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 S1 surface in the condensed phase have been investigated by an integrated quantum mechanical approach, focused 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 nonplanar local C2 minimum, which we propose to be the fluorescent state. Crossed a barrier of ≈120 cm(-)(1), a steeper path drives to a more stable C1 minimum S1-Min, corresponding to a pseudoperpendicular twisted intramolecular charge transfer (TICT) state. CAS(6,6) optimization allows for locating the lowest energy S1/S0 conical intersection in the isomerization path which is reached from S1-Min by an increased asymmetry of the two rings and a marked pyramidalization at one N center. The S1 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.