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 absorption of the chromophores in CHCl3 after irradiation into their pi-->pi* absorption band is easily measured, we were not able to detect spectral changes characteristic of isomerization in the solid films during irradiation. The quantum efficiency for trans-->cis isomerization in LB films is reduced at least by a factor of similar to 10(-4) from that in organic solvent. We conclude that aggregation of the chromophores in LB films limits conformational changes after photoexcitation, in distinction from the situation when aminoazobenzene derivatives are embedded in thin polymer or liquid-crystal films. Hence, the subsequent reorientation is probably driven by a mechanism that does not encompass the cis isomer. The temperature dependence of the reorientation indicates thermal activation with an activation energy Delta E = 0.24 eV. The irradiation intensity dependence shows that the reorientation is a collective process. It is distinctly different from what would be expected if photoexcitation resulted in reorientation of individual dye molecules and is simulated in a simple model from which the intensity threshold, I-th = 0.95 +/- 0.2 mW/cm(2), is retrieved. From the experimental observations we postulate that the vibronic coupling of excited states may open a radiationless deactivation channel in which the electronic excitation energy is thermalized and distributed across neighboring molecules of the excited chromophore. The excited chromophore may hence rotate by an infinitesimal amount within the steric potential of its neighbors. This process ceases only when the chromophores are rotated to an orientation in which they are no longer excited by the irradiating light beam.