Rotation and isomerization of diphenylbutadiene and 4-(methanol)stilbene in supercritical CO2 -: A re-examination

The rates of excited-state rotation and isomerization of trans,trans-1,4-diphenylbutadiene (DPB) and 4-(methanol)stilbene (HMS) in supercritical CO2 (T = 36 degreesC, rho = 0.4-2rho(c)) were measured using time-resolved emission methods. These measurements were prompted by a discrepancy between rotation times recently simulated for these systems (Patel, N.; Biswas, R.; Maroncelli, M. J. Phys. Chem. B 2002, 106, 3306-3318) and earlier experimental results. The present data show the rotation times of the two solutes to be approximately equal and linear functions of (bulk) solvent density, consistent with the simulations. The rate of isomerization of HMS decreases approximately linearly with solvent density, whereas the isomerization of DPB is remarkably insensitive to density.