Solvent Effect on Dual Fluorescence and the Corresponding Excited State Dynamics

Abstract

Ultrafast depopulation dynamics of highly excited states of near-infrared (NIR) tri-carbocyanine dyes is presented in primary alcohols using the femtosecond fluorescence upconversion technique. Pronounced solvent dependence is observed in the dynamics of the highly excited states. Typically, the tri-carbocyanine dyes have strong absorption and fluorescence with a single peak in the NIR region, however, on tuning the excitation wavelength to the visible region, two distinct emission bands are seen with a large peak wavelength difference. Such dual fluorescent peaks correspond to the respective transitions: S2 → S0 for the visible region and S1 → S0 for the NIR region. The exact fluorescent band positions or their intensities strongly depend on the viscosity and polarity of the solvents. The fluorescence decay timescales measured using fluorescence upconversion techniques with femtosecond time resolution also vary significantly as a function of solvent polarity. The faster decay on the ~250 fs and slower decay on the order of picosecond timescale was found which are strongly depending with increasing the chain length of alcohol. Specifically, for example, in the IR144 dye, the faster decay time constant (τ1) increase with the increase in the viscosity and chain length of the alcohol while, in the case of the IR140 dye, we noticed that the τ1 value decreases with increasing viscosity and chain length of alcohols. We invoke the ion-pair formation of IR140 dye with alcohols to explain such behavior.