Differential scanning calorimetry in studies of solid-state reactions in photochromic materials

Differential scanning calorimetry (DSC) was employed to study the kinetics of reverse thermally driven reactions in polycrystalline samples of three photochromic compounds. Results obtained on two dihydropyridine derivatives were critically re-examined and compared with results of the experiments performed with an acridizinium salt. Heat flow curves, measured on samples previously irradiated with UV, exhibited broad exothermic anomalies at elevated temperatures (above ca. 320 K for dihydropyridines and above ca. 490 K for the acridizinium salt), observed only once after irradiation and attributed to bleaching reactions. The activation energies of bleaching reactions in the dihydropyridine derivatives, determined from fractional heating experiments, amount to ca. 90-170 kJ/mol. Exact determination of the distribution of activation energies proved difficult due to uncertainties with a proper setting of baselines. (C) 2002 Elsevier Science B.V. All rights reserved.