A theoretical study on the factors influencing cyanine photoisomerization: the case of thiacyanine in gas phase and in methanol

The effects influencing cyanine photoisomerization on the $S_1$ surface in the condensed phase have been investigated by an integrated quantum mechanical approach, focussed mainly on 3,3'-diEt-2,2'-thiacyanine. After excitation, a barrierless motion, involving the torsion coupled to bond skeletal deformation, leads to a slightly non-planar local $C_2$ minimum, which we propose to be the fluorescent state. Crossed a barrier of $\approx$ 120 cm$^{-1}$ a steeper path drives to a $C_1$ global minimum $S_1$-Min, corresponding to a pseudo-perpendicular twisted intramolecular charge transfer (TICT) state. CAS(6,6) optimization allows to locate the lowest energy $S_1/S_0$ conical intersection which is reached from $S_1$-Min by an increased asymmetry of the two rings and a marked pyramidalization at one N center. $S_1$ surface is rather flat in the Franck-Condon region and suggests that other paths can be competitive with the minimum energy one. The comparison among different cyanines shows how variation of the molecular scaffold and/or of its substituents modulate the dynamics of the photoisomerization. All the indications coming from our computations are in line with and provide an explanation to the available experimental results.