Photochromism of Dihydroazulene Embedded in Solid Polymer Matrices

Photochromic dihydroazulene/vinylheptafulvene systems have been extensively investigated in solution, whereas their behavior in solid polymer matrices remains much less understood despite their relevance for photonic and optically responsive devices. Here, a phenyl-substituted dihydroazulene derivative (Ph-DHA) was embedded in nine optically transparent polymer matrices that differ in polarity, chemical functionality, rigidity, and segmental mobility. The resulting films were examined by in situ optical transmission measurements under continuous 365 nm irradiation and during subsequent thermal recovery. UV exposure induced pronounced coloration of all films, accompanied by a red shift of the optical absorption edge from approximately 2.7-2.9 eV to 1.8-1.9 eV, indicating formation of the photoinduced vinylheptafulvene form in the solid state. The initial photoinduced changes in absorbance followed a stretched exponential time dependence with the stretch exponent β approaching unity (pure exponential). The minimal time constants (fastest responses) were observed in the 460-500 nm spectral region. Thermal recovery at elevated temperatures was also adequately described by a stretched exponential function but with greater β variation (0.6-1.0). The characteristic relaxation times showed weaker spectral dependence and generally decreased upon heating. No simple correlation between the kinetic parameters and any single polymer property, such as polarity, molecular weight, or chemical functionality, was observed. Instead, the results suggest that a broad distribution of local microenvironments and relaxation pathways within the polymer host governs the rate of photochromic response of Ph-DHA in solid films. This systematic comparison demonstrates that Ph-DHA retains robust and thermally reversible photochromism across chemically diverse solid polymer matrices and provides a phenomenological basis for designing polymer-based photoresponsive films.