High pressure studies of the Kramers turnover behavior for the excited-state isomerization of 2-alkenylanthracene in alkane

The isomerization rate of 2-(2-propenyl)anthracene (22PA) in the lowest exc ited singlet (S-1) state was investigated in supercritical fluids (SCF); et hane and CO2, as well as in a series of liquid n-alkanes at high pressures. Combining the present results with our preceding ones which had been exami ned in compressed liquid n-alkane solvents [J. Phys. Chem. A101, 2240 (1997 )] we completed the curve of forward isomerization rate constant (k(f)) aga inst solvent viscosity (eta) over the entire friction range. We also discus s the dependence of k(f) on the inverse of solvent self-diffusion coefficie nt (D-1), which is used as a measure of solvent collisional frequency. The Kramers turnover behavior of 22PA thus observed was compared with our previ ous result of 2-vinylanthracene [J. Chem. Phys. 103, 5548 (1995)]. The k(f) values of both isomerization reactions around the turnover viscosity regio n never attains those which are predicted by transition state theory. (C) 1 999 American Institute of Physics. [S0021-9606(99)00303-7].