Unusual UV (λexc = 303 nm) and visible (λexc = 574 nm) activated photochromism of an indeno-fused naphthopyran

Abstract

In this work a completely novel photochromic mechanism exhibited by an indeno-fused naphthopyran (chromene) is presented. The studied molecule is the first case of a chromene-type compound exhibiting both photochromism and thermochromism. These phenomena were investigated in ethanol. Thermochromism was detected by monitoring absorption spectra in the 285–320 K temperature range: the enthalpy of the thermocoloration reaction was determined (ΔH = 25 kJ mol⁻¹) and the equilibrium constant was estimated (K_eq ≈ 10⁻³). For the photochemical investigation, carried out in the 200–270 K temperature range, continuous monochromatic light was used for excitation. The photocoloration was observed under two distinct stimulations: UV-irradiation (λ_exc = 303 nm), which is normally used for photochromic chromene activation, and visible-irradiation (λ_exc = 574 nm) of the thermally equilibrated solution. Two coloured species are involved in the photochemical and thermal processes. One of them, P, is photochemically produced by UV irradiation of the colourless form, while the other, T, is present in thermal equilibrium with the closed form. When the thermally equilibrated solution is irradiated with visible light, T is converted to P. This system not only results in a cyclic on–off chromogenic device, which can be switched-on by UV radiation, but it also exhibits on–off functionality using lower energy visible light. Such behaviour makes this photochromic system exceptionally efficient upon exposure to sunlight.