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Therefore the photochromic reaction of diarylethene derivatives can be controlled by the addition of acid, giving ester hydrolysis. 8 S Me Me O Me Me Me Me R N S R R R = Me or Ph Chemically gated diarylethene derivatives. 9 Me X >130 K Ph S Me S R′ SAc Gated photochromism controlled by thermal means. 9. Here the ring-opening process is suppressed with decreasing temperature, leading to the complete absence of the photoreaction below a cut-off temperature of around 120–130 K. By contrast, the reverse ring-closure process to the coloured form shows no significant temperature dependence.

48(a),(b) But there is still a long way to go before practical applications are realised. 4 Biological Photochromes The visual system is highly dependent on a complex photochromic photocycle, where the chromophore 11-cis-retinal is attached to the protein opsin via a protonated Schiff base of a lysine residue. 13. Bacteriorhodopsin (BR) is a seven-helical transmembrane protein, also with the retinal co-factor, found in the so-called ‘‘purple membranes’’ of bacterium H. salinarium. It converts the energy of ‘‘green’’ light (500–650 nm, max 568 nm) into an electrochemical proton gradient, which in turn is used by a second membrane protein, ATP synthase, to generate chemical energy in the form of ATP.

25) move bathochromically, the effect being most marked in the latter case. 6. 27a). 7. 26c). 26c ) R2 R1 Me R3 O O O O O O O O O A O O O O O O C(CN)2 O O B Effect of substituents of the properties of fulgides and derivatives. 7 R Vis X O R1 S Me MeS coloured R2 F O R= or F F F F X = O or NR Photochromic behaviour of dithiophenylethenes. thiophene ring can be annulated with a benzene ring or replaced with indoles, furans and thiazole rings. The reversible electrocyclic interconversion between the colourless ringopened state and the coloured ring-closed state on irradiation with light occurs at well-separated wavelengths.

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