Solvent viscosity determines excited state lifetimes

A series of earlier posts have considered how the excited dynamics of a range of organic dyes is determined by the viscosity of the solvent.
This essentially determines the friction associated with torsional motion on the excited state potential energy surface.

Seth Olsen brought to my attention a very nice paper from 1987.
Torsional dynamics of molecules on barrierless potentials in liquids. II. Test of theoretical models by Ben-Amotz, D. and Harris, C.

They test three alternative models, comparing to experimental data for auramine O and Crystal Violet. Thus this paper is very similar to the 2000 paper by van der Meer, Zhang, and Glasbeek that I discussed in an earlier post.

They find experimentally that the excited state lifetime scales with the solvent viscosity. This scaling is required by the Smoluchowski equation (for overdamped stochastic motion in an external potential), regardless of the form of the potential energy.

The experimental data is most consistent with model (a) above.