Local medium effects in the photochemical behavior of substituted stilbenes immobilized on quartz surfaces

Several substituted trans-stilbene derivatives have been prepared and immob ilized onto a quartz surface. A number of immobilization methods has been t ried including the silanization technique, cross-linking with cyanuric chlo ride, surface activation with cyanogen bromide and surface smoothing with c oating proteins. Direct cross-linking failed, but other immobilization tech niques were successful. Studies of solvent polar effects on the fluorescenc e spectrum of the immobilized stilbenes indicate that the maximum wavelengt h of the fluorescence emission is not very sensitive to solvent polarity. T he apparent local polarity of the medium in the vicinity of the stilbene la bel was estimated, and E-T(30) value was found to be close to 50 kcal mol(- 1). The trans-cis photoisomerization kinetics of the stilbene derivatives i n the immobilized and free state in a medium of different viscosity was mon itored with the fluorescence technique at constant-illumination conditions. The apparent photoisomerization rate constant of the process was calculate d using steady-state approximations. It was found to be 3-4 times less for the immobilized label than in a free state which indicates that the surface and protein itself sterically hinder the rotation of the stilbene fragment around the olefinic double bond in the excited state. Investigation of the microviscosity effect on the photoisomerization of the immobilized and fre e stilbene label was carried out by changing the relative concentration (% v/v) of glycerin in a glycerin-water mixture used as a solvent. With the ap propriate calibration, the microviscosity in the vicinity of the stilbene l abel was estimated. (C) 1999 Elsevier Science S,A. All rights reserved.