EVIDENCE OF ADIABATIC CHANNELS IN THE SINGLET PHOTOISOMERIZATION OF CIS-1,2-DIARYLETHENES - A FLUOROMETRIC STUDY

The deactivation of the first excited singlet state (S1) of some cis isomers of 1,2-diarylethenes was studied by fluorimetric and photochemical measurements. When at least one of the aryl groups is a large polycyclic group of low S1 energy (as in the case of pyrenyl and anthryl derivatives) the minimum at 90-degrees (typical of stilbene) in the potential energy curve as a function of the twisting coordinate may disappear. This leads to an unusual fluorescence emission at room temperature and favours the adiabatic 1cis* --> 1trans* isomerization mechanism instead of the well-known diabatic pathway, which implies internal conversion to the ground state from the 1perp* configuration.