DOI: 10.1002/adma.200502131

A Nonclassical Approach to Oligoacenes: CrossconjugatedNonplanar Poly(acene) Precursors**

By Hengbin Wang, Christine Schaffner-Hamann, Filippo Marchioni, and Fred Wudl*

As a special type of conjugated polymers, conjugated ladderpolymers have a ribbon- or laddertype 2D framework that lim-its their conformational freedom and dramatically reduces thesteric inhibition of electron delocalization. Conjugated ladderpolymers normally possess a high resistance to mechanical,thermal, and chemical degradation; however, the main prob-lem associated with their special structure is very poor solubil-ity in organic solvents. In general, solubilizing sidegroups arenecessary to render ladder polymers processible.[1]

Ladder polymers are generally synthesized by either step-wise formation of the double strand through the cyclization of afunctionalized single-strand polymer precursor, or polymeriza-tion of multifunctionalized monomers to generate both strandsof the ladder structure in a single reaction. In both cases, quan-titative conversion of monomer functional groups is necessaryto achieve a structurally well-defined ladder polymer with asfew defects as possible. Most of the conjugated ladder polymersreported in the literature were synthesized by the stepwise pro-cedure. Some required multistep reactions to finish the cycliza-tion and aromatization, which lowered the efficiency and addedmore defects to the conjugated ladder polymer.

Linear poly(acenes) are a special class of conjugated ladderpolymers. Because of their delocalized p-electron system oli-goacenes exhibit very high charge-carrier mobilities, a prop-erty useful for organic field-effect transistors (OFETs).[2,3]

Among the members of the oligoacene family, tetracene andpentacene are the most studied, particularly in OFETs. Long-er acenes are expected to show a smaller band gap, which maymake them more promising than pentacene for some applica-tions, such as organic photovoltaics. However, because oftheir high sensitivity towards photo-oxidation, dimerization,and polymerization only the successful synthesis of heptacenehas been reported in the literature.[4] Recently, we reportedthe successful synthesis of a “twistacene” that corresponds to

a heptacene derivative, and very recently a diyne derivative ofheptacene was isolated as well.[5] Even with the extremetwisted structure of twistacenes, some electronic properties ofthe polyacenes are apparently retained. A multilayer organiclight emitting diode (OLED) device showed that when thiscompound was incorporated into polyfluorene, a bright whitelight was emitted.[6] Here, we report the synthesis of a nonpla-nar, crossconjugated ladder polymer by a palladium-catalyzedcrosscoupling reaction. In this procedure, two strands of theladder polymer are synthesized stepwise in a one-pot reaction.Smaller units of the ladder polymer could also be used as oli-goacene precursors.

In 1998, de Meijere’s group reported the synthesis of a se-ries of 9,10-diaryl-9,10-dihydroanthracenes by a palladium-catalyzed crosscoupling (Heck) reaction of 2-bromo-stil-bene.[7] They found that when they tried to synthesize o-disty-rylbenzene from o-dibromobenzene and styrene (under modi-fied Heck reaction conditions), o-distyrylbenzene was formedonly as a minor product. The major product was 9,10-diben-zyl-9,10-dihydroanthracene (a mixture of syn and anti iso-mers). When o-bromo-trans-stilbene was used as the startingmaterial, the yield of 9,10-dibenzyl-9,10-dihydroanthracenewas improved to 88 %. From the proposed mechanism, the in-tramolecular cyclization step is favored compared to the sec-ond intermolecular coupling process.

Based on de Meijere’s discovery, we proposed that a poly-merization process would be achieved when a bis-functional-ized monomer is used in the reaction (Scheme 1). Indeed, whena dimethylformamide (DMF) solution of compound 5a was putinto a pressure vessel together with Pd(OAc)2, LiCl, K2CO3,and nBu4NBr at 110 °C for 24 h, a brown precipitate formedduring the reaction. A yellow-brown polymer, partially solublein chloroform, was obtained after work-up. The molecularweight of the soluble part, measured by using gel-permeationchromatography against polystyrene standards, was around8000 Da (1 Da = 1.66054 × 10–27 kg), with a relatively high poly-dispersity index (PDI) that was probably due to the nature ofthe polymerization, including insolubility of the larger chains.To increase the solubility of the polymer, the alkoxy-functional-ized monomer 5b was synthesized. Indeed, the polymerizationoccurred smoothly and a brown polymer with a molecularweight of 30 000 Da (PDI = 2.6) was obtained. This polymerwas found to be very soluble in various organic solvents.

Both aromatic and aliphatic peaks were clearly observed inthe 1H NMR spectrum of polymers 6a and 6b, but no sharppeaks (other than the solvent chloroform) were observed.Electron spin resonance (ESR) spectroscopy confirmed the

CO

MM

UN

ICATI

ON

558 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19, 558–560

–[*] Prof. F. Wudl, Dr. C. Schaffner-Hamann, Dr. F. Marchioni

Department of Chemistry and BiochemistryUniversity of California Los Angeles607 Charles E. Young Dr. East, Los Angeles, CA 90095-1569 (USA)E-mail: wudl@chem.ucla.eduDr. H. WangMitsubishi Chemical Research and Innovation Center601 Pine Ave., Suite C, Goleta, CA 93117 (USA)

[**] We are indebted to the NSF for support through GrantDMR 0209651 as well as the Air Force for a MURI Grant N000014-01-1-0757 and the ARO through MURI Grant DAAD19-99-1-0316.Supporting Information is available online from Wiley InterScienceor from the author.

existence of radicals in polymers 6a and 6b. These open-shellspecies were very stable to various reducing reagents, such ashydrazine and zinc. Unfortunately, we could not further char-acterize the species responsible for the ESR spectrum and thebroadening of the NMR lines.

To facilitate the characterization of the polymer, variousoligomers were synthesized by “polymerizing” monomer 5a inthe presence of excess 3a (Scheme 2). When one equivalent

of monomer 5a was allowed to react withsix equivalents of monomer 3a, oligomers7–10 could be obtained and purifiedby using silica-gel chromatography withmethylene chloride as eluent. Whereas iso-mers 7a and 7b were separated by recrys-tallization from hexane, as reported inthe literature,[7] isomers of oligomers 8–10were not separable under a variety of con-ditions.

The oligomers 7–10 are yellow-brownishsolids that are very soluble in various or-ganic solvents such as CHCl3, CH2Cl2, tet-rahydrofuran (THF) and toluene. As men-tioned, the 1H NMR and 13C NMR spectraof oligomer 7a and 7b had been reported inthe literature[7] and are relatively simple tointerpret; that was not the case for oligo-mers 8–10, all of which give very compli-cated 1H NMR spectra because of the exis-tence of multiple isomers. Their molecularweights were confirmed by using mass spec-trometry. According to differential scan-ning calorimetry (DSC), only oligomer7a has a melting transition at 196 °C; noclear thermal transition from oligomers7b–10 was observed. From both DSC and

thermogravimetric analysis (TGA) studies, these oligomersare stable up to 200 °C under nitrogen (see Supporting Infor-mation for details).

Figure 1 shows absorption spectra of these oligomers andpolymer 6; all the compounds give very similar absorptionprofiles with the maximum absorption around 350 nm. As thesize of the oligomer increases its absorption is only slightlyred-shifted, indicative of only slight p-orbital overlapping be-

tween two quinodimethane units. This isunderstandable because these mole-cules are crossconjugated and very like-ly have a zig-zag conformation (Fig. 2),as determined from quantum mechani-cal calculations (Gaussian View, optimi-zation/semi empirical/restricted/AustinModel 1 (AM1)). Finally, in analogy toanthraquinone, which is not fluorescent,none of the oligomers or polymers arefluorescent.

Oligomer 7a and 7b gave exactlyequivalent cyclic-voltammetry profiles,consisting of three irreversible oxida-tion waves at 1.73, 1.47, and 1.28 V andone irreversible reduction at –1.91 Vversus the standard calomel electrode.These are comparable with the litera-ture data.[7] Compared to oligomer 7,oligomers 8 and 9 gave similar butmuch less well-defined irreversible oxi-

CO

MM

UN

ICATIO

N

Adv. Mater. 2007, 19, 558–560 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 559

Scheme 1. Synthesis of polymers 6a and 6b by Pd-catalyzed crosscoupling polymerization. Et:ethyl; DMF: N,N-dimethylformatide; Ac: acetate; Bu: butyl.

Scheme 2. Synthesis of oligomers 7–10 by Pd-catalyzed crosscoupling reaction.

dation waves. Interestingly, there are two irreversible reduc-tion processes in the case of oligomer 8 and three in the caseof oligomer 9. The exact reduction potentials of –1.68 and–1.79 V for oligomer 8 and –1.68, –1.80, and –1.94 V for oligo-mer 9 are extracted from differential pulse voltammetry(Fig. 3).

In conclusion, two poly(acene quinodimethane) crossconju-gated zig-zag ladder polymers as well as three oligomerswere synthesized by a palladium-catalyzed polycondensationreaction. The structure and physical properties of thesecompounds indicate that the dominant feature is the anthra-quinodimethane unit. Current research in our laboratory

is directed towards conversion of the quinodimethanes toacenes.

Received: October 6, 2005Revised: August 19, 2006

Published online: January 25, 2007–[1] a) L. Yu, M. Chen, L. R. Dalton, Chem. Mater. 1990, 2, 649. b) U.

Scherf, J. Mater. Chem. 1999, 9, 1853. c) K. Oyaizu, T. Mikami,F. Mitsuhashi, E. Tsuchida, Macromolecules 2002, 35, 67. d) S. A.Patil, U. Scherf, A. A. Kadashchuk, Adv. Funct. Mater. 2003, 13, 609.e) A. Babel, S. A. Jenekhe, J. Am. Chem. Soc. 2003, 125, 13 656. f) S.Qiu, P. Lu, X. Liu, F. Shen, L. Liu, Y. Ma, J. Shen, Macromolecules2003, 36, 9823. g) J. Bouchard, S. Wakim, M. Leclerc, J. Org. Chem.2004, 69, 5705.

[2] a) E. Clar, Polycyclic Hydrocarbons, Vol. 1&2, Academic Press, Lon-don 1964. b) T. Fang, Ph.D. Thesis, University of California, Los An-geles 1986. c) R. G. Havey, Polycyclic Aromatic Hydrocarbons, Wi-ley-VCH, New York 1997. d) J. Roncali, Chem. Rev. 1997, 97, 173.e) Y. Geerts, G. Klärner, K. Müllen, in Electronic Materials: TheOligomer Approach (Eds: K. Müllen, G. Wagner), Wiley-VCH,Weinheim 1998, Ch. 1. f) M. Bendikov, F. Wudl, D. F. Perepichka,Chem. Rev. 2004, 104, 4891.

[3] a) D. Biermann, W. J. Schmidt, J. Am. Chem. Soc. 1980, 102, 3163.b) R. A. Pascal, W. D. McMillan, D. V. Engen, R. G. Eason, J. Am.Chem. Soc. 1987, 109, 4660. c) N. Smyth, D. V. Engen, R. A. Pascal, J.Org. Chem. 1990, 55, 1937. d) X. Qiao, D. M. Ho, R. A. Pascal, An-gew. Chem. Int. Ed. Engl. 1997, 36, 1531. e) C. D. Dimitrakopoulos,P. R. L. Malenfant, Adv. Mater. 2002, 14, 99. f) T. Tokumoto, J. S.Brooks, R. Clinite, X. Wei, J. E. Anthony, D. L. Eaton, S. R. Parkin,J. Appl. Phys. 2002, 92, 5208. g) F. A. Hegmann, R. R. Tykwinski,K. P. Lui, J. E. Bullock, J. E. Anthony, Phys. Rev. Lett. 2002, 89,227 403. h) H. Meng, M. Bendikov, G. Mitchell, R. Helgeson, F. Wudl,Z. Bao, T. Siegrist, C. Kloc, C.-H. Chen, Adv. Mater. 2003, 15, 1090.

[4] M. M. Payne, S. R. Parkin, J. E. Anthony, J. Am. Chem. Soc. 2005,127, 8028.

[5] H. M. Duong, M. Bendikov, D. Steiger, Q. Zhang, G. Sonmez, J. Ya-mada, F. Wudl, Org. Lett. 2003, 5, 4433.

[6] Q. Xu, H. M. Duong, F. Wudl, Y. Yang, Appl. Phys. Lett. 2004, 85, 3357.[7] A. de Meijere, Z. Z. Song, A. Lansky, S. Hyuda, K. Rauch, M. Nol-

temeyer, B. König, B. Knieriem, Eur. J. Org. Chem. 1998, 2289.

CO

MM

UN

ICATI

ON

560 www.advmat.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19, 558–560

Figure 1. UV-vis. absorption spectra of polymers 6a, 6b, and oligomers7–10 in diluted CHCl3. Absorptions between 220 and 250 nm (CHCl3)were artificially truncated.

Figure 2. Quantum mechanical calculation (Gaussian View, optimiza-tion/semi-empirical/restricted/AM1) of the conformation of oligomer 9(side and front view).

Figure 3. Differential pulse voltammetry profiles of oligomers 8 and 9.