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 i somers of 1,2-diarylethenes was studied by fluorimetric and photochemi cal measurements. When at least one of the aryl groups is a large poly cyclic group of low S1 energy (as in the case of pyrenyl and anthryl d erivatives) the minimum at 90-degrees (typical of stilbene) in the pot ential energy curve as a function of the twisting coordinate may disap pear. This leads to an unusual fluorescence emission at room temperatu re and favours the adiabatic 1cis --> 1trans* isomerization mechanism instead of the well-known diabatic pathway, which implies internal co nversion to the ground state from the 1perp configuration.