Excited-state properties of a photochromic spirooxazine: double pathways for both fluorescence emission and camphorquinone-sensitized reaction

In this article, we report a study on the singlet and triplet excited-state properties of a spirooxazine (1,3-dihydro-3,3-dimethyl-1-isobutyl-6'-(2,3-dihydro-1H-indol-1-yl)spiro[2H-indole-2,3'-3H-naphtho[2,1-b][1,4]oxazine]). The singlet state of this molecule is photoreactive: upon UV light stimulation, it produces a colored merocyanine that thermally reverts to the starting compound. A double-way radiative relaxation path was found for singlet-state excitation. Experimental observations on the absorption and fluorescence spectra were in excellent agreement with TD-DFT calculations for the singlet state. The triplet state, which could not be directly populated by intersystem crossing from the singlet, when reached by energy transfer from a suitable sensitizer (camphorquinone), yielded the colored merocyanine with quantum yield close to unity. However, the donor/acceptor interaction also originated a new photochromic system as a consequence of the competition of hydrogen abstraction with energy transfer in the interplay of the sensitizer with the substrate. The newly produced photochrome was structurally, spectrally, and photochemically characterized. It exhibited excellent colorability in both directly excited and triplet-sensitized photoreactions by virtue of high photoreaction quantum yield and rather slow bleaching rate of the colored form but also underwent significant degradation in the presence of oxygen that led to the destruction of the photochromic functionality.