Photochromism of spirooxazines with elements of lipid complementarity in solution and liposomes

Two new phenanthryl-based spirooxazines that contain elements of lipid complementarity have been synthesized. The effect of substitution on their photochromic properties and relative orientation within the lipid bilayer of dipalmitoylphosphatidylcholine large unilamellar vesicles (LUVs) has been examined. Absorption spectroscopy was used to determine the rate constants for thermal ring closure in LUVs, and solvents of varying polarity and viscosity. The rate constant for thermal ring closure was shown to increase with increasing solvent polarity. However, when solvent polarity was kept relatively constant, a slight decrease in this rate constant with increasing solvent viscosity was observed for a tetradodecoxy-substituted spirooxazine. Upon inclusion of these lipid-like spirooxazine analogues in LUVs, the rate constant for thermal ring closure was significantly smaller when the degree of substitution was high. As well, the relative orientation of these photochromes in the lipid bilayer was determined based on their solvatochromic properties, and also correlated to differences in substitution.