Effect of solvent polarity on a rotational isomerization mechanism of rhodamine-B in normal alcohols

Abstract

Experiments were performed to investigate the effects of solvent polarity and solvent viscosity on the nonradiative rates of rhodamine-B in a series of normal alcohols. The fluorescence quantum yields and lifetimes for rhodamine-B in these solvents were measured and used to calculate the nonradiative rates. From a modified kinetic model, the nonradiative rate can be deduced to be the conversion rate for crossing over a potential barrier from an excited planar state to an excited twisted state. Both solvent viscosity and solvent polarity can play key roles in this mechanism because the former may hinder the rotational motion and the latter may change the barrier height. Our results show that the variation in the nonradiative rates is due to solvent polarity changes rather than to solvent viscosity changes. The effect of solvent viscosity on the excited-state lifetimes of rhodamine-B in a series of methanol–poly(ethylene oxide) mixed solvents was also investigated. Our results indicate that the measured lifetimes are essentially independent of solvent viscosities, which in turn indicates that solvent viscosities have no significant effect on the nonradiative rates.