The last few years have seen an explosion of interest in traditional photochromic systems not only for their applications in variable transmission devices, which continues to attract commercial interest, but also for the ability of these molecules to undergo structural and electronic reorganisation which has been seized upon by materials scientists for the development of switches, logic gates, photoinduced molecular motions such as rotors and fibrils, and photoregulation of drug availability. This comprehensive review examines, for the first time, the 'dark side of photochromism'; negatively photochromic systems which are coloured in their ground state and reversibly photobleach upon exposure to electromagnetic radiation with a wavelength over ca. 400 nm i.e. visible light responsive systems. This review is organised by structural class and examines their synthesis, structure, key spectroscopic data for coloured and bleached species, structure-switching relationships and applications. The usefulness of these negatively photochromic systems is only gradually coming to the fore with the advantages of low energy activation c.f. conventional Uvactivated switches, enabling the modulation of a plethora of useful optical and physical properties and the design of new materials with broad ranging applications.

1-Naphthol reacts with 1,1-diarylprop-2-yn-1-ols 5a,b, under alumina catalysis, by two pathways to give the photochromic naphtho

A range of polymerisable photochromic naphthopyrans were prepared via the synthesis of an acrylate moiety on either the naphthalene ring (R) and/or the di-aryl (Ar) rings on the 3-position of the naphthopyran. The monomers obtained (NA) were copolymerised with methyl methacrylate (MMA) and methyl acrylate (MA) by the RAFT (reversible addition fragmentation chain transfer) process in order to control the structure of the polymeric chains and accurately control the number of photochromic molecules per chain. Films were prepared with the synthesized photochromic polymers and their photochromic properties were assessed. We found that the fading rate of the photochromic dye was slower in a matrix of PMMA (higher T g ) than in a matrix of PMA (lower T g ). By producing block copolymers of PMMA-b-poly(MA-co-NA), the fading rate was increased, and the production of PS-b-poly(MA-co-NA) further enhanced the speed of switching, as phase separation occurred between the poly(MA-co-NA) block and the PS block. This study presents an example of tuning the photochromic properties of a dye via variation of the architecture of its polymeric support.

Photochromic fuzzy logic systems have been designed that extend human visual perception into the UV region. The systems are founded on a detailed knowledge of the activation wavelengths and quantum yields of a series of thermally reversible photochromic compounds. By appropriate matching of the photochromic behaviour unique colour signatures are generated in response differing UV activation frequencies.

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Heating 4-methoxy-1-naphthol with a 1,1-diarylprop-2-yn-1-ol gave the 2,2-diaryl-6-methoxy-2H-naphtho[1,2-b]pyran together with the novel merocyanine, (E)-2-[3',3'-bis(aryl)allylidene]-4-methoxynaphthalen-1(2H)-one. Brief UV-irradiation of the pyran favoured the formation of the (Z)-merocyanine with longer irradiation and/or acidic conditions favouring the (E)-isomer.