Effect of surface microenvironment on bound spiropyran photochemistry

Spiropyrans are a group of organic molecules that undergo a reversible photoinduced transformation (i.e. photochromism) from a colorless, non-planar spiropyran form to a colored, planar merocyanine form. Photochromism is accompanied by a large change in the structure and the dipole moment. These changes suggest that such molecules might be useful in light-controlled, 'smart surface' applications. This study examines the effect of the microenvironment near the surface-bound spiropyran on its photochemistry. The surfaces were designed to exhibit a mixture of hydrophobic and hydrophilic components using a mixed silane chemistry on a silica substrate, and the spiropyran was covalently bound to the surface using a carbodiimide linking technique. The solvatochromic behavior of spiropyran derivatives was studied in solution using UV-Vis absorption spectroscopy and fluorescence spectroscopy for comparison with the surface-bound species. Spiropyrans in solution and on the surface both exhibited negative solvatochromism. Based on linear solvation energy relationships using the Kamlet-Taft polarity scales, hydrogen bonding appears to play a prominent role in solvent stabilization of spiropyrans in solution and on surfaces. Correlations between emission maxima of the spiropyrans and Reichardt _τ(30) polarity scale revealed that both the solvent and the substituent groups on the spiropyran affected the spiropyran photochemistry. The solvatochromic behavior of the surface-bound spiropyran was similar to that of a model spiropyran in solution, being very sensitive to the polarity of the surrounding liquid and not significantly affected by the surface components, except in solvents of low polarity where the surface seemed to have an increased influence on the spiropyran's solvation shell.