Photochromism of nitrospiropyrans: effects of structure, solvent and temperature

Abstract

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 (τ_t-Sp) at 25°C ranges from 2 s for 5 in methylcyclohexane to 10⁴ s for 7 in ethanol. This large variation in τ_t-Sp is due to changes in both the activation energy (E_a = 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 (Φ_col = 0.3–0.8) and decreases (<0.2) with increasing polarity. The observed merocyanine triplet state is rather short-lived (<10 μs) and the precursor of the trans- and a cis-merocyanine. The same intermediates, the triplet state and the cis-isomer, were observed when the trans-merocyanine was excited at 530 nm. The thermal cis → trans isomerization takes place in the μs–ms range and has an activation energy of E_{c → t} = 30–40 kJ mol⁻¹. The effects of substituents and medium properties are described and the mechanism of photochromism is discussed.