Photochromism of nitrospiropyrans: Effects of structure, solvent and temperature
The thermal and photochemical ring opening and ring closure of the spiropyran/merocyanine couple of three nitro-substituted spiropyrans (6-NO2: 6, 7 and 8-NO2: 8) were studied and the results were compared with those of other 6-NO2 spirobenzopyranindolines (BIPS) (1-5). The photocolouration, which occurs in the triplet manifold throughout, and the photochemical conversion and thermal relaxation of the two observable photomerocyanines into the closed spiropyran (Sp) form (decolouration) were quantified as a function of solvent polarity and temperature. The relaxation time (τt-Sp) at 25°C ranges from 2 s for 5 in methylcyclohexane to 104 s for 7 in ethanol. This large variation in τt-Sp is due to changes in both the activation energy (Ea = 75-105 kJ mol-1, increasing with polarity) and the pre-exponential factor (A = 1012-1015 s-1). The quantum yield of colouration with 308 nm pulses is substantial in solvents of low polarity (φcol = 0.3-0.8) and decreases (c→t = 30-40 kJ mol-1. The effects of substituents and medium properties are described and the mechanism of photochromism is discussed.
Goerner
p. 416 - 423
(2007/10/03)
1-OCTADECYL- AND 3,3-DIPHENYL-SUBSTITUTED PHOTOCHROMIC INDOLINOSPIROCHROMENES
A number of photochromic indolinospirochromenes that contain an octadecyl substituent in the 1 position were synthesized.The introduction of an octadecyl group in place of a methyl group increases the stability of the merocyanine form in solution but does not change its spectral characteristics.On the other hand, the introduction of phenyl groups in place of methyl groups in the 3 position does not change the stability of merocyanine but gives rise to a bathochromic shift of the long-wave absorption band.
Samoilova, N.P.,Bobyleva, G.K.,Gal'bershtam, M.P.
p. 41 - 44
(2007/10/02)
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