Anisotropic Rotational Relaxation in Rigid Media by Polarized Photoselection

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Abstract
A study is made of anisotropic rotational relaxation of solute molecules in rigid glasses. Assuming the relaxation process to be Brownian diffusion, equations are derived which connect the rate of depolarization in a photoselection experiment with the components of the rotational-diffusion tensor. These equations are an extension to symmetric- and asymmetric-top cases of equations previously derived only in the spherical-top limit. These new equations indicate the possibility of determining two distinct relaxation times for the molecule. In the light of these equations previous measurements are extended to allow measurement of a a second relaxation time for N,N,N′,N′-tetramethylparaphenylenediamine in various mixtures of 3-methylpentane and isopentane at 77°K. These results allow a more sensitive examination of the character of the local environment of solute molecules in such media. In particular, the predictions of Stokes' law for an ellipsoid rotating in a continuous viscous fluid are compared with the experimental results. A discrepancy of a factor of as much as 5 is found, and the implications of this are discussed.