Picosecond kinetics of trans-cis-photoisomerisations: From jet-cooled molecules to compressed solutions

The understanding of photoisomerisation reactions involving large amplitude motion is discussed with special emphasis on the merits of systematic studies over wide ranges of physical conditions. It appears that the friction beta in such cases is proportional to macroscopic solvent viscosity eta and the original Kramers theory is sufficient to describe the effect of friction in the range from intermediate to strong damping, i.e. from low viscosity fluid to compressed high viscosity liquid solution. This one-dimensional model may be connected to multidimensional unimolecular rate theory to obtain a representation of the density dependence of the rate coefficient down to the thermal collisionless regime. The apparent failure of this approach in the case of trans-stilbene can be traced to modifications of the excited state PES by the solvent. This interpretation is consistent over the entire range of physical conditions studied, starting at the jet-cooled isolated molecule and solvent cluster and ending in compressed liquid solution at very high viscosities. It is shown that detailed information about the PES is urgently needed to provide safer ground for a profound understanding of the photoisomerisation dynamics.