The competitive effect of intramolecular charge transfer on the photochromism of spiro[cyclohexadiene-indolines] studied by ps-spectroscopy

The photochromism of spiro[cyclohexadiene-indolines], the reversible bond cleavage at the spiro-carbon resulting in a colored merocyanine, is affected by solvent polarity. Photoexcited intramolecular charge transfer within the preserved spiro form of the molecule is favored in polar solvents, explaining the drop in merocyanine yield (less than 1% in acetonitrile compared to 18% in cyclohexane). Charge transfer is expressed by a strongly solvatochromic fluorescence corresponding to an increase of the dipole moment of more than 20 D in the excited state with respect to a charge separation over a distance of 4 Angstrom. Picosecond-resolved measurements of the fluorescence and the transient absorption confirm the picture of photoisomerization via the first excited singlet state and a vibronically hot ground state of the merocyanine. The time constant (similar to 100 ps) for the merocyanine formation is hardly sensitive to the solvent polarity and is independent of the lifetime of the charge transfer state of the spiro[cyclohexadiene-indoline] (5 ps to 1.4 ns, increasing with solvent polarity).