Photochromic Polyphosphazenes with Spiropyran Units

Photochromic spiropyrans were linked to polyphosphazenes through diethyleneoxy or triethyleneoxy spacer units. The reversion of photogenerated merocyanine groups to spiropyran units in these polymers in various solvents or in the solid state was investigated. The reversion rate in the polymeric solid state was much slower than in solution. In solution the rate decreased as the solvent polarity decreased. A phosphazene polymer with spiropyran units as the only side groups present showed a slower merocyanine to spiropyran reversion rate than did a mixed-substituent polyphosphazene with a lower loading of the chromophores. Presumably this is a consequence of steric effects. A polymer with spiropyran units linked via diethyleneoxy spacer groups underwent the merocyanine to spiropyran reversion at a slower rate than did a polymer with the spiropyran units connected through a triethyleneoxy spacer unit. Unlike the behavior of the free small-molecule spiropyran, the merocyanine relaxation in the polymeric species showed deviations from a first-order relationship in solution and in the solid state. Relaxations in THF and in the solid state were modeled by a biexponential process. The overall phenomena were explained by the solvation or aggregation of the polymer chains in solution as well as by the existence of two different environments for the merocyanine units.