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

Several substituted trans-stilbene derivatives have been prepared and immobilized onto a quartz surface. A number of immobilization methods has been tried including the silanization technique, cross-linking with cyanuric chloride, surface activation with cyanogen bromide and surface smoothing with coating proteins. Direct cross-linking failed, but other immobilization techniques were successful. Studies of solvent polar effects on the fluorescence spectrum of the immobilized stilbenes indicate that the maximum wavelength of the fluorescence emission is not very sensitive to solvent polarity. The apparent local polarity of the medium in the vicinity of the stilbene label was estimated, and E³⁰_T value was found to be close to 50 kcal mol^-1. The trans-cis photoisomerization kinetics of the stilbene derivatives in the immobilized and free state in a medium of different viscosity was monitored with the fluorescence technique at constant-illumination conditions. The apparent photoisomerization rate constant of the process was calculated 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 free 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 appropriate calibration, the microviscosity in the vicinity of the stilbene label was estimated.