Photoisomerization of diphenylpolyenes: A prototype unimolecular reaction?

The photoisomerization dynamics of diphenylpolyenes, in particular of trans-stilbene and E,E-diphenyl-(1,3)-butadiene, continue to be the subject of controversial discussion. A key question addresses the time scale of intramol. vibrational energy redistribution (IVR) in conjunction with the validity of applying statistical rate theory to these reactions. We present new exptl. results from fluorescence up-conversion and pump-probe absorption spectroscopy in supercrit. solvents. Our studies confirm that in fluid soln. IVR is complete within about a hundred femtoseconds. The subsequent time evolution critically depends on the electronic level structure of the diphenylpolyene and excitation wavelength. It is governed by coupling among electronically excited states, vibrational cooling, and photoisomerization of vibrationally hot mols., until after a couple of picoseconds the regime of a thermal reaction in the S1-state is reached. The question whether reaction rates of vibrationally hot and thermalized mols. may be described by statistical rate theory is addressed employing a new harmonic model based on ab initio calcns. [on SciFinder(R)]     «

The photoisomerization dynamics of diphenylpolyenes, in particular of trans-stilbene and E,E-diphenyl-(1,3)-butadiene, continue to be the subject of controversial discussion. A key question addresses the time scale of intramol. vibrational energy redistribution (IVR) in conjunction with the validity of applying statistical rate theory to these reactions. We present new exptl. results from fluorescence up-conversion and pump-probe absorption spectroscopy in supercrit. solvents. Our studies confir...     »