Thermal Formation of a Magenta Merocyanine in Ethanol: A Microsolvated SP→TCC→TTC Pathway in a Nitrospiropyran

Solutions of 5’-methoxy-1’,3’,3’-trimethyl-6-nitrospiro[chromene-2,2’-indoline] in alcohols are intensely pink in the dark, become colorless under white light and recover their color thermally, with Arrhenius activation energies of 101.1± 0.5, 104.9± 1.6 and 105.3± 1.4 kJ mol−1 in ethanol, methanol and acidified ethanol, respectively. The measured process is therefore the thermal formation of the merocyanine (MC) from the photogenerated spiropyran (SP). The absorbance of the equilibrated ethanol solution, combined with the calculated oscillator strength, indicates that MC constitutes only a small fraction of the compound, of the order of a few per cent. With continuum solvation alone no cis-merocyanine intermediate (TCC) is found, whereas in a complex with five explicit ethanol molecules a fast SP⇌TCC pre-equilibrium precedes a considerably slower isomerization to the trans isomer (TTC). The reversible kinetic model shows that the measured rate constant reflects the position of the