Photochromism of nitrospiropyrans: effects of structure, solvent and temperature

The thermal and photochemical ring opening and ring closure of the spiropyr an/merocyanine couple of three nitro-substituted spiropyrans (6-NO2: 6, 7 a nd 8-NO2: 8) were studied and the results were compared with those of other 6-NO2 spirobenzopyranindolines (BIPS) (1-5). The photocolouration, which o ccurs in the triplet manifold throughout, and the photochemical conversion and thermal relaxation of the two observable photomerocyanines into the clo sed spiropyran (Sp) form (decolouration) were quantified as a function of s olvent polarity and temperature. The relaxation time (tau (t-Sp)) at 25 deg reesC ranges from 2 s for 5 in methylcyclohexane to 10(4) s for 7 in ethano l. This large variation in tau (t-Sp) is due to changes in both the activat ion energy (E-a = 75-105 kJ mol(-1), increasing with polarity) and the pre- exponential factor (A = 10(12)-10(15) s(-1)). The quantum yield of colourat ion with 308 nm pulses is substantial in solvents of low polarity (Phi (col ) = 0.3-0.8) and decreases (<0.2) with increasing polarity. The observed me rocyanine triplet state is rather short-lived (< 10 mus) and the precursor of the trans- and a cis-merocyanine. The same intermediates, the triplet st ate and the cis-isomer, were observed when the trans-merocyanine was excite d at 530 nm. The thermal cis --> trans isomerization takes place in the mus -ms range and has an activation energy of Ec --> t = 30-40 kJ mol(-1). The effects of substituents and medium properties are described and the mechani sm of photochromism is discussed.