Chromophore aggregation and photoinduced dichroism in sol-gel films

Photoinduced dichroism (PID) and UV-visible spectroscopic studies have been performed in silica-based thin films prepared by the sol-gel technique, containing Disperse Red 1 (DR1) as trans-cis photoisomerizable chromophore and carbazole as a plasticizer. The absorbances parallel and perpendicular to the induced anisotropy axis, A∥ and A⊥, have been monitored during illumination. Some of the samples show an increase of the average absorbance T0 = (A∥ + 2A⊥)/3 over the initial value. This effect can be interpreted by the presence of aggregates of DR1 molecules, breaking down during illumination and increasing the absorption at the probe wavelength. The formation of the aggregates is evidenced by a blue-shift in the absorption spectra. The carbazole apolar group acts as spacers between the DR1 molecules, effectively avoiding the formation of aggregates. Model equations have been proposed to describe the kinetics of the system and the initial PID data have been fitted by using a simple phenomenological model. © 2007 Elsevier B.V. All rights reserved.