A closer look at the photochromism of vinylidene-naphthofurans

Vinylidene-naphthofurans are a new class of photochromic molecules, easily synthetized, with a unique structure combining an allene group linked to a dihydroftiran ring. These uncoloured molecules show acidochromism in solution and photochromic properties when adsorbed in silica gel or dissolved in acidified alcoholic solutions but not in common solvents or in the solid state. A mechanism for their thermally reversible photochromic behaviour is proposed based on NMR analysis of UV-irradiated CH3OD + THF-d8 acidified solutions: the UV light promotes the addition of methanol to the naphthofuran affording a set of non-coloured photoproducts which evolve towards a non-coloured compound P-2. In the presence of acid, the later is quickly converted into a cationic violet dye that returns thermally to the initial closed naphthofuran in the dark. This photochromic system switches between the uncoloured and violet state after UV or sunlight exposure (15 s) and returns thermally to the initial uncoloured state in 2-8 min, in the dark, at room temperature. (C) 2016 Elsevier Ltd. All rights reserved.