areno-condensed annulenes – extended discotic mesogens€¦ · photochemistry 1 introduction...

PAPER 1213

Areno-Condensed Annulenes – Extended Discotic MesogensAreno-Condensed AnnulenesH. Meier*Institute of Organic Chemistry, Johannes Gutenberg University, Mainz, GermanyFax +49(6131)3925396; E-mail: [email protected] 24 February 2002

Synthesis 2002, No. 9, 01 07 2002. Article Identifier: 1437-210X,E;2002,0,09,1213,1228,ftx,en;C10502SS.pdf. © Georg Thieme Verlag Stuttgart · New YorkISSN 0039-7881

Abstract: [n]Annulenes (n �12) condensed with 2–4 aromatic ringsystems (benzenes, naphthalenes, anthracenes, phenanthrenes,chrysenes, pyrenes) can be prepared by cyclocondensation reac-tions or ring transformations. Due to the local arene aromaticity, themolecules can be regarded as aromatic islands, which are connectedby olefinic bridges. The compounds are non-planar, but the majorityof the systems shows a fast inversion of the central macrocyclicring, so that the molecules appear on average as large planar discs,which consist of extended � systems. The aggregation tendency (�stacking) of the compounds can be strengthened by the attachmentof flexible saturated chains on the periphery. The discs representthen mesogens for columnar (or nematic) discotic LC phases. Be-cause of the stilbenoid character, the compounds show a variety ofinteresting photophysical and photochemical properties. They canbe applied in photoconductive liquid crystalline phases Colh and inradiation-induced imaging techniques.

Key words: annulenes, condensation, ring closure, aggregation,photochemistry

1 Introduction

Annulenes 1 (Figure1) represent a highly interesting classof compounds in synthetic as well as in theoretical chem-istry. 1–6 However, because of their low thermal and pho-tochemical stability, annulenes are hardly suitable forapplications in materials science, although some promis-ing attempts have been made.7–9

Figure 1

The condensation of benzene or higher arene ring systemson the annulene perimeter enhances significantly the ther-mal stability. In the series of [18]annulene (2),10 a varietyof benzo-condensed compounds has been studied, for ex-ample the monobenzo system 3,11 the dibenzo systems 412

and 5,13 the tetrabenzo system 6a,14 the hexabenzo system7,15 which represents a hexa-m-phenylene, the ‘nonaben-zo’ system 816 and the ‘dodecabenzo’ system 9,17 well-known as kekulene (Figure 2).

Figure 2 [18]Annulenes condensed with benzene ring systems

Apart from the fact, that all these hydrocarbons are disc-like compounds with a central 18-membered ring, thechemical character of the compounds is different. Themacrocyclic aromaticity (diatropicity), present in 2, is re-duced in the monobenzo derivative 311 and disappears to-tally in the higher condensed systems 4, 5, 6a, etc. in favorof the local benzenoid aromaticity.12–17

We were mainly interested in areno-condensed annuleneslike 6a, which represent extended discotic mesogens andconsist of stilbenoid units with interesting photophysicaland photochemical properties for materials science, par-

1214 H. Meier PAPER

Synthesis 2002, No. 9, 1213–1228 ISSN 0039-7881 © Thieme Stuttgart · New York

ticularly for columnar liquid crystals.18 The general for-mula 10/10� and the aromatic building blocks listed inTable 1 characterize these molecules. The annelation ofthe aromatic ring systems with the central annulene ring isalways on the shorter (concave) section of the circumfer-ence of the arene. The size of the central annulene ringdiscussed in this article varies mainly from 12 to 24; buteven in this range, many possible combinations of 10/10�with the aromatic building blocks shown in Table 1 werenot yet synthesized.

2 Synthetic Methods

The synthetic strategy towards the areno-condensed annu-lenes 10/10� is mostly based on the preparation of appro-priately functionalized arenes. The final step consists thenof the generation of CC double bonds, which are formedin cyclocondensation reactions (type A–B or AA–BB)(Equation 1).

Linear condensation products are always formed in com-petitive processes. Due to the end groups, the linear prod-ucts have a higher polarity and can be easily separated.Although the generation of the linear products reduces the

yields of the cyclic products, cyclocondensation reactionsrepresent a convenient route for the preparation of the tar-get compounds. Alternatives are ring transformations ofpreformed cyclic compounds, for example the final intro-duction of olefinic double bonds by elimination reactions.

2.1 Siegrist Reactions

The majority of areno-condensed annulenes was preparedby applying the Siegrist reaction,19–21 which is a conden-sation process of N-arylimino groups and activated me-thyl groups in a strongly alkaline medium. For theapplication discussed here, both reactive groups are com-monly fixed on the same starting material so that two,three or four such molecules can form a cyclocondensa-

Equation 1

Table 1 Selection of Aromatic Building Blocks for the Areno-Condensed Annulenes 10/10�

CondensationType

Benzene Naphthalene Anthracene Phenanthreneand Higher Arenes

[a]

[a,b]

[a,b,c]

[a,b,c,d]

[a,b,c,d,e]

[a,b,c,d,e,f]

PAPER Areno-Condensed Annulenes 1215


tion product. Due to the strict anti elimination of aniline,the kinetically controlled reaction is highly stereoselec-tive. The trans/cis ratio amounts to about 1000:1, which isfar beyond the thermodynamic equilibrium.18 Scheme 1demonstrates that the size of the generated central ring de-pends on the relative position of the two reactive groupsand on the number of involved arene molecules.

Figures 3 and Figure 4 summarize further examples ofareno-condensed [16]-, [18]- and [24]annulenes whichwere prepared by Siegrist ractions. The yields vary be-tween 67% for 6b and 3% for 23, which is a byproduct inthe preparation of 17e.

2.2 McMurry Reactions

In the series of benzo-condensed [12]- and [16]annulenes,terminal dialdehydes can be successfully applied for ringclosure reactions. Scheme 2 shows three typical examplesof one- and twofold intramolecular McMurry reactions.Compound 16a (Figure 3) is also accessible by a McMur-ry reaction.24

2.3 Pinacol Coupling Followed by the Corey–Win-ter Procedure

An alternative route for the intramolecular CC coupling ofdialdehydes makes use of the pinacol formation. The re-action of 28 with VCl3(THF)3 and zinc leads to a 2:1 mix-ture of the threo-diol 29 and the erythro-diol 30(Scheme 3). The threo isomer can be converted into theerythro form by successive Swern oxidation and reduc-tion with NaBH4. The reaction of 30 with thiocarbonyldi-imidazole (TCDI) yields the thionocarbonate 31, which istransformed to all-cis-tetrabenzo[a,e,i,m][16]annulene16c by the reaction with 1,3-dimethyl-2-phenyl-1,3,2-di-azaphospholidine (DMPD) in refluxing benzene.33 Theoverall yield for 28 � 16c is lower than for the McMurrystep 26 � 16b, but the modified Corey–Winter procedure

Scheme 1 Preparation of tetrabenzo[ab,f,jk,o][18]annulene (6a),14

trinaphtho[3,4,5-abc:3,4,5-ghi:3,4,5-mno][18]annulene (13a)22 andtriphenanthro[3,4,5,6-abcde:3,4,5,6-ijklm:3,4,5,6-qrstu][24]annule-ne (15a):23 i) KOC(CH3)3, DMF

Figure 3 Further areno-condensed [16]-, [18]annulenes, which were prepared by Siegrist reactions22, 24–28

1216 H. Meier PAPER


gives selectively the all-cis-configuration 16c – in con-trast to 16b, which has the all-trans-configuration. Anal-ogous reaction sequences can be used for the preparationof all-cis-tribenzo[a,e,i][12]annulene 25d and all-cis-

pentabenzo[a,e,i,m,q][20]annulene 32a,33 which aredrawn in Scheme 3 in the (also for 16c) more realisticcrown conformation.

Figure 4 Further areno-condensed [18]- and [24]annulenes, which were prepared by Siegrist reactions23, 24, 27–31



2.4 Wittig–Horner Reactions

Intramolecular Wittig–Horner reactions represent anotherwell-established method for the generation of areno-con-densed annulenes. Interestingly, phosphonium salts andphosphonates exhibit different cyclization tendencies.Whereas 33 yields 26% of the (Z,Z)-dimer 34 and onlytraces of the (E,E,E)-trimer 25e,34 the related phosphonate35 gives a mixture of all-(E)-configured cyclooligomersup to the 36-membered ring 39 (Scheme 4).24 The pre-ferred formation of (E)-configurations excludes the gener-ation of 34; moreover, the [12]annulene 25e cannot bedetected in the mixture of cyclooligomers.24 Figure 5shows the MALDI-TOF spectrum of the benzo-con-densed [n]annulenes (n = 16, 20, 24, 28, 32, 36). The ma-jor products in both reactions have linear structures; thepreparative value of these processes is limited, since theseparation of the cyclooligomers is rather laborious.

Instead of cyclocondensation reactions of the AB type,AA + BB processes can be applied with superior results.(E,E,E)-Tribenzo[a,e,i][12]annulene 25e can be obtainedby the reaction of phthaldialdehyde 40 and the diphospho-nium salt 41.35 The additionally formed (E,E,Z)-isomerrearranges spontaneously by an intramolecular [2� + 2� +2�] process to yield a polycyclic compound, namely tetra-cyclo[6.4.0.0.4,1205,9]dodeca-2,6,10-triene (See Scheme 9,vide infra).36 The Wittig reaction of 40 and 42 furnishestetrabenzo[a,e,i,m][16]annulene (16b).37 The two an-thracene derivatives 43 and 44 yield the dianthra[14]an-nulene 4538 (Scheme 5).

2.5 Ring Transformations

Apart from cyclization reactions in the final step, the syn-thesis of areno-condensed annulenes 10/10� can be

Scheme 2 Benzo-condensed [12]- and [16]annulenes prepared byintramolecular McMurry reactions : i) TiCl3/Zn�Cu, DME

24, 32 Scheme 3 Preparation of the tetrabenzo[a,e,i,m][16]annulene 16cby pinacol coupling followed by the Corey–Winter procedure. (Ana-logous reaction sequences can be used for the preparation of 25d and32a)

1218 H. Meier PAPER


achieved by transformation reactions of preformed (mac-rocyclic) ring systems. The generation of three olefinicbridges –CH=CH– by bromination and dehydrobromina-tion of three saturated bridges –CH2–CH2– can be used forthe preparation of (E,E,E)-tribenzo[a,e,i][12]annulene(25e).39 On the other hand, acetylenic bridges –C�C– canbe partially hydrogenated with Lindlar catalyst to olefinicbridges. Interestingly (E,Z,Z)-tribenzo[a,e,i][12]annulene

Scheme 4 Benzo-condensed annulenes obtained by Wittig and Hor-ner reactions of the AB type

Scheme 5 Wittig reactions of the AA-BB type for the preparationof benzo- and anthra-condensed [12]-, [14]- and [16]annulenes

Figure 5 MALDI-TOF spectrum of the cyclic products obtained by the Horner reaction of phosphonate 35



(25f) was obtained by this method (Scheme 6).40 Thermalor photochemical isomerization of 25f yield the (E,E,Z)-configuration which spontaneously rearranges to give thepolycyclic compound mentioned above. An example,where the olefinic bridges already exist, but the con-densed aromatic ring system has to be changed, is pub-lished for the formation of 48 by the twofoldphotoyclization of 47.40 The cyclic trimer 49 of benzocy-clobutadiene served as a highly efficient precursor fortribenzo[a,e,i][12]annulene. The all-cis arrangementpresent in 49 is almost completely retained in the intramo-lecular [2� + 2� + 2�] process.41

3 Molecular Structures

The parent annulenes with (4n + 2) � electrons (n = 3, 4,5, 6) exhibit a diatropic behavior. They have a planar oralmost planar structure, which corresponds to a compro-mise between the repulsion of the inner hydrogen atomsand the resonance stabilization of the macrocyclic ring.4,5

The condensation of the macrocyclic rings with benzeneor higher arene ring systems provokes a change of themacrocyclic aromaticitiy to the ‘local’ aromalticity of thearenes. Thus, one can consider the areno-condensed annu-lenes as molecules, which consist of aromatic ‘islands’,which are connected by olefinic bridges. This statementholds for all systems irrespective of the number of � elec-trons in the central macrocyclic ring. A typical conse-

quence of the ‘local’ benzenoid aromaticity is the fact,that the inner (olefinic) protons Hi have somewhat higher� values in the 1H NMR spectra than the outer olefinicprotons Ho – provided that there is no fast exchange be-tween Hi and Ho. The macrocyclic ring current, which ischaracteristic for the parent annulenes1–6 and permits aneasy destinction between 4n and (4n + 2) � electron sys-tems, is not present in the areno-condensed annulenes dis-cussed here.42

The existence of all-trans configurations in the areno-condensed annulenes 10/10� is a necessary but not suffi-cient precondition for on average planar, disc-like mole-cules. One or more cis configurations as in 16c, 25d, 25f,32a and 48 cause crown- or boatlike structures, which donot have the capability of planarization. Concerning theconformation of the all-(E)-configured compounds 10/10�, the trinaphtho-condensed [18]annulene 13a mayserve here for a detailed discussion.22 The conformationalmobility of 13a is guaranteed by torsions around the CCsingle bonds on both sides of the olefinic double bonds.Figure 6 illustrates this mobility by the variation of thetorsion angle �. The planar conformations A and E, eachpossessing C3h symmetry, ensure maximum conjugation,yet incorporate high steric energy, which is caused bystrong H–H interactions. According to force field calcula-tions (MMX, Serena, PCM Version 4), A and E representtransition regions, whereas conformer B (� = 32°) is theglobal minimum (�Hf = 164.3 kcalmol

–1).22 The H–H in-teractions are minimized in the saddle region C (� = 90°),but the conjugation is completely interrupted. Further tor-sion can lead back to A via the energetically high lyingminimum D (� = 140°, �Hf = 174.1 kcalmol

-1) and thetransition region E. Of course, the rotation does not haveto be synchronous in all three positions indicated in A.The force field calculation reveals a further minimum B�(�Hf = 165.3 kcalmol

–1) almost isoenergetic to B, inwhich two inner olefinic protons are standing upwardsand one downwards. Since B (C3) and B� (C1) are chiral,the corresponding enantiomers have to be included in theconformational dynamics.

A part of the calculated energy hypersurface is shownin Figure 7.43 The contour plot of the energy illustrates,that smooth diastereoisomerization routes B(�1 = �2 = �3 = –32°) � B� (�1 = �3 = –32°, �� = +32°)and B � B�� (�1 = �2 = +32°, �3 = –32°) and enantiomeri-zation routes B� � B�� exist, which are characterized bylow activation barriers (Ea 2 kcalmol

–1). Thus the com-pound 13a appears to be planar on average (de facto C3hsymmetry). The inner olefinic protons of 13a show theirresonance at lower field than the outer olefinic protons[�(Hi) = 8.21, �(Ho) = 7.58]. The assignment was estab-lished by deuterium labeling and NOE difference spec-troscopy including the two-dimensional ROESYtechnique.22 Due to the macrocyclic aromaticity, the un-substituted [18]annulene exhibits the opposite behavior[�(Hi) = –2.99, �(Ho) = 9.28]

1. Moreover, inner and outerprotons do not exchange in 13a, because conformation Dis too unfavorable to be populated.

Scheme 6 Preparation of areno-condensed annulenes by differentring transformation reactions

1220 H. Meier PAPER


The other [18]annulenes condensed with phenanthrenes(17, 18) pyrenes (19) or chrysenes (20) show very similarconformational dynamics.22,27–31,43 Thus the areno-con-densed [18]annulenes are specially suitable as disc-likemesogens with a great diameter.

Can the design of the mesogen be still improved, if largerannulenes (n >18) are used? Let us consider the areno-condensed [24]annulenes. They can be devided in twocategories: the triareno-[abcde]-fused systems 15 and 21and the tetraareno-[abc]-fused systems 22 and 23. Theconformational dynamics of the first series includes C3and C1 species, and fast flip processes like in the areno-condensed [18]annulenes lead on average to a pseudo C3hsymmetry. The second series shows a different behavior.The force field calculation reveals S4 conformations asglobal minima (Figure 8). The conformational dynamicsinclude C1 and C2 species; however, the molecules cannever adopt a plane of symmetry. Thus, the geminal pro-tons of the OCH2 groups in 22 and 23 are diastereotopic.The S4 conformation (the lack of planarity) is not a resultof the steric repulsion of the inner hydrogen atoms; thedifferent molecular structures and dynamics in the seriesof the areno-condensed [24]annulenes are a consequenceof different perimeter sequences of trans-fixed, cis-fixedand flexible bonds of the 24-membered rings.28 The 1HNMR-spectra of the OCH2 protons provide in systemswith alkoxy sidechains a proof for the fast planarization ofthe central ring or its absence. Interestingly, the enan-tiotopic OCH2 protons in the series 13, 15, 17–21 did notbecome diastereotopic on cooling, nor did the diaste-

reotopic OCH2 protons of 22 and 23 become enantiotopicon heating. 28,30 Consequently, only the ring systems of thefirst series 15 and 21 of [24]annulenes are suitable as dis-cotic mesogens.

Figure 8 S4 conformation of [24]annulene condensed with fournaphthalene units (MMX calculation: Serena, PCM Version 4)28

An important point concerns the exchange of inner andouter (olefinic) protons – a process that is well known forthe parent annulenes. As mentioned above, this exchangedoes not occur in the series of the areno-condensed[18]annulenes 13 and because of similar reasons also notin the series 17–20. Nevertheless, such molecular dynam-

Figure 6 Illustration of the conformational dynamics of compound13a

Figure 7 Contour plot of the MMX energies (in kcal�mol–1) of sel-ected conformations of 13a;43 (The torsional angles �1 and �2 weresystematically varied, �3 and the other molecular parameters were op-timized without symmetrization.) Isomerization routes between dia-stereoisomers (––) and between enantiomers (...)



ics can be observed in the series 15, 16, 21 and 25.23,24,32

The exchange mechanism is independent of the pla-narization. Whereas for example 15a and 25e exhibit bothfeatures, 16a and 16b preserve diastereotopic OCH2 pro-tons, although the fast exchange of inner and outer protonsprovokes a singlet signal for the olefinic protons.

Semiemperical calculations (AM1, PM3) agree well withthe conformations found in 1H NMR measurements; nev-ertheless, we performed also an ab initio Hartree-Fockcalculation at the 3-21 G level for the molecule 6a.14 Ac-cording to this calculation, the free molecule has C2h sym-metry. The planar D2h geometry represents the compoundin the ‘time average’. Concerning the bond lengths and thebond angles, the parameters obtained in the crystal struc-ture analysis of 6a show a very good agreement with thecalculated data. However, the torsion angles are muchsmaller in the crystalline state that means the molecule ismuch flatter than calculated. Table 2 gives a comparisonof the calculated parameters for the free molecule and thedata measured in the crystal.

4 Aggregation

On the theoretical basis of the TURBOMOLE ab initioprogram package,44 dimeric aggregates of 6a were calcu-lated.14 The most stable molecular pairs have a C2h or a Cisymmetry (Figure 9). The gas phase calculation indicatesin both cases an energy gain of 4.7 kcalmol–1 for the in-termolecular attraction.

Figure 9 Intrastack molecular pairs of 6a with C2h (left side) and Cisymmetry (right side); The Å values indicate the distances betweenneighbouring olefinic bonds.14

Within the C2h molecular pair, the upper molecule is shift-ed with respect to the lower molecule so as to keep thecommon mirror plane; in the Ci pair, the shift is mainly ina direction perpendicular to that realized in the C2h pair.Both arrangements are principally suitable for the genera-tion of higher aggregates, for example in columnar me-sophases of compounds with long flexible sidechains or inthe crystalline state, where a herringbone design(Figure 10) was established by high resolution electronmicroscopy and X-ray analysis.14 The observed intrastackdimeric pair has Ci symmetry and an average distance of3.58 Å between the olefinic bonds.

Table 2 Selected Molecular Parameters of Compound 6a: Ab initio Calculation/Crystal Structure Analysis14

Bond Lengths/pm

2–3 3–4 4–5148.3/147.7 132.4/132.2 148.1/147.7

Bond Angles/°

1–2–3 2–3–4 3–4–5 4–5–6120.7/120.5 124.4/126.2 124.8/125.4 121.3/121.5

Torsion Angles/°

1–2–3–4 3–4–5–6–41.6/–20.9 +35.9/+11.1

Figure 10 Model structure obtained for the high-resolution imagein the electron diffraction measurement of crystals of 6a14

1222 H. Meier PAPER


An aggregation tendency was found for the majority ofareno-condensed annulenes. Long saturated chains at-tached to the periphery of the disc-like moleculesstrengthen the aggregation. It is not always easy to detectthe aggregation in solution experiments. Concentration-dependent intensities in UV/Vis spectra are reliable indi-cations – even when the effects are small as shown inFigure 11 for compound 15b.30 A concentration – depen-dent fluorescence can be due to ground state aggregates,which are maintained in the first excited singlet state, orto excimers.

Apolar solvents like cyclohexane provoke a stronger ag-gregation of the disc-like compounds with high � electrondensity than polar solvents, but in some cases the aggre-gation can even be seen in chloroform. Figure 12 exhibitsthe 1H NMR spectra of 15b at different concentrations inCDCl3. The line broadening at higher concentrations isdue to the formation (and dissociation) of aggregates andthe restricted mobility of molecules and molecule seg-ments in aggregates.45

Other proofs of aggregation make use of fluorescencespectra, fluorescence excitation spectra or of dynamiclight scattering experiments.28 However, all these mea-surements may exhibit intriguing features. Let us considercompound 19a. Pyrene is an excellent example for a dualfluorescence. Time-resolved measurements revealed, thatadditionally to the monomer fluorescence F, a fluores-cence band F� at longer wavelengths develops in concen-trated solutions on a ns time scale after the excitationpulse. After about 100 ns the measured spectrum corre-sponds to the steady-state fluorescence.46 The delay in theexcimer emission is due to the diffusion of a pyrene mol-ecule in the excited state S1 to a ground-state molecule S0(Equation 2) and to the geometrical change of the encoun-ter complex to an excimer, which is characterized by aparallel arrangement of the planar molecules in a distanceof about 3.0–3.5 Å.47

Equation 2

The tripyreno[18]annulene 19a shows also some depen-dence of the fluorescence spectrum on the concentration.However, there is no time-dependent evolution of theband shape.45 Despite the aggregation tendency, there isvirtually only one emitting species with an average life-time between 7.3 and 8.2 ns.45 The aggregates exist in theground-state S0 as well as in the excited singlet state S1,

Figure 12 400 MHz 1H NMR spectra of the triphenanthro[24]annu-lene 15b in CDCl3: bottom: 6.0�10–4 M, top: 6.0�10–3 M solution (Thesignals in the range 9.5 > � > 7.5 belong to the aromatic ABX spin sy-stem and the superimposed olefinic A2 singlet at 7.80 ppm, the signalat � = 4.3 to the OCH2 groups, the signals at 2.0 > � > 1.5 to the otherCH2 groups and the signal at � = 0.9 to the CH3 groups.)

Figure 11 Absorbance A of the triphenanthro[24]annulene 15b in methylcyclohexane; The measurements with constant c�d = A/� give dif-ferent absorption curves (with four isosbestic points) and thus reveal the influence of the concentration on the aggregation.



but their electronic properties are similar to those of themonomer (Equation 3). The slight dependence of the flu-orescence spectra and the fluorescence lifetimes on theconcentration can be explained by different intermolecu-lar interactions in different loose aggregates. (DifferentFranck–Condon factors).45

5 Thermotropic Liquid Crystals

Although the areno-condensed annulenes adopt non-pla-nar conformations, the majority of the compounds has, asdiscussed in Section 3, a de facto symmetry with the mo-lecular plane as symmetry plane. The inversion of the cen-tral macrocyclic rings is in all these cases fast in terms ofthe NMR time scale.

Extended � electron systems with the shape of planardiscs represent an ideal precondition for discotic me-sogens. The � stacking is supported by van der Waals in-teractions of long flexible chains attached on theperiphery: Moreover, alkoxy chains enhance significantlythe solubility of the compounds. One has to assume mi-crosegregation between the discotic mesogens and therange of the saturated sidechains.

The association of discotic mesogens is crucial for the for-mation of columnar liquid crystals, which we have inves-tigated by X-ray small-angle diffraction, differentialscanning calorimetry (DSC), and polarization microsco-py. In fact, many liquid crystalline phases have beenfound in the class of areno-condensed annulenes.14,22–32,43,48–52 The triphenanthro-condensed [18]annulenes 17and 18 (Figure 3, Figure 4) may serve for a more detaileddiscussion. It is evident, that the four methoxy sidechainsin 17a are too short for the generation of a mesophase;however, four hexyloxy groups, present in 17c, are suffi-cient to form a nematic discotic phase ND. Figure 13 (up-per part) shows the typical texture obtained in a polarizingmicroscope. In the lower part of Figure 12 one can see forcomparison the picture of the crystals of 17d, which con-tains nine hexyloxy chains and forms crystals with a sharpmelting point at 328 °C.48

On the basis of X-ray scattering measurements, a model ofthe ND phase was conceived, in which pairs of molecules

Equation 3

Figure 13 Measurements in the polarization microscope;48 Upper part: LC texture caused by the birefringence of the ND phase of 17c mea-sured at 190 °C; lower part: crystals of 17d, which melt at 328 °C (Scale 340:1)

1224 H. Meier PAPER


17c, double discs, are arranged in a preferred orienta-tion.27,48 The model is closely related to the arrangementof the molecules in the crystal. The unit cell contains twoenantiomeric molecules 17c, which are twisted by 60°against each other. The distance of the main molecularplanes amounts to 3.6 Å and is enhanced to about 4.3 Å inthe molecular pairs of the ND phase.

27 Figure 14 illustratesthe arrangement in the crystal and the model for the liquidcrystal.

Figure 14 X-ray investigations of the crystalline phase and of theND phase of compound 17c (The hexyloxy sidechains are omitted.)

Compound 18a, which has three hexyloxy sidechains,does not form an LC phase. Obviously the number, thelength and the position of the sidechains are decisive forthe generation of LC phases.

From the viewpoint of materials science, the most inter-esting discotic mesophases have a columnar arrangement.Colho and Colhd phases are the most common LC phases inthe series of areno-condensed annulenes. Since the X-raysignal for the distance between the discs in a hexagonalcolumnar phase (h) is often broad, it is a question of defi-nition, whether such phases are called ordered (index o) ordisordered (index d). A schematic diagram for a hexago-nal columnar phase based on areno-condensed annulenediscs is shown in Figure 15.

Compound 17e, a triphenanthro[18]annulene with ninedodecyloxy sidechains forms such a hexagonal discoticarrangment. Figure 16 shows the extremely broad temper-ature range, in which the mesophase exists.50 The phasetransitions to the isotropic phase (clearing points) are in

some cases above 300 °C, so that the decomposition startsearlier.23

6 Interaction with Light and its Applications in Materials Science

Stilbenoid chromophores are distinguished by their inter-esting photophysics and photochemistry.18 The majorityof areno-condensed annulenes is photostable in the crys-talline state. An exception was found for compound 6a,which reacts in a topochemical control to the ‘shifted’dimer 50.14,25 It could be assumed, that the photodimeriza-tion corresponds to the calculated molecular pair with C2hsymmetry (Figure 9). According to the electron diffrac-tion, the crystals contain a similar arrangement with ashortest distance of 3.46 Å between two olefinic doublebonds.14 The irradiation in solution as well the prolongedirradiation in the solid state lead to oligomers of 6a. Thederivative 6b behaves opposite; it forms oligomers in thesolid state, whereas the solution photolysis yields the beltcyclophane 5125,51 (Scheme 7). Obviously the geometryof excimers and/or pericyclic minima has a decisive influ-ence on the photodimerization.

Scheme 7 Photodimer 50, obtained in a topochemically-controlled

Figure 15 Schematic diagram of a hexagonal columnar phase gene-rated by areno-condensed annulenes (The saturated side-chains areomitted.)



reaction of 6a in the crystalline state, and photodimer 51 obtainedfrom 6b in solution (0.4�10–3 M in benzene)

A D2h arrangement of a molecular pair 6b is a prerequisitefor the formation of 51; however, such a pair does not cor-respond to an exothermic molecular pair in the groundstate. Therefore the formation of suitable excimers has tobe claimed.

Belt cyclophanes, a new phane type, in which the originalannulene ring has generated the belt structure by severalregio- and stereoselective photocycloaddition reactions,were obtained from many areno-condensed annu-lenes.14,23,25,28,31,43,45,50,51 The arene moieties attached atthe edges of the belt are not in a complete face-to-face ori-entation, because of the geometry, which is forced by the4-membered rings; nevertheless, �-� interactions can beobserved in the UV, the fluorescence and the 1H NMRspectra. The photodimerization of trans stilbene works insolution at concentrations c �10–2 M. Surprisingly, muchlower concentrations down to 10–6 M can be used in theseries of the areno-condensed annulenes. According tofluorescence lifetime measurements with the single-pho-ton timing technique, the average lifetime � of the in-volved singlet states S1 ranges between 2 and 16 ns.

28,45

The Smoluchowski equation for the diffusion indicates,that such a lifetime is much too short to find a dimeriza-tion partner M(S0) in diluted solutions (150 °C). A quantitativedimerization leads then to the same belt cyclophanes,which can be obtained in solution at room tempera-ture.28,49,50–52

How can this enormous temperature effect of the photore-action be rationalized? As discussed above, the discs aretwisted against each other and also somewhat laterallyshifted. Nevertheless, the �-� interaction of the olefiniccenters has to occur within the average lifetime � of the S1state in order to induce a photocyclodimerization. Thiscondition demands high mobility of the discs in the col-umn, particularly a fast uniaxial rotation. We performed asolid-state 2H NMR investigation with compound 18k.51

The quadrupole splitting constant Q is directly related tothe mobility of the discs. In the crystalline state at –73 °Ca Q value of 123 kHz is measured in the pake spectrum.As soon as the phase transition to the hexagonal columnarphase is reached at 0 °C, the pake spectrum changes to aline spectrum. With increasing temperature the lines be-come sharper and the Q values smaller. At 187 °C, where

Figure 16 DSC diagram of compound 17e with the phase transitions between the crystalline phase c, the hexagonal discotic phase LC andthe isotropic phase i; The phase transition enthalpies are given in J�g–1.50

1226 H. Meier PAPER


an efficient photodimerization takes place, the Q value isreduced to 19 kHz. This value corresponds to a fast uniax-ial rotation, whereby the columnar axis is orthogonal tothe magnetic field vector. Such an arrangement representsthe energy-lowest state in the macroscopic orientation ofthe Colh phase in the magnetic field.

53,54 From Q = 19 kHzcan be deduced, that the C–D bonds in 18k form an angleof 60–65° with the axis of the column – a result, whichagrees very well with the inverting pyramidal structure ofthe compound. Thus, the temperature effect of the photo-dimerization is an effect of the columnar topochemistryand its dynamics.

The application of columnar phases as photoconductivematerials depends of course on the photostability. Thephotodimerization results in an insulator – but fortunatelythe process does not work at temperatures below 150 °C.

On the other hand, it is interesting to have LC systems,which loose this property by irradiation – either irrevers-ibly for imaging techniques or reversibly for opticalswitching. Photodimerization and photocrosslinking reac-tions are processes, which can serve for the first purpose.Figure 17 shows an irreversible degradation of 18j by ir-radiation. The original mosaic texture of the Colh phasemeasured in the polarization microscope disappears. Theirradiated area becomes black, an isotropic (non birefrin-gent) phase is isothermally formed by a photochemicaltransformation.51 Monochromatic light ( = 366 nm),which is absorbed by the S0 � S1 transition, induces thecyclophane formation. Prolonged irradiation periods orUV light of short wavelengths ( = 254 nm) lead tocrosslinking in the original columns and between the col-umns. The statistical CC bond formation in stilbenoidcompounds is a radical process, whereby the radicals havelifetimes, which are several orders of magnitude longerthan the average S1 lifetimes in the concerted dimeriza-tions.18

Figure 17 Mosaic texture of the LC phase of 18j obtained at 230 °Cand its photochemical degradation by irradiation in the right upperarea (Scale 340:1)50

The formation of dimers or crosslinked oligomers in thephotochemistry of stilbenoid compounds is an irreversibleprocess.18 Open-chained stilbenoid compounds oftenshow a reversible cis/trans photoisomerization, which canbe used for the optical switching between LC phases andisotropic phases.49 Is it possible to perform such switchingprocesses with areno-condensed annulenes? Numerous ir-radiation experiments revealed that the majority of thecompounds 6, 13, 15–23 does not show a trans � cis pho-toisomerization, at most traces of cis isomers can be de-tected in the 1H NMR spectra. An exception was found inthe series of the tribenzo[a,e,i][12]annulenes.39,55 The(E,E,E)-configuration in 25e is transformed by light to the(E,E,Z)-form of 25g, which rearranges spontaneously tothe polycyclic hydrocarbon 52. Further irradiation leadsagain in a sequence of a photochemical and a thermal step(retro-Diels–Alder) to the cleavage of the compound tonaphthalene (54) and anthracene (55) (Scheme 9). Thewhole process however can not be applied to the LC-forming [12]annulene 25c.24

Scheme 8 Photochemical processes in solution and in columnar LC phases at different temperatures



Scheme 9 Consecutive photochemical and thermal steps for thecleavage of tribenzo[a,e,i][12]annulenes (25e)

7 Conclusion and Outlook

Different cyclocondensation reactions represent the majorroutes for the preparation of annulenes, which are con-densed with benzene, naphthalene, anthracene, phenan-threne, chrysene or pyrene ring systems. In contrast to theparent compounds, these areno-condensed annulenes arethermally highly stable. They have non-planar structures.Nevertheless, they appear to be planar on average, if theinversion of the central macrocyclic ring has a low activa-tion barrier – a condition, which is fulfilled for the major-ity of the systems. Therefore the compounds are‘superdiscs’, which represent extended � systems with ahigh aggregation tendency. This property makes themuseful as discotic mesogens in columnar liquid crystals.Due to the stilbenoid character, the areno-condensed an-nulenes exhibit interesting photophysical and photochem-ical properties. Special emphasis is put onphotoconductive LC systems with high charge carrier mo-bilities and on imaging techniques, in which birefringentLC phases are transformed by light and without heating toisotropic phases. Regio- and stereoselective photodimer-ization reactions in solution and at high temperatures inLC phases lead to a new type of phanes, which is calledbelt cyclophanes.

Recently an extension of this research area to heterocyclicsystems was successful.56 The incorporation of three ni-trogen atoms in the areno-condensed 1,7,13-triaza[18]an-

nulenes opens the door to molecular recognition and toswitching processes with metal complexes. Moreover,many compounds 10/10� with arene ring systems listed inTable 1 are still unknown and should be studied with re-gard to applications in materials science.

References

(1) Lloyd, D. Non-Benzenoid Conjugated Carbocyclic Compounds; Elsevier: Amsterdam, 1984.

(2) Balaban, A. T. Annulenes: Benzo-, Hetero-, Homo-Derivatives, and Their Valence Isomers; CRC: Boca Raton, 1986.

(3) Garratt, P. J. Aromaticity; Wiley: New York, 1986.(4) Lloyd, D. The Chemistry of Conjugated Cyclic Compounds;

Wiley: Chichester, 1989.(5) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y.

Aromaticity and Antiaromaticity; Wiley: New York, 1994.(6) Nakagawa, M. The Chemistry of Annulenes From the

Standpoint of Organic Chemistry; Osaka University Press: Suita, 1996.

(7) Wada, T.; Ojima, J.; Yamada, A.; Garito, A. F.; Sasabe, H. Proc. SPIE – Int. Soc. Opt. Eng. 1990, 1147; Nonlinear Opt. Prop. Org. Mater. 2 1990, 286; Chem. Abstr. 1990, 112, 207234 .

(8) Sasabe, H.; Wada, T.; Hosoda, M.; Ohkawa, H.; Hara, M.; Yamada, A.; Garito, A. F. Proc. SPIE – Int. Soc. Opt. Eng. 1990, 1337; Nonlinear Opt. Prop. Org. Mater. 3 1990, 62; Chem. Abstr. 1991, 115, 17597.

(9) Kamiyama, T.; Isoda, S.; Ogura, A. Jpn. Kokai Tokky Koho JP 6319 854, 1988; Chem. Abstr. 1988, 109, 103235.

(10) Sondheimer, F.; Wolovsky, R.; Amiel, Y. J. Am. Chem. Soc. 1962, 84, 274.

(11) Staab, H. A.; Meissner, U. E.; Gensler, A. Chem. Ber. 1979, 112, 3907.

(12) Michels, H.-P.; Nieger, M.; Vögtle, F. Chem. Ber. 1994, 127, 1167.

(13) Staab, H. A.; Meissner, U. E.; Meissner, B. Chem. Ber. 1976, 109, 3875.

(14) Yu, R.; Yakimansky, A.; Voigt-Martin, I. G.; Fetten, M.; Schnorpfeil, C.; Schollmeyer, D.; Meier, H. J. Chem. Soc., Perkin Trans. 2 1999, 1881.

(15) Staab, H. A.; Binnig, F. Chem. Ber. 1967, 100, 293.(16) Staab, H. A.; Bräunling, H.; Schneider, K. Chem. Ber. 1968,

101, 879.(17) Staab, H. A.; Diederich, F. Chem. Ber. 1983, 116, 3487.(18) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399;

Angew. Chem. 1992, 104, 1425; and references cited therein.(19) Siegrist, A. E. Helv. Chim. Acta 1967, 50, 906.(20) Siegrist, A. E.; Meyer, H. R. Helv. Chim. Acta 1969, 52,

1282.(21) Siegrist, A. E.; Liechti, P.; Meyer, H. R. Helv. Chim. Acta

1969, 52, 2521.(22) Meier, H.; Kretzschmann, H.; Kolshorn, H. J. Org. Chem.

1992, 57, 6847.(23) Meier, H.; Fetten, M.; Schnorpfeil, C. Eur. J. Org. Chem.

2001, 779.(24) Fetten, M. Ph. D. Thesis; University of Mainz: Germany,

1998.(25) Meier, H.; Fetten, M.; Schnorpfeil, C.; Yakimansky, A. V.;

Voigt-Martin, I. G. Tetrahedron Lett. 1999, 40, 4791.(26) Meier, H.; Kretzschmann, H. In Photochemical Key Steps in

Organic Synthesis; Mattay, J.; Giesbeck, A., Eds.; VCH: Weinheim, 1994, 236.

(27) Kretzschmann, H.; Müller, K.; Kolshorn, H.; Schollmeyer, D.; Meier, H. Chem. Ber. 1994, 127, 1735.

1228 H. Meier PAPER


(28) Müller, K.; Meier, H.; Bouas-Laurent, H.; Desvergne, J. P. J. Org. Chem. 1996, 61, 5474.

(29) Müller, K. Ph.D. Thesis; University of Mainz: Germany, 1995.

(30) Schnorpfeil, C. Ph.D. Thesis; University of Mainz: Germany, 2000.

(31) Schnorpfeil, C.; Fetten, M.; Meier, H. J. Prakt. Chem. 2000, 342, 785.

(32) Meier, H.; Fetten, M. Tetrahedron Lett. 2000, 41, 1535.(33) Kuwatani, Y.; Yoshida, T.; Kusaka, A.; Iyoda, M.

Tetrahedron Lett. 2000, 41, 359.(34) Brown, C.; Sárgent, M. V. J. Chem. Soc. (C) 1969, 1818.(35) Staab, H. A.; Graf, F.; Junge, B. Tetrahedron Lett. 1966,

743.(36) See also: Banciu, M. D.; Simion, I.; Banciu, A.; Petride, A.;

Draghici, C. Rev. Roum. Chim. 1991, 36, 1101; Chem. Abstr. 1991, 117, 191445.

(37) Griffin, C. E.; Martin, K. R.; Douglas, B. E. J. Org. Chem. 1962, 27, 1627.

(38) Akiyama, S.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1971, 44, 3158.

(39) Tausch, M. W.; Elian, M.; Bucur, A.; Cioranescu, E. Chem. Ber. 1977, 110, 1744.

(40) (a) Thulin, B.; Wennerstroem, O. Acta Chem. Scand. Ser. B. 1976, 30, 369. (b) Thulin, B.; Wennerstroem, O. Acta Chem. Scand. Ser. B. 1983, 37, 589.

(41) Iyoda, M.; Kuwatani, Y.; Yamauchi, T.; Oda, M. J. Chem. Soc., Chem. Commun. 1988, 66.

(42) See also: Sigel, J. S.; Baldridge, K. K. Angew. Chem., Int. Ed. Engl. 1997, 36, 745; Angew. Chem. 1997, 109, 765.

(43) Meier, H.; Lehmann, M.; Schnorpfeil, C.; Fetten, M. Mol. Cryst., Liq. Cryst. 2000, 352, 85.

(44) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Chem. Phys. Lett. 1989, 162, 165.

(45) Schnorpfeil, C.; Meier, H.; Irie, M. Helv. Chim. Acta 2001, 84, 2467.

(46) Yoshihara, K.; Kasuya, T.; Inoue, A.; Nagakura, S. Chem. Phys. Lett. 1971, 9, 469.

(47) Michl, J.; Bonacic-Koutecky, V. Electronic Aspects of Organic Photochemistry; Wiley: New York, 1990, 274.

(48) Meier, H.; Kretzschmann, H.; Lang, M. J. Photochem. Photobiol. A: Chem. 1994, 80, 393.

(49) Meier, H.; Lifka, T.; Müller, K. J. Inf. Rec. Mats. 1994, 21, 457.

(50) Meier, H.; Müller, K. Angew. Chem., Int. Ed. Engl. 1995, 34, 1437; Angew. Chem. 1995, 107, 1598.

(51) Meier, H.; Müller, K.; Fetten, M. J. Inf. Recording 1996, 22, 421.

(52) Petermann, R.; Schnorpfeil, C.; Lehmann, M.; Fetten, M.; Meier, H. J. Inf. Recording 2000, 25, 259.

(53) Slichter, C. P. Principles of Nuclear Magnetic Resonance; Springer Verlag: Berlin, 1980.

(54) Werth, M.; Leisen, J.; Boeffel, C.; Dong, R. Y.; Spiess, H. W. J. Phys. II 1993, 53.

(55) Staab, H. A.; Graf, F.; Doerner, K.; Nissen, A. Chem. Ber. 1971, 104, 1159.

(56) Meier, H.; Schnorpfeil, C.; Fetten, M.; Hinneschiedt, S. Eur. J. Org. Chem. 2002, 537.

Areno-Condensed Annulenes – Extended Discotic Mesogens

areno-condensed annulenes – extended discotic mesogens€¦ · photochemistry 1 introduction...

Documents