EFFECT OF THE NATURE OF THE AROMATIC-GROUPS ON THE LOWEST EXCITED-STATES OF TRANS-1,2-DIARYLETHENES

The photophysical and photochemical behaviour of a series of diarylethenes has been investigated experimentally. For the same compounds the potential-energy curves of trans --> cis photoisomerisations and the ethenic bond orders in the S1 and T1 states have been calculated. The purpose of the study is to assess which properties of the aryls govern the photophysical behaviour of the diarylethenes. It is found that the relative energies of the L(a) and L(b) states of the larger aryl with respect to the ethene B(u) state determine to a large extent the photophysical and photochemical properties of the diarylethenes.