Cyanine dyes have a broad range of application because of their dual fluorescent properties. Still, there is much that is unknown regarding the excited state dynamics of these dyes, and even less known about their molecular dynamics in upper excited states. Chirp-manipulation, steady-state measurements, time-correlated single photon counting, kinetics modeling and theoretical calculations all aided in the understanding of differentiated linear and non-linear activated excited state relaxation pathways that led to altered S2/S1 dual fluorescence for both IR144 and IR140. Non-linear excitation takes place in response to the dual action of a high viscous environment and high photon density with decreased second order dispersion. This non-linear excitation is best understood as two-photon excitation. By altering viscosity and pulse chirp, linear and non-linear processes within the excited states of these molecules can be selectively controlled. It is observed that two-photon excitation elicits increased S2/S1 dual fluorescence ratios in both cyanine dyes and that this increase is maximized further in higher viscosity solvents. It is important to conclude that there is not only an increase in the S2/S1 fluorescence ratio with higher viscosity solvents, but there is a multiplicative effect, as chirp is manipulated.
Read
