X. Shi, P.M. Burrezo, S. Lee, W. Zhang, B. Zheng, G. Dai, J. Chang, J.T. López Navarrete, K.-W. Huang, D. Kim, J. Casado and C. Chi
Chem. Sci., 5, pp. 4490-4503, (2014)
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 bisindenothienoacene (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.