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Theoretical study of structure, electronic properties, and photophysics of cyano-substituted thiophenes and terthiophenes.

In this paper, quantum chemical calculations for various cyano derivatives of thiophene and terthiophenes at the density functional theory (DFT) and ab initio Møller-Plesset (MP2) levels of theory are presented. In the case of the studied terthiophenes, CN groups located in the central part of the molecule lead to a preference of cis-cis geometry over trans-trans conformation. For alpha-substituted dicyano terthiophene, the investigation of torsional dependences shows that the highest energy barrier occurs at the perpendicular orientation of the aromatic rings. On the other hand, the dicyano substitution in the central part of terthiophene molecule exhibits the lowest energy barrier. Excitation energies were calculated using time-dependent density functional theory (TD-DFT). The obtained theoretical results show that the CN groups in alpha and beta positions have a distinct effect on the excitation energies and corresponding oscillator strengths. A CN group located in the alpha position causes a larger bathochromic shift than a CN group in the beta position. Besides, a CN group in the beta position has negligible influence on the position of the first absorption maximum.