© Frieder Jäkle, all rights reserved Project B 1

Frieder Jäkle CONJUGATED ORGANOBORANE OLIGOMERS, MACROCYCLES AND Project B POLYMERS FOR ORGANIC ELECTRONICS AND SENSORS Conjugated polymers have emerged as an important class of synthetic materials that are useful, for example, in plastic electronics, photovoltaics, smart electrochromic windows, charge-dissipating coatings, and in the broad area of biological and chemical sensors. All these important applications can be traced back to the extended conjugated structure of the polymer backbone. Further functionalization of conjugated organic polymers with main group elements and transition metals is a topic of much current interest. Especially attractive about organoboron polymers is their electron-deficient character due to the empty p-orbital on boron. Overlap of this empty p-orbital with conjugated π-systems leads to unusual optical and electronic properties, and the importance of organoboranes. The incorporation of electron-deficient organoborane groups into polymers has the potential for discovery of new solution-processible materials for light emitting devices (LEDs), field-effect transistors (FETs), and photovoltaics, and of sensors for anions and other nucleophiles, including toxic small molecules and chemical warfare agents. In our work, we have pursued to distinct designs of conjugated organoboranes: (a) We have developed a new modular synthetic route that provides highly selective and controlled access to conjugated organoboron oligomers, macrocycles, and polymers that feature boron in the main chain. Not only are we able to tune the properties through facile substitution reactions, but also to control the chain length and architecture and hence the degree of extended conjugation. (b) We have also investigated conjugated polymers and block copolymers that feature electron-deficient borane moieties attached to an organic conducting polymer backbone. In this case the properties of the conducting polymer are directly influenced by the electron-deficient nature of the boryl substituents.

I. Incorporation of Boron into the Main Chain. We have introduced tin-boron exchange reactions as a facile and selective method for the preparation of main chain-type conjugated organoborane oligomers and polymers. In early work we prepared a family of main chain polymeric Lewis acids that contain Lewis acidic boron groups embedded into a polythiophene backbone. When aromatic groups such as phenyl and pentafluorophenyl are attached to boron, blue and green luminescence is observed, respectively, while the attachment of ferrocenyl substituents leads to a characteristic red color. The incorporation of readily accessible highly Lewis acidic groups into the conjugated polymer backbone provides an opportunity for sensing of Lewis basic substrates. For instance, treatment of the polymers containing phenyl substituents on boron with pyridine leads to efficient quenching of the fluorescence, while polymers containing ferrocenyl groups change color from red to light orange (vide infra).

More recently, we have succeeded in the preparation of a “universal” conjugated organoborane scaffold that is based on a highly luminescent fluoreneborane polymer backbone; the presence of reactive Br substituents on boron allows not only for facile tuning of the optical properties but also enhances the thermal and environmental stability of these materials.

In collaboration with the Wagner group in Frankfurt we have studied redox-active ferrocene-bridged main-chain organoboron polymers, in which the electronic communication throughout the polymer chain can be further influenced by reversible redox chemistry of the ferrocene iron atoms. In a related project, we introduced boratabenzene-derived metallocene analogues as versatile new building blocks for metallopolymers. Poly-merization of Fe(C5H5B-C≡CH)2 was achieved via Sonogashira-Hagihara coupling and click-type polymerization.

BBr

BBr

"Universal Organoborane Polymer"

n

! !

© Frieder Jäkle, all rights reserved Project B 2

The Question of Extended Delocalization. One of the most important questions with regard to the properties of conjugated organoboron polymers is: how effective is the extended conjugation through the conjugated π-system and the empty p-orbital on boron? An experimental approach to this problem that has been successfully applied to various other conjugated polymer systems, including main group polymers such as polysilanes and polystannanes, is to prepare well-defined oligomers of exact chain length and to compare their photophysical properties. With extension of the conjugation length typically a gradual bathochromic shift is observed in the absorption spectra and the absorption maximum eventually approaches that of the high molecular weight polymer itself. We have developed a new iterative method for the synthesis of monodisperse conjugated oligomers that takes advantage of the different reactivity of arylsilane vs arylstannane functionalities. We then studied the effect of chain extension from monomer to hexamer on the photophysical properties, the electro-chemical characteristics, and the fluoride anion binding behavior. Our system is unique in that it offers the opportunity to compare theoretical and experimental data for conjugated organoborane oligomers and polymers.

Conjugated Organoborane Macrocycles. Conjugated macrocycles present a particularly attractive class of materials for optoelectronic applications as they comprise discrete, monodisperse structures, representative of an

infinite polymer chain without any end groups. Another desirable feature is their well-known ability to self-assemble into tubular supramolecular structures and to form well-defined and highly symmetric arrays upon deposition on surfaces. Numerous conjugated organic cyclics have been explored. In late 2011, we introduced the first example of an electron-deficient conjugated bora-cyclophane with fluorene as the bridging π-system, a feat that has generated much interest and was highlighted in C&E News and in Angewandte Chemie. Theoretical calculations provide clear evidence

of extended delocalization throughout the macrocycle. Expanding on this discovery, we recently succeeded in the preparation of the first ambipolar macrocycle, which contains nitrogen as donor and boron as acceptor sites, bridged by π-conjugated phenylene groups. This new type of macrocycle may be viewed as a π-expanded borazine; however, introduction of the phenylene bridges results in remarkably different properties in comparison to borazine, including strong blue fluorescence, solvatochromic emission, and redox processes that reflect the ambipolar structure of this unique D-π-A type macrocycle.

II. Side-Chain Functionalization of Conjugated Organic Materials. Side-group functionalization of conjugated polymers with electron-deficient boron centers represents an interesting alternative to the embedding of boron into the conjugated backbone described above. Importantly, the tunability of these systems is further enhanced since two of the valencies on boron are available and only one is needed for attachment to the polymer backbone. The synthetic approach builds on the successful borylation of silylated

© Frieder Jäkle, all rights reserved Project B 3

polystyrene discovered by our group, which is applied here to conjugated polymers. Different silylated conjugated polymer precursors were prepared by organometallic coupling reactions. Using Stille coupling of stannylated and iodine-substituted bithiophene derivatives we prepared a soluble silylated polythiophene with a molecular weight of ca. Mn = 20,000 and relatively low polydispersity of ca. PDI = 1.5-1.6. Similarly, Suzuki coupling reactions were used to synthesize copolymers that feature disilyl-bithiophene moieties alternating with fluorene and carbazole derivatives, respectively. These polymers are highly soluble and can be prepared on a multi-gram scale. Upon treatment with mesityl copper we obtained the respective dimesitylboryl substituted polymers, in which the boron groups are directly attached to the conjugated polymer backbone and thus have a distinct impact on the electronic properties of these polymers. Extensive studies on the optical properties and electronic structure have been performed and they clearly demonstrate that the LUMO levels are lowered due to boron incorporation, ultimately resulting in lower band gaps.

A challenge that we have recently started to tackle is to find controlled polymerization procedures that lead to regioregular polythiophenes and ultimately should allow for preparation of rod-coil block copolymers that feature

organoborane groups attached to the conjugated block segments. We first studied the Grignard metathesis (GRIM) polymerization of 2,5-dibromo-3-heptynylthiophene. The corresponding alkynyl-substituted polymer is deep red colored with an absorption maximum at λmax = 517 nm, which is at significantly longer wavelength compared to 3-alkyl substituted polythiophenes (440-460 nm). Interestingly, the absorption maximum is similar to that reported for head-to-head polythiophenes, which are prepared by Stille-type polymerization of 3,3’-dialkynyl-2,2’-bithiophenes and adopt a

coplanar conformation. We then introduced Mes2B groups into the polymer side chains via hydroboration with Mes2BH. After this polymer modification the polymer exhibits improved solubility and a distinct blue shift of the absorption maximum to λmax = 478 nm. We also explored the direct GRIM polymerization of 3-vinylborane-functionalized thiophene monomers, which were readily prepared by hydroboration of the respective alkynyl-functionalized precursor. Reaction of the alkyne with Mes2BH led to almost equal amounts of two different vinylborane isomers that correspond to the α- and β-borylation products, which could be separated by reverse phase column chromatography.

III. Conjugated Organoborane Oligomers and Polymers as Sensory Materials. One of the key challenges in the design of an efficient sensor system is to have individual recognition sites along the polymer backbone act in a cooperative fashion. We have synthesized bifunctional conjugated organoboranes that represent a fragment of a conjugated organoboron polymer and thus allow us to study cooperative effects between adjacent boron centers. NMR spectroscopic and luminescence studies clearly indicated that the individual boron centers indeed strongly interact with each other. When applied to larger oligomers, an amplified quenching effect is observed, which we attribute to energy transfer to low lying charge transfer states that are generated upon anion binding to one of the Lewis acid sites (LA).

Alk Alk

NAr

1. 2n BBr32. 4n MesCu

S

HexS

Hex

SS

BMes2

Mes2B n

!-system

SS

SiMe3

Me3Si n

!-system!-system

B as Acceptor

LA LA!! ! LA !!LA

- D + DLA = Lewis acid (e.g. borane)D = Donor

LA LA!! ! LA !!LAD

Luminescent polymer

Luminescent quenched or modified

© Frieder Jäkle, all rights reserved Project B 4

We have also studied the anion binding to our borylated polythiophenes and corresponding quarterthiophene model systems. During the course of these investigations we discovered a desirable fluorescence turn-on response. The binding of anions to the boryl groups in the quaterthiophene (shown) converts the electron-deficient borane moieties into electron-rich borate groups. This change in the electronic structure and steric demand of the substituent ultimately results in a dramatic change in the absorption and emission as well as the electrochemical characteristics, thereby leading to a sensory signal that can be easily observed with the naked eye or by using simple voltammetric measurements. The luminescence turn-on effect upon binding of fluoride or cyanide is illustrated here. Relevant Publications:

Reviews: F. Jäkle, Chem. Rev. 2010, 110, 3985-4022; A. Doshi and F. Jäkle, Chapter 1.30 in Comprehensive Inorganic Chemistry II (Editors: Jan Reedijk, Kenneth Poeppelmeier) 2012, in press; F. Jäkle, Coord. Chem. Rev. 2006, 250, 1107-1121; Y. Qin, G. Cheng, K. Parab, A. Sundararaman, F. Jäkle, Macromol. Symp. 2003, 196, 337-345.

Topic I: P. Chen, R. A. Lalancette and F. Jäkle, Angew. Chem. Int. Ed. 2012, in press (designated as VIP); P. Chen, F. Jäkle, J. Am. Chem. Soc. 2011, 133, 20142-20145 (Featured in JACS editorial “Advances at the Frontiers of Photochemical Sciences” J. Am. Chem. Soc. 2012, 134, 8289-8292; C&E News, December 5, 2011, Vol. 89, Number 49, pp35 (Science Concentrates); JACS Spotlight; ACIE highlight by Gabbai); P. Chen, R. A. Lalancette, F. Jäkle, J. Am. Chem. Soc. 2011, 133, 8802-8805; F. Pammer, R. A. Lalancette, F. Jäkle, Chem-Eur. J. 2011, 17, 11280-11289 (Featured in ACS Noteworthy Chemistry, October 31, 2011); H. Li, F. Jäkle, Macromol. Rapid Commun. 2010, 31, 915-920; M. Scheibitz, H. Li, J. Schnorr, A. Sanchez-Perucha, M. Bolte, H.-W. Lerner, F. Jäkle, M. Wagner, J. Am. Chem. Soc. 2009, 131, 16319-16329; A. Lorbach, M. Bolte, H. Li, H.-W. Lerner, M. C. Holthausen, F. Jäkle, M. Wagner, Angew. Chem. Int. Ed. 2009, 48, 4584-4588; H. Li, F. Jäkle, Macromolecules 2009, 42, 3448-3453; H. Li, F. Jäkle, Angew. Chem. Int. Ed. 2009, 48, 2313-2316; Sundararaman, H. Li, R. Varughese, L. N. Zakharov, A. L. Rheingold, F. Jäkle, Organometallics 2007, 26, 6126-6131; J. B. Heilmann, Y. Qin, F. Jäkle, H.-W. Lerner, M. Wagner, Inorg. Chim. Acta 2006, 359, 4802-4806; J. B. Heilmann, M. Scheibitz, Y. Qin, A. Sundararaman, F. Jäkle, T. Kretz, M. Bolte, H.-W. Lerner, M. C. Holthausen, M. Wagner, Angew. Chem. Int. Ed. 2006, 45, 920-925; A. Sundararaman, M. Victor, R. Varughese, F. Jäkle, J. Am. Chem. Soc. 2005, 127, 13748-13749.

Topic II: F. Pammer, R. A. Lalancette, and F. Jäkle, 2012, Macromolecules, accepted pending minor revision; F. Pammer and F. Jäkle, 2012, Chem. Sci., in press; H. Li, F. Jäkle, Polym. Chem. 2011, 2, 897-905; H. Li, A. Sundararaman, K. Venkatasubbaiah, F. Jäkle, J. Am. Chem. Soc. 2007, 129, 5792-5793 (Featured in ACIE highlight by Bazan et al).

Topic III: H. Li, R. A. Lalancette, F. Jäkle, Chem. Commun. 2011, 47, 9378-9380; P. Chen, F. Jäkle, J. Am. Chem. Soc. 2011, 133, 20142-20145; A. Sundararaman, K. Venkatasubbaiah, M. Victor, L. N. Zakharov, A. L. Rheingold, F. Jäkle, J. Am. Chem. Soc. 2006, 128, 16554-16565.

SS

S

B

B

S

Me

Me MesMes

MesMes

+F +Cl +CN - - -