The symmetric and asymmetric thiophene-fused benzocarborane: structures and first hyperpolarizabilities.

The unusual properties of thiophene-fused benzocarborane have attracted a lot of interest in recent years due to their wide applications in photonics and optoelectronics. In the present work, nine molecules [M, N] (M, N are labeled as the number of thiophene rings on the left and right part, respectively) on the basis of thiophene-fused benzocarborane were considered. The first hyperpolarizability (β tot) values of three synthesized symmetric molecules [1, 1], [2, 2], [3, 3] (M=N, Chem. Eur. J 2012. 18, 11251-11257) and six asymmetric molecules [1, 2], [1, 3], [1, 4], [1, 5], [2, 3], [2, 4] (M≠N) were investigated, β tot values of symmetric molecules show the order: 39 of [1, 1]< 800 of [2, 2]< 903 au of [3, 3], which indicate that β tot value increases with increasing the number of thiophene ring for symmetric molecules. The other order of β tot values can be observed: 39 of [1, 1]< 800 of [1, 2]< 3553 of [1, 3]< 7998 of [1, 4]< 13049 au of [1, 5] and 66 of [2, 2]< 3240 of [2, 3]< 8029 au of [2, 4]. Interestingly, when sum of M and N is constant, larger difference between M and N is, larger β tot value is: 800 au of [2, 2]< 3553 au of [1, 3]; 3240 au of [2, 3] < 7998 au of [1, 4]; 903 au of [3, 3]< 8029 au of [2, 4]< 13049 au of [1, 5]. Significantly, [1, 5] with six thiophene rings has the largest β tot value (13049 au) which is greatly larger than 903 au of [3, 3] with six thiophene rings. Furthermore, the natural bond orbital (NBO) charge populations, the nucleus-independent chemical shift (NICS), the bond length alternation (BLA) of the nine molecules and crucial transition were studied in our work. We hope that the present work will be beneficial for future theoretical and experimental studies on the electro-optical properties of thiophene-fused benzocarborane molecules.