Phenacenes (cata-fused acene analogues) are of emerging interest in organic electronics, demonstrating potential as semiconducting materials and OLED emissive layers whilst maintaining oxidative stability.(1) In this study we introduce a cross coupling strategy for the orthogonal, A2B2, tetrasubstitution of chrysene allowing effective tuning of its photophysical properties.
Though our recently developed BHQ reaction we were readily able to access 4,10-dichlorochrysene.(2) Iridium catalysed C-H borylation(3) of 4,10-dichlorochrysene resulted in diborylation, affording as a single regioisomer the 2,8-diboryl-4,10-dichlorochrysene. The chlorine acts as removable steric directing group(4, 5) enabling ready access to 2,8- and 4,10- A2B2- tetrasubstituted chrysene derivatives.
After the initial C-H borylation, substitution of the 2,8- positions could then be performed with preservation of the 4,10-dichloro functionality, installing 4-(n-hexyl)phenyl, 4-(n-hexyloxy)phenyl, 4-(diphenylamino)phenyl and trifluoromethyl substituents. The remaining 4,10-dichloro positions were then accessed in subsequent palladium catalysed Kumada couplings in three examples, firstly installing phenyl and 2-thienyl substituents in the 4-(n-hexyl)phenyl substituted chrysene and then functionalising the 2,8-trifluoromethlated chrysene with 4-(diphenylamino)phenyl substituents to generate an orthogonally substituted donor-acceptor chrysene. This was of particular note as it was computationally predicted (TD-DFT) to possess a charge-transfer(6) transition from the HOMO to the LUMO due to its donor-acceptor structure.
The chrysene derivatives synthesised featured broadened UV-vis spectra particularly in the 2,8-Bis(4-(diphenylamino)phenyl) chrysene and the donor-acceptor substituted chrysene. Estimations of the HOMO and LUMO levels based on UV-vis optical band gap and CV data indicated increased HOMO levels and decreased band gaps, comparable with other contemporary p-type materials such as Pentacene, DNTT and Rubrene.(7) This is particularly notable in the case of the donor-acceptor chrysene and in the more electron rich derivatives featuring 2-thienyl and 4-(diphenylamino)phenyl functionalities.
In conclusion we present a novel and versatile strategy for the orthogonal synthesis of A2B2- tetrasubstituted chrysenes utilising a regioselective C-H borylation, resulting in a variety of derivatives with properties comparable to literature benchmarks, with possible applications either as p-type semiconductors or as tuneable emissive materials in OLEDs.
References:
1. (a) Y. Kubozono, X. He, S. Hamao, K. Teranishi, H. Goto, R. Eguchi, T. Kambe, S. Gohda, and Y. Nishihara, Eur. J. Inorg. Chem., 2014, 3806-3819; (b)
T. Wu, H. Chou, P. Huang, C. Cheng and R. Liu, J. Org. Chem., 2014, 79, 267-274.
2. (a) J. A. Bull, M. G. Hutchings and P. Quayle, Angew. Chem. Int. Ed., 2007, 46, 1869-1872; (b) M. Little, H. Lan, J. Raftery, J. J. Morrison, J. J.
W. McDouall, S. G. Yeates and P. Quayle, Eur. J. Org. Chem., 2013, 6038-6041.
3. For a recent reviews see: (a) A. Ros, R. Fernández and J. M. Lassaleta, Chem. Soc. Rev., 2014, 43, 3229-3243; (b) H. Shinokubo, Proc. Jpn. Acad., Ser. B, 2014, 90, 1-11.
4. G. A. Chotana, M. A. Rak and M. R. Smith III, J. Am. Chem. Soc., 2005, 127, 10539-10544.
5. S. Hitosugi, Y. Nakamura, T. Matsuno, W. Nakanishi and H. Isobe, Tetrahedron Lett., 2012, 53, 1180-1182.
6. J. A. Marsden, J. J. Miller, L. D. Shirtcliff and M. M. Haley, J. Am. Chem. Soc., 2005, 127, 2464-2476.
7. C. Wang, H. Dong, W. Hu, Y. Liu and D. Zhu, Chem. Rev., 2012, 112, 2208-2267.