Ultrafast Rationless Deactivation of Orgnic Dyes: Evidence for a Two-state Two-mode Pathway in Polymethine Cyanine

CASSCF quantum chemical calculations (including dynamics) have been used to investigate the ultrafast photoisomerization of three symmetric cyanine dye models of different chain lengths. For the 'model' trimethine cyanine, the 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 charge- transfer (TICT) minimum state with an adjacent conical intersection. The path is consistent with both biexponential decay of fluorescence without rise 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 bonds is shown to be an activated process, where the torsional reaction path terminates, again, at a TICT state and the decay takes place at a twisted S1/S0 conical intersection. In agreement with the experimental results, the activation energies increase with the length of the polymethine chain. In contrast to the differenCeS in the potential energy surface between short and long cyanines, we demonstrate that the excited state evolution of these systems can be understood in terms of the same two-state two-mode model of the reaction coordinate previously reported for the (isoelectronic) retinal protonated Schiff base models.