Rotational Relaxation in Rigid Media by Polarized Photoselection

Publisher Website
Google Scholar
Abstract
A study is made of the rotational relaxation of solute molecules in media ranging in viscosity from 106 to 1012 P. Viscosities are determined for various mixtures of 3‐methylpentane and isopentane at 77°K The solute relaxation is observed by two distinctly different methods both based on polarized photoselection. At the high viscosities a dichroism is produced by polarized photochemistry and the rotational relaxation is followed by monitoring the collapse of the dichroism. At lower viscosities the steady‐state depolarization of long‐lived phosphorescence is observed. The latter technique is completely analogous to previous work at much lower viscosities where the depolarization of fluorescence is measured. A theory is presented which links the general photoselection equations to proper solutions of the rotational diffusion equation for an isotropic body. The theory is in a form which permits a variety of relaxation studies where solute molecules may be photoselected along different combinations of molecular axes and the photoproduct or the emission viewed along different combinations of molecular axes. Both the steady‐state and transient solutions to the theory are satisfactorily tested here and also a Stokes'‐law‐type relationship between viscosity and relaxation times appears to hold. With further refinement, especially experimental, such studies hold out promise for considerable insight regarding the local environment of solute molecules in such media.