iso)-indomethene BODIPYdyes 204are independent of thesolvent polarity. The fluorescence quantumyieldof thebicyclic-BODIPY dyes 203 is much higher than the one forthecorrespondingdi(iso)indometheneBODIPYdyes 204.Other indomethene dyes including 172190and 206-209190,225have been prepared via a different route. Substituted 2-acylace-tophenones 205, obtained from 2-hydroxyacetophenones andhydrazines,226werecondensedwithammoniatogivethedibenzopyrromethene. Theseweretreatedwithborontri-fluoride to give the 3,4:3,4-dibenzopyrrometheneborondifluoride core (Scheme 15). The new dyes exhibit very longwavelength absorption and emission bands, and are relativelystable to photobleaching.9.2.2.DyesBasedOnBenz[c,d]indoleBenz[c,d]indole and its derivatives have been used to formdeeply colored dyes. Compound 210 (reaction 19), forinstance, was obtained by condensation of 2-methylthiobenz-[c,d]indolium iodide and 2-methylbenz[c,d]indolium iodide,followedbycomplexationwithborontrifluorideetherate.Its fixed planar structure exhibits a sharp absorption band at618 nm, and fluoresces at 625 nm.227,2289.2.3.Porphyrin-FusedSystemsSystems 212-214 have edge-shared porphyrin and BO-DIPYparts. Theyweresynthesizedfromtheformylatedpyrroloporphyrin 211 via acid-catalyzed condensation with2,4-dimethylpyrrole. Their UVspectracontainfour quiteintense bands in the 300-750 nmregion, and do notresemblethesumof thespectraof tetraphenylporphyrin(TPP) and of BODIPY. Systems 212 and 214 emit, respec-tively, at 693 and 714 nm upon excitation over all the 240-700 nm region with low quantum yields, and 213 did notshow any significant fluorescence.1210.Aza-BODIPYDyes10.1.TetraarylSystemsSynthesis of the azadipyrromethene chromophore 215(without boron, but with aryl substituents) was first describedin the 1940s.229-231This, as described below, is the frame-workfor a veryinterestingset of dyes calledthe aza-BODIPYs. Two general methods to prepare these compoundswere developed. In one, 2,4-diarylpyrroles were convertedinto their 5-nitroso derivatives, then condensed with a secondScheme 15. Synthesis of Di(iso)indomethene Dyes from2-Acylacetophenone Derivatives4920 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessmolecule of pyrrole (Scheme 16a). Scheme 16b shows thesecond method, in which Michael addition products D fromchalcones and nitromethane,229or cyanide E,230,231was react-ed with formamide (or other ammonia-sources) to give thecore 215. In the first instance, these reactions were performedneat, that is, without solvent. Soon after, it was realized thatuse of alcohol solvents usually causes the azadipyr-romethenestoprecipitatefromthereactionmixture, thus,enhancing the ease of isolation and yields. The postulatedmechanism for formation of the azadipyrromethene core fromthe nitromethane adducts is shown in Scheme 16c. Itproceeds via pyrrole 217, which is nitrosylated in situ to give218 that then condenses with another molecule of the pyrrole.It is also possible to prepare the azadipyrromethenechromophore from cyanide Michael addition products E asdescribed in Scheme 16d. Ammonium formate serves as asource of ammonia in the pyrrole-forming condensation step.The2-aminopyrroleintermediates 219readilyconverttothe azadipyrromethene core on heating in the air. Switchingfromacetatetoformateinthedrymelt process for thesynthesis of 215 nearly doubled the yield to give ap-proximately 50%.231Formation of this chromophore seemsto be most facile when there are four phenyl substituents,but it is possible to obtain the diphenyl products 222 (below)by slight modification of the reaction conditions.231The first reactions of azadipyrromethenes with boron elec-trophiles were reported in the early 1990s (Scheme 17).131,232Scheme 16. Formation of AzapyrrometheneaaFormation from (a) nitroso pyrrole and (b) nitrobutyrophenones D and keto-nitrile E; (c) postulated mechanism for the formation of azapyrromethenefrom nitrobutyrophenones D and (d) from keto-nitrile E.BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4921The aza-BODIPY 221 was made from the 3,5-tetraphenylazapyrromethene 220 in that way. It was reported that theless substituted aza-BODIPY 223 could not be obtained aftertreatment of the corresponding aza-dipyrromethene 222 withboron trifluoride;232however, this might have been an artifactof the particular conditions screened.Beginning with research largely from OSheas group from2002 onward, there has been a resurgence of interest in aza-BODIPYdyes, andthishasresultedinsynthesesofdyeslike 224-227.233-235In this latest era of the field, the azadi-pyrromethene skeletons are still prepared from nitromethaneadducts to the corresponding chalcone according to the routedescribed in Scheme 16c, but butanol, rather than methanolor solvent-free conditions, was the preferred medium.234Thesyntheses were completedbyaddingBF3OEt2at roomtemperature;theintermediateazadipyrromethanesprecipi-tatedout inhighpurities andwerenot purifiedbeyondwashing with ethanol.233,234UV absorption maxima of the tetraarylazadipyrromethene-BF2 chelates strongly depend on the Ar-substituents. para-Electron donating groups on the 5-Ar substituents giveincreasedextinctioncoefficientsandsignificant redshiftsin the maxabs (149 nm for dimethylamino vs H, cf. 227 vs221).235para-Substitution with an electron donating groupon the 3-aryl ring has less impact, but still gives abathochromic shift (cf. 224 vs 225).234The UV absorption maxima of the aza-BODIPY dyes arerelatively insensitive to solvent polarity; only small blue shiftstend to be observed (6-9 nm) when switching solvents fromtoluene to ethanol. Their absorptions are sharp, with a fullwidth at half-maximum height (fwhm) varying from 51 to67 nmin aqueous solution with an emulsifier calledCremophor EL, and 47-57 nm in chloroform indicating thatthe dyes do not aggregate under those conditions. Theextinctioncoefficientsrangefrom75 000to85 000M-1cm-1, whichismuchgreater thansubstitutedporphyrins(3000-5000 M-1cm-1) for instance. This strong absorbanceis one of the factors that facilitate efficient singlet-oxygengeneration; hence, thesemolecules arepotentiallyusefulphotosensitizers for photodynamic therapy.236-239Fluorescence emission spectra of the aza-BODIPY dyesreported to date are also relatively insensitive to the solventpolarity. Compounds 221 and 224-226 have high fluores-cence quantum yields. In the case of 226, there is a bromineatom attached to the phenyl substituent, but no significantdecrease inthe quantumyieldwas observed.234Ontheother hand, introduction of bromine directly into the core ofthe dye, for example, compounds 228-230results inaScheme 17. First Syntheses of Aza-BODIPY Dyes4922 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgesssignificant decrease of the fluorescence quantumyields,indicating a larger heavy-atom effect and an increased singletoxygen production.234A quantum yield was not reported for227, but the compound became much more fluorescent underacidicconditions duetovariationintheinternal chargetransfer.235Upon addition of acid, the absorption band at 799nm disappears, and a new band at 738 nm appears indicatingthe monoprotonated form. The later disappears with additionof moreacid, andanewbandcharacteristicof thebis-protonated species appears at 643 nm.Aza-BODIPYdyesmostlyhavebeendiscussedinthecontext of agents for photodynamic therapy, but chemosen-sors have also been made from these compounds. Compound231 is highly selective for mercuric ions that become chelatedbetween the 2-pyridyl groups.240Mercuric ion complexationred-shifts both the UV-absorption and fluorescence emissionmaxima. The dissociation constant was determined to be 5.4 10-6M, with a 1:1 binding stoichiometry.Compounds 232 and 233 are photoinduced electron-transfer crown ether chemosensors featuring aza-BODIPYchromophore;241theyareusedas visiblesensors for theparalytic shellfish toxin Saxitoxin. Saxitoxin contains guani-dine groups, and it is these functional groups that interactwith the crown ether part of these molecules. In the absenceof Saxitoxin, PET from the crown ether to the fluorophorequenches the fluorescence. Upon complexation of the toxin,PETcannolongerhappenandfluorescenceisturnedon.At 1:1 toxin/crown stoichiometry, the fluorescence enhance-ment was over 100% for compound 232. The average bindingconstant for 232 to Saxitoxin, 6.2 105M-1, was amongthehighest observedfor anychemosensor of that toxin.Compound 233, which has a larger crown ether, bound thetoxin less strongly (1.4 104M-1) and gave insignificantfluorescence enhancement. The putative PET mechanism inthese systems involves -stacking of one of the Saxitoxinsguanidiniums to the fluorophore.10.2.ExtendedAza-BODIPYSystemsStronglyelectron-donatinggroups,rigidifyingstructuralmodifications, and embellishments to extend the conjugationof theBODIPYdyestendtored-shift their fluorescenceemissions. Predictablythen, similar modificationscanbeused to push the emission spectra of aza-BODIPY systemsinto the near-IR.The extended aza-BODIPYsystems 234-246 weresynthesized from 2,4-diarylpyrroles. These starting materialswereobtainedingoodoverall yieldoverfourstepsfroman alkene via (i) addition of iodo azide, (ii) dehydro-halo-genation, (iii)pyrrolysis(azirineformation), and(iv)car-banion-induced pyrrole formation as shown in Scheme18.242,243The conformationally restricted aza-BODIPY dyes werethen prepared as shown in Scheme 19 via condensation ofthepyrrolegeneratedinScheme18withanitrosopyrrolegeneratedinsitu, followedbycomplexationwithborontrifluoride. Attempts to condense 2,4-dimethylpyrrole withanitrosatedrestricted2,4-diarylpyrrolefailedtogiveanyproduct, suggesting that the 2-aryl substituent in the pyrroleis essential for the formation of these restricted aza-BODIPYdyes.All the conformationally restricted systems 234-246absorb over 650 nm. Dyes 234-236 with both sidesincorporated in carbocyclic rings have narrow, intense (high) absorption bands at long wavelengths. In comparison tothe non-constrained tetraaryl-azaBODIPY 224, there is abathochromic shift of up to 52 nm and a concomitant halvingofthefwhm(27.1nmfor236vs52.0nmfor224). Thenovel dyes were reported to possess excellent chemical andScheme 18. Synthesis of Cyclized/Restricted2,4-DiarylpyrrolesScheme 19. Synthesis of (a) Symmetrical and (b)Asymmetrical Conformationally Restricted AzaBODIPYDyesBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4923photostability. Furthermore, the fluorescence is insensitiveto solvent polarity.242,243There are also several details regarding the spectralproperties of the dyes inthis series. Substitutionat the3-position of the core does not affect the -value of the dyes(compare 234-236). Introduction of electron-donating groupsresults in a small blue shift and a slightly higher fluorescencequantum yield (234 vs 235). Shorter abs, lower, broaderabsorption band, and lower fluorescence quantum yield wereassociatedwiththesulfur-containingdye237. Thedehy-drogenatedcarbocyclicrestrictedringinsystem239de-creases its quantum yield relative to 234. Aza-BODIPYs withonly one side restricted have much lower extinction coef-ficients (240-246vs234). Thequantumyields of non-symmetric aza-BODIPY dyes are highly dependent on thesubstituents on the aromatic ring. Electron donating para-substituents give higher quantum yields (240 and 241), withshorter abs, indicatingthat thephenyl rings aretwisted.ortho-Electron donating groups (e.g., in 242) result in shortabs, low, and broad absorption bands, indicating that thephenyl rings in 242 are twisted. Unexpectedly shorter abs,low, broader absorption bands and lower quantum yieldswere observed when two methoxy groups were present (243and 244). The sharp and intense absorption, high quantumyield obtained in the special case of 245 wherein a 2-meth-oxy-1-naphthyl substituent is present indicates that thenaphthyl ringisdistortedsothat electrontransferissup-pressed.10.2.1.DyesBasedonBenz[c,d]indoleSynthesisofcompound248involvesrefluxing2-benz-[c,d]indolamine hydroiodide with 2-(methylthio)benz[c,d]-indole-hydroiodidein1,2-dichlorobenzeneinpresenceoftriethylamine, giving the amine 247. Subsequent treatmentwithborontrifluorideetherateaffordsthedesiredproduct248 (reaction 20).244Whilethebenz[c,d]indole-basedBODIPYdye 210dis-playsared-shiftedabsorptionandfluorescence, theaza-4924 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessBODIPY analogue 248 shows a blue-shifted absorption at540 nm. Unfortunately, no fluorescence properties have beenreported.11.OtherAnaloguesoftheBODIPYs11.1.Biimidazol-2-ylBF2ComplexesBiimidazol-2-yls difluoroborate complexes 249 have beenobtained by (i) reacting glyoxal bisulfite, 2,3-butanedione,and concentrated aqueous NH3 (Scheme 20a);245,246(ii) con-densation of 1,2-cyclohexanedione with 40% aqueous glyoxalin ethanol saturated with dry ammonia gas;246and, (iii) con-densation of oxamide with o-phenylenediamine in refluxingethylene glycol (Scheme 20c).247,248It would seem logicalthat similar dyes of the type 250 and 251 could be prepared,but attempts to do so have been reported as unsuccessful.249Theparent, non-B-coordinatedheterocyclicsystemsformolecules 249 are not significantly fluorescent, but the BF2complexes display strong fluorescence as shown in thediagrams. Predictably, red shifts for both the absorption andemission maxima, and increased extinction coefficients areobserved with increased conjugation and rigidity.11.2.Pyridine-BasedSystemsThe dipyridyl-2-yl-boron complexes 252-256 have beenprepared from reactions of bipyridines with boron electro-philes.250They absorb between 302 and 322 nm, with theexceptionofthecomplex 255, whichabsorbsat371nm.No fluorescence was detected for these complexes.Thepyridomethene-BF2complex 257wassynthesizedas shown in reaction 21 from the 2,2-dipyridylmethane byfusionwithsodiumborohydride.251Theproduct showsastrongabsorptionat468nm,withaextinctioncoefficientof 17 783 M-1cm-1in chloroform, but does not fluoresce.The 10-azapyridomethene-BF2 complex 258a was obtain-ed via condensation between dipyrid-2-ylamine and boron tri-fluoride.232Similarly, 10-azapyridomethene-B(Et)2 258b wasobtained by condensation with triethylboron (reaction 22).Complex 258a absorbs at 382 nm (log) 4.47) in ethanol,and strongly fluoresces at 422 nm ( ) 0.81).Treatment of dipyrid-2-ylamine with methyl iodide givesthe methyldipyrid-2-ylamine, which can be converted to the10-methyl-10-azapyridomethene-BF2 complex 259 by treat-ment with boron trifluoride. The homologous complex 260Scheme 20. Synthesis of Biimidazol-2-yls-BF2 ComplexesBODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 4925was obtainedbycondensationbetween2-amino-6-meth-ylpyridineand2-chloro-6-methylpyridine, andsubsequenttreatment with boron trifluoride. Both these complexes arefluorescent, but theneutral one, 260, fluorescesat longerwavelength.11.3.2-KetopyrroleComplexes252The 2-ketopyrrole complexes 261-265 were isolated asminor byproducts (less than5%) inthesynthesis of thecorresponding BODIPY analogues.252The yield of 261-265could be improved by increasing the amount of acid chlorideusedinthe synthesis. 2-Ketopyrrole complexes are lessconjugatedthanBODIPYdyes; hence, their absorption,extinction coefficients, and emission wavelengths are shorter/smaller. The Stokes shifts for these compounds, however,are greater thanfor BODIPYsystems (48-154nm, cf.approximately 10-15 nm for BODIPY dyes).11.4.AzobenzeneDerivativesAzobenzenes are the most common chromophores incommercial dyes and, because of photoinduced isomeriza-tion, they are used as photoresponsive molecular switches.A consequence of facile photoisomerization is that quantumyields for these compounds are so low that fluorescence tendsonly to be observed in a rigid matrix at low temperature. Arecentinnovationinthisfield,however,isfixationofthe-conjugatesystemsusingboryl-entitiestogivethefluo-rescent compounds 266-268 (reaction 23).253Azobenzene dyes 266 and 267 display a remarkable red-shifted absorbance (386 and 439 nm, respectively) relativeto unsubstituted (E)-azobenzene ( ) 315 nm). Irradiationof 266and 267withasuper-high-pressuremercurylampdid not cause photoisomerization. The azobenzene derivative268, bearingafluorinatedsubstituent ontheboronatom,showed almost no fluorescence. We speculate that this couldbe due to intramolecular charge transfer fromthe leastelectronricharyl substituent intotheexcited-stateofthecomplexed azobenzene system.11.5.MiscellaneousN,N-BidentateDiphenylBoronChelatesLuminescent N,N-bidentate diphenylboron chelates 269-281254,255have emission maxima that vary with the natureof the parent organic system. Limited information wasprovidedregardingthe spectroscopic properties of thesemolecules, but thedatapresentedareinterestingbecausesome of the dyes have high quantum yields and/or fluores-cenceemissions around600nm. Thesesystems perhapswarrant further attention.Avarietyofmethodswereusedtoproducetheparentheterocyclesforthedyesshownabove(Scheme21). The2-(2-pyridyl)indole- and 2-(2-thiazolyl)indole-based ligandswere synthesized by a two-step Fischer synthesis (Scheme21, panels a and b, respectively). Scheme 21c illustrates howa Negishi coupling256was used for the synthesis of the 2-(8-quinolyl)indole or -azaindole. Treatment of the productheterocycles shown in Scheme 21 with triphenylboron (1:1ratio) in toluene afforded the boron complexes 269-281 ingood yields.4926 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgess11.6.Boryl-SubstitutedThienylthiazolesFinally, therearecompoundsthatareevenlesscloselyrelated to the BODIPY core, formed by intramolecular B-Ncoordination in N-heteroaromatic systems. This new conceptis illustrated for the development of new electronic materials.The interaction between the Lewis acid (boron) and Lewisbase (nitrogen) not only constrain the -conjugated frame-work in a coplanar fashion, but also lower the LUMO level(Figure 8). Thus, (3-boryl-2-thienyl)-2-thiazole 282 wassynthesizedfrom(3-bromo-2-thienyl)-2-thiazoleasshownin Scheme 22.257Compound 282 can easily be functionalizedtothe tin283or iodo284derivatives via the lithiatedintermediates. Lithiation occurs regioselectively at the 5-po-sition of the thiazole ring.The corresponding tin and iodo derivatives can be usedas building blocks in metal-catalyzed coupling reactions toprepareextended-electronsystemssuchasthehead-to-head (H-H), head-to-tail (H-T), and tail-to-tail (T-T)dimers 285-287 (Scheme 23). The dimers display red-shiftedabsorption and emission relative to the monomer. The boryl-substitutedthienylthiazoles 282and 285-287showweakfluorescence emission in the range 452-492 nm.257Scheme 21. Two-Step Fischer Synthesis of (a) 2-(2-Pyridyl)-indole, (b) 2-(2-Thiazolyl)indole, and (c) Synthesis of 2-(8-Quinolyl)indole or Aza-indole via Negishi CouplingFigure 8. Intramolecular B-N Coordination of N-heteroaromaticsystems.BODIPYDyesandTheirDerivatives Chemical Reviews,2007,Vol.107,No.11 492712.ConclusionApplications of BODIPY dyes outnumber, by far, studiesof their fundamental chemistry and spectroscopic properties.This is unfortunate because they are such powerful tools forimaging, chemosensors, lasing materials, and so forth. Therewould be even more applications if some deficiencies in thearea were addressed. For instance, there are very few water-soluble BODIPYdyes. In this review, we mentionedcompounds53-56and69d. Theonlyotherexamplesofwater-soluble BODIPYdyes we found were lipophilicBODIPYs made water-soluble by activation with a sulfonatedN-hydroxysuccinimide (NHS) derivative. 4-Sulfo-2,3,5,6-tetrafluorophenyl (STP) BODIPY esters derivatives have alsobeen synthesized, and proved to be more water-soluble andmore reactive with a primary amine than the correspondingN-hydroxysuccinimidyl esters.258Oneconspicuousareaforfurtherresearchisthedesignand synthesis of BODIPY derivatives that emit further intothenear-IR. Stepstowardthiscanbetakenbyattachingaromatic groups, preferably with electron-donating substit-uents(e.g., 87), rigidifyingtheir structure(e.g., 202), orproducingring-fusedsystems like172. Oneof themostScheme 22. Synthesis of the Dimesityl-boryl SubstitutedThienylthiazole 282Scheme 23. Synthesis of (a) Head-to-Head, (b) Head-to-Tail,and (c) Tail-to-Tail Dimers4928 Chemical Reviews,2007,Vol.107,No.11 LoudetandBurgessinnovative synthetic procedures for doing this involvescondensation of BODIPY methyl substituents to give dyessimilar to ones that Invitrogen markets such as BODIPY630/650 and BODIPY 650/665.Another synthetic innovation is the chlorination of certainBODIPYs to give 3,5-dichloro-derivatives. These productsmay be functionalized via Pd-mediated couplings orSNAr reactionstogiveproductsthat tendtofluoresceatlonger wavelengths. Compounds 65 and 110d are examplesof the products that can be formed by this approach.Many papers have been published on BODIPY-likesystems in which fluorine atoms on the BODIPY dyes arereplaced by a variety of C- or O-based nucleophiles to giveproductsF. Thesemodificationshaveprovenuseful asameans to make energy transfer systems. Substitutions of thiskind may alter the quantum yields and fluorescence proper-ties of the dyes, but they do not tend to shift their emissionsto the red. Conversely, incorporation of a cyanide group atthe meso-position gives a dramatic shift toward the near-IR.SubstitutionoftheboronatominBODIPYdyesgivesfluorescent Zn, In, and Ga derivatives, but other metals tendto quench the fluorescence. Quenching in such derivativespresumably occurs via electron-trasnfer mechanisms involv-ing the dyes in their excited states.Replacement of C-8 in the BODIPY with a nitrogen givesthe so-called AzaBODIPYs. These are an extremelyinterestingset ofdyesbecauseoftheirlongfluorescenceemissions wavelength. No water-soluble derivatives of thesehave been prepared, and nearly all the substituted compoundsfeature aryl groups in the 3,5-positions.Overall, anabundanceofrelativelystraightforwardap-plicationsofBODIPYdyeshavebeenreported. Someofthe most significant synthetic procedures tend to be writtenin cryptic styles that are so often seen in the patent literature,but there have been some recent milestones in this area. Themost important future developments with respect to applica-tions in biotechnology will involve synthesis of water-soluble, easilyfunctionalizedsystems, particularlythosefluorescing above 600 nm.13.References(1) Nakanishi, J.; Nakajima, T.; Sato, M.; Ozawa, T.; Tohda, K.;Umezawa, Y. Anal. Chem. 2001, 73, 2920.(2) Yamada, K.; Toyota, T.; Takakura, K.; Ishimaru, M.; Sugawara, T.New J. Chem. 2001, 25, 667.(3) Frangioni, J. V. Curr. Opin. Chem. Biol. 2003, 7, 626.(4) Weiss, S. Science 1999, 283, 1676.(5) Moerner, W. E.; Orrit, M. Science 1999, 283, 1670.(6) Ha, T. Methods 2001, 25, 78.(7) Nie, S.; Zare, R. N. Annu. ReV. Biophys. Biomol. Struct. 1997, 26,567.(8) Tamarat, P.; Maali, A.; Lounis, B.; Orrit, M. J. Phys. Chem. 2000,104, 1.(9) Ross, A. H.; Daou, M.-C.; McKinnon, C. A.; Condon, P. 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