Spin–orbit coupling between electronic states
of different
multiplicity can be strongly coupled to molecular vibrations, and
this interaction is becoming recognized as an important mechanism
for controlling the course of photochemical reactions. Here, we show
that the involvement of spin–vibronic coupling is essential
for understanding the photophysics and photochemistry of heptamethine
cyanines (Cy7), bearing iodine as a heavy atom in the C3′ position
of the chain and/or a 3H-indolium core, as potential
triplet sensitizers and singlet oxygen producers in methanol and aqueous
solutions. The sensitization efficiency was found to be an order of
magnitude higher for the chain-substituted than the 3H-indolium core-substituted derivatives. Our ab initio calculations
demonstrate that while all optimal structures of Cy7 are characterized
by negligible spin–orbit coupling (tenths of cm–1) with no dependence on the position of the substituent, molecular
vibrations lead to its significant increase (tens of cm–1for the chain-substituted cyanines), which allowed us to interpret
the observed position dependence.