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.