The photoisomerization of spiro[cyclohexadiene-indoline] via an intramolecular charge transfer state

The coloration of spiroindolines in solution under UV excitation is an indication of photochromic behavior. The occurrence of dual fluorescence is due to two different photoisomers — the spiro form and the merocyanine form of the molecule. Stokes shifts of the spiroindoline fluorescence up to 8000 cm−1 show the charge transfer character of the excited singlet state, where relaxation leads either to a charge separation between the indoline and the cyclohexadiene moiety (emitting state of the spiro compound, SCT*) or the C–C bond cleavage in the cyclohexadiene (precursor of the merocyanine formation, XCT*). Fluorescence and photoisomerization are observed in time-resolved measurements on a picosecond time scale. The charge separation is stabilized in acetonitrile, where the longest lifetime for the SCT* state (580 ps) and the lowest yield of merocyanine was observed. In nonpolar cyclohexane the fluorescence decay and the formation of merocyanine proceed within 10 ps.