Ultrafast radiationless deactivation of organic dyes: Evidence for a two-state two-mode pathway in polymethine cyanines

CASSCF quantum chemical calculations (including dynamics) have been used to investigate the ultrafast photoisomerization of three symmetric cyanine dy e models of different chain lengths. For the "model" trimethine cyanine, th e photochemical isomerization path can be divided into two phases: initial barrierless skeletal stretching coupled with torsional motion and the decay process that takes place in the region of the twisted intramolecular charg e-transfer (TICT) minimum state with an adjacent conical intersection. The path is consistent with both biexponential decay of fluorescence without ri se time at short wavelengths and the rise time followed by monoexponential decay at long wavelengths observed in time-resolved experiments. For penta- and heptamethine cyanines, the photoisomerization about different C-C bond s is shown to be an activated process, where the torsional reaction path te rminates, again, at a TICT state and the decay takes place at a twisted S-1 /S-0 conical intersection. In agreement with the experimental results, the activation energies increase with the length of the polymethine chain. In c ontrast to the differences in the potential energy surface between short an d long cyanines, we demonstrate that the excited state evolution of these s ystems can be understood in terms of the same two-state two-mode model of t he reaction coordinate previously reported for the (isoelectronic) retinal protonated Schiff base models.