title: A Theoretical Study on the Factors Influencing Cyanine Photoisomerization:  The Case of Thiacyanine in Gas Phase and in Methanol.
abstract: 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.
authors.affiliation: Istituto di Biostrutture e Biommagini del CNR, via Mezzocannone 6, I-80134 Napoli, Italy, and Istituto per i Processi Chimico-Fisici del CNR, Area della Ricerca del CNR di Pisa, via Moruzzi 1, I-56124 Pisa, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Improta
authors.affiliation: Istituto di Biostrutture e Biommagini del CNR, via Mezzocannone 6, I-80134 Napoli, Italy, and Istituto per i Processi Chimico-Fisici del CNR, Area della Ricerca del CNR di Pisa, via Moruzzi 1, I-56124 Pisa, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Santoro