EFFECT OF MEDIA POLARITY ON THE PHOTOISOMERIZATION OF SUBSTITUTED STILBENE, AZOBENZENE AND IMINE CHROMOPHORES

The influence of substituents and media polarity on the photoinduced E -->Z geometrical isomerisation of the stilbene, azobenzene and N-benzy lideneaniline chromophores has been compared and assessed. The efficie ncy of the process in all three systems is markedly dependent on the p resence and characteristics of electron-donor and electron-acceptor su bstituents at the 4- and 4'-positions. The results are discussed in te rms of relaxation of the E-excited singlet state. In the absence of a nitro substituent, relaxation to the S-1 orthogonal state competes eff ectively with non-productive intramolecular electron transfer; in the presence of a nitro substituent, the T-1 orthogonal state is formed fr om inter-system crossing. For systems with a 4-nitro and a 4'-electron -donor substituent, access to the triplet state is inhibited by polar solvents promoting formation of the inactive charge-transfer state fro m the S, state, and no isomerisation is observed. Similar effects are observed in both solution and polymer films. Such variations in behavi our have important implications for the utilisation of the chromophore s in nonlinear optical phenomena including photorefractivity.