Antiaromatic bisindeno-[n]thienoacenes with small singlet biradical characters: Syntheses, structures and chain length dependent physical properties

Abstract
Recent studies demonstrated that aromaticity and biradical character play important roles in determining the ground-state structures and physical properties of quinoidal polycyclic hydrocarbons and oligothiophenes, a kind of molecular materials showing promising applications for organic electronics, photonics and spintronics. In this work, we designed and synthesized a new type of hybrid system, the so-called bisindeno-[n]thienoacenes (n = 1-4), by annulation of quinoidal fused α-oligothiophenes with two indene units. The obtained molecules can be regarded as antiaromatic systems containing 4n π electrons with small singlet biradical character (y0). Their ground-state geometry and electronic structures were studied by X-ray crystallographic analysis, NMR, ESR and Raman spectroscopy, assisted by density functional theory calculations. With extension of the chain length, the molecules showed a gradual increase of the singlet biradical character accompanied by decreased antiaromaticity, finally leading to a highly reactive bisindeno[4]thienoacene (S4-TIPS) which has a singlet biradical ground state (y0= 0.202). Their optical and electronic properties in the neutral and charged states were systematically investigated by one-photon absorption, two-photon absorption, transient absorption spectroscopy, cyclic voltammetry and spectroelectrochemistry, which could be correlated to the chain length dependent antiaromaticity and biradical character. Our detailed studies revealed a clear structure-aromaticity-biradical character-physical properties-reactivity relationship, which is of importance for tailored material design in the future. This journal is

Citation
Shi, X., Burrezo, P. M., Lee, S., Zhang, W., Zheng, B., Dai, G., … Chi, C. (2014). Antiaromatic bisindeno-[n]thienoacenes with small singlet biradical characters: syntheses, structures and chain length dependent physical properties. Chem. Sci., 5(11), 4490–4503. doi:10.1039/c4sc01769b

Acknowledgements
C.C. acknowledges financial support from MOE AcRF Tier 1 grants (R-143-000-510-112 and R-143-000-573-112) and NUS Start Up grant R-143-000-486-133. The work at the University of Malaga was supported by MINECO of Spain (CTQ2012-33733) and by the Junta de Andalucia (Project P09-FQM-4708). The work at Yonsei Univ. was supported by the Mid-career Researcher Program (2010-0029668) and the Global Research Laboratory (2013K1A1A2A02050183) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (Information and Communication Technologies) and Future Planning. K. H. acknowledges financial support from KAUST. During the reviewing process of this manuscript, a paper on a similar system was published as a Just Accepted article.32

Publisher
Royal Society of Chemistry (RSC)

Journal
Chem. Sci.

DOI
10.1039/c4sc01769b

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