MECHANISM OF PHOTOREORIENTATION OF AZOBENZENE DYES IN MOLECULAR FILMS

Aminoazobenzene photoreorientation is investigated in systems in which hydrophobically substituted chromophores are organized in molecular ( Langmuir-Blodgett) films on glass substrates. Whereas the cis absorpti on of the chromophores in CHCl3 after irradiation into their pi-->pi absorption band is easily measured, we were not able to detect spectra l changes characteristic of isomerization in the solid films during ir radiation. The quantum efficiency for trans-->cis isomerization in LB films is reduced at least by a factor of similar to 10(-4) from that i n organic solvent. We conclude that aggregation of the chromophores in LB films limits conformational changes after photoexcitation, in dist inction from the situation when aminoazobenzene derivatives are embedd ed in thin polymer or liquid-crystal films. Hence, the subsequent reor ientation is probably driven by a mechanism that does not encompass th e cis isomer. The temperature dependence of the reorientation indicate s thermal activation with an activation energy Delta E = 0.24 eV. The irradiation intensity dependence shows that the reorientation is a col lective process. It is distinctly different from what would be expecte d if photoexcitation resulted in reorientation of individual dye molec ules and is simulated in a simple model from which the intensity thres hold, I-th = 0.95 +/- 0.2 mW/cm(2), is retrieved. From the experimenta l observations we postulate that the vibronic coupling of excited stat es may open a radiationless deactivation channel in which the electron ic excitation energy is thermalized and distributed across neighboring molecules of the excited chromophore. The excited chromophore may hen ce rotate by an infinitesimal amount within the steric potential of it s neighbors. This process ceases only when the chromophores are rotate d to an orientation in which they are no longer excited by the irradia ting light beam.