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Spiropyrans are a group of organic molecules that undergo a reversible photoinduced transformation (i.e., photochromism) from a colorless, nonplanar spiropyran form to a colored, planar merocyanine form. Photochromism is accompanied by a large change in the structure and in 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 by using a mixed silane chemistry on a glass substrate, and the spiropyran was covalently bound to the surface via amide linkages. 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. Correlations between emission maxima of the spiropyrans and Reichardt's ET(30) polarity scale revealed that the surface-bound spiropyran experienced lower polarity than a solution model in solvents of low and medium polarities. Linear solvation energy relationships using the Kamlet−Taft polarity scales showed that hydrogen bonding played a prominent role in solvent stabilization of surface-bound spiropyrans in hydrogen-bonding solvents. The surface design used causes the spiropyran to interact significantly with the surface in solvents of lower polarity and to behave as if it were dissolved in solution in more polar, hydrogen-bonding solvents.
Rosario et al. (Wed,) studied this question.