Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kasha's rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular non radiative decay processes occurring in condensed phase. Here we tracked the S-2 and S-1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S-2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S-1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S-2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S-2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.