Photoprocesses in spiropyrans and their merocyanine isomers: Effects of temperature and viscosity

The spectroscopic properties of five spiro[2H-1-benzopyran-2,2'-indoline] compounds containing a nitro group in the B-position of the benzopyran moiety (1-5) were studied as a function of temperature in media of different polarity, e.g., methylcyclohexane, 2-methyltetrahydrofuran (MTHF), glycerol triacetate, and ethanol. UV excitation of a spiropyran at ambient temperature leads to a triplet state of the merocyanine ((3)t*) prior to population of the trans and cis isomers. The yields of formation of (3)t* and the merocyanine ground states decrease in a similar manner on decreasing the temperature. In highly viscous media, where the photochemical ring opening is suppressed, phosphorescence and the T-T absorption of the spiropyran could be detected: e.g., lambda(TT) = 451 nm, tau(T) = 0.2 s for 3 (R 1': CH3, R-8: OCH3) in MTHF at temperatures below 100 K. Upon excitation at 530 nm photoisomerization into a less stable cis isomer occurs. The quantum yield is reduced on decreasing the temperature, whereas the yields of fluorescence and population of the 3t* state increase in a similar manner. A semilogarithmic plot of the inverse triplet life-time vs. 1/T shows two linear sections, indicating rotation about the C=C bond in fluid solution and a strong increase in triplet life-time due to the hindrance of rotation at very high viscosities. The (3)t* state, which is an intermediate in the photocolouration, is not directly involved in trans --> cis photoisomerization. (C) 1997 Elsevier Science B.V.