The complex photochromic behaviour of 5,6-benzo(2H)dimethylchromene in 3-methylpentane solution

The photokinetic behaviour of 5,6-benzo(2H)dimethylchromene was investigated in 3-methylpentane solution by spectrophotometric methods using monochromatic irradiation wavelengths in the UV and visible regions. This molecule only exhibits photochromism at low temperature (T < 240 K), since the rate of the back thermal reaction (bleaching) is very fast (k_Δ > 10² s⁻¹) at ambient temperature. The colour-forming kinetics were followed under continuous irradiation using UV light (318 nm) in two low temperature ranges (160–140 and 235–219 K). In the 160–140 K interval, the temperature is low enough to reduce the thermally activated processes and UV irradiation produces one coloured form, C1 (Φ_C1 = 0.42). Visible light irradiation converts C1 (Φ_−C1 = 0.10) to a photoproduct, P, which is thermally and photochemically stable in that temperature range. In the 235–219 K temperature interval, both thermal and photochemical processes were detected: two coloured forms, C1 and C2, are produced in consecutive steps, one of them is thermoreversible, whereas the other is photoreversible. Quantum yields of the photoreactions and kinetic parameters (rate coefficient and activation energy) of the thermal ring-closure reaction were determined. Based on the results obtained, a plausible reaction mechanism is proposed.