Linear and Nonlinear Optical Processes Controlling S₂ and S₁ Dual Fluorescence in Cyanine Dyes

We report on the changes in the dual fluorescence of two cyanine dyes IR144 and IR140 as a function of viscosity and probe their internal conversion dynamics from S-2 to S-1 via their dependence on a femtosecond laser pulse chirp. Steady-state and time-resolved measurements performed in methanol, ethanol, propanol, ethylene glycol, and glycerol solutions are presented. Quantum calculations reveal the presence of three excited states responsible for the experimental observations. Above the first excited state, we find an excited state, which we designate as S-1', that relaxes to the S-1 minimum, and we find that the S-2 state has two stable configurations. Chirp-dependence measurements, aided by numerical simulations, reveal how internal conversion from S-2 to S-1 depends on solvent viscosity and pulse duration. By combining solvent viscosity, transform-limited pulses, and chirped pulses, we obtain an overall change in the S-2/S-1 population ratio of a factor of 86 and 55 for IR144 and IR140, respectively. The increase in the S-2/S-1 ratio is explained by a two-photon transition to a higher excited state. The ability to maximize the population of higher excited states by delaying or bypassing nonradiative relaxation may lead to the increased efficiency of photochemical processes.