The effect of a sulphur bridge on the photochromic properties of indeno-fused naphthopyrans

The synthesis of four new 2,2-diphenyl-2H-naphtho[1,2-b]pyrans with a fused indeno group at the f face and a sulphur junction between the 2,2-phenyl groups is described. The photochromic properties in solution of these novel compounds were investigated under continuous irradiation. Compared to known indeno-fused naphthopyrans, these new compounds showed a significant bathochromic shift in the spectra of the open forms, faster ring closure kinetics and an expected decrease in the colourabilities.     

Novel synthesis of 2,2-dialkyl-3-dialkylamino-2,3-dihydro-1H-naphtho[2,1-b]pyrans

2,2-Dialkyl-3-dialkylamino-2,3-dihydro-1H-naphtho[2,1-b]pyrans were prepared from 2-naphthol, a secondary amine, and 3-hydroxy-2,2-dialkylpropanal in the presence of a catalytic amount of p-toluenesulfonic acid. This one-pot reaction involves retro-aldol disintegration of 3-hydroxy-2,2-dialkylpropanal followed by formation of a Mannich base intermediate from 2-naphthol, a secondary amine, and formaldehyde (retro-aldol product). This Mannich base then disproportionates into a quinone methide intermediate and the secondary amine is regenerated. It then forms an enamine intermediate with 2,2-dialkylacetaldehyde (another retro-aldol product). Finally, the quinone methide intermediate undergoes electrocyclic ring closure with enamines to produce the title compounds.     

Spectrokinetic studies on new bi-photochromic molecules containing two naphthopyran entities

A range of new bi-photochromic molecules containing two identical (3a-d) or two distinct naphthopyran units (6a-d), linked through the phenyl substituents located on the sp³ hybridised pyran ring carbon atom, using conjugated and non-conjugated spacers, have been synthesised from bis-propynols and (substituted)naphthols. Study of the spectrokinetic properties of these compounds under near UV-vis continuous irradiation conditions revealed that the two naphthopyran units are stimulated independently leading to open forms with higher colourabilities but without affecting the individual bleaching kinetics. Compared to the individual photochromic components and to model mono-photochromes it was observed that the nature of the bridge has a small effect on the photochromic properties of each system.     

Synthesis and photochromic properties of substituted 3H-naphtho[2,1-b]pyrans

The synthesis and spectroscopic properties of novel 3H-naphtho[2,1-b]pyrans are described. Subtle variation of the colour of the photo-generated merocyanine dyes derived from these naphthopyrans can be accomplished by controlling the steric interactions between a terminal pyrrolidine donor group and a proximal substituent.     

Remarkable thermally stable open forms of photochromic new N-substituted benzopyranocarbazoles

New N-substituted benzopyranocarbazoles were synthesized and their photochromic properties, under continuous near-UV irradiation, are described. Besides a remarkable sensitiveness to solar light, N-methyl and N-benzyl derivatives give rise to particularly stable coloured open forms. The structure of these photoisomers was characterized through NMR spectroscopy. According to our results, it was possible, after UV irradiation, to accumulate more than 90% of the long-lived TT-isomer. At ambient temperature, coloured TT state was fully photobleachable by visible light and no significant degradation was observed during cycling. This remarkable property opens a range of potential applications for these N-substituted benzopyranocarbazoles in photoswitchable devices.     

The synthesis and electronic absorption spectra of 3-phenyl-3(4-pyrrolidino-2-substituted phenyl)-3H-naphtho[2,1-b]pyrans: further exploration of the ortho substituent effect

Introduction of a substituent into a sterically demanding 2-position of a 3-(4-pyrrolidinophenyl) ring of a 3,3-diaryl-3H-naphtho[2,1-b]pyran results in the generation of an additional short wavelength absorption band leading to organic photochromes that appear as dull shades of orange and red.     

Photoswitch based on remarkably simple naphthopyrans

Photochromic switches built around 5-carbonyl-3,3-diphenyl-3H-naphtho[2,1-b]pyran unit have been synthesized. Their open forms are constituted of a 1:1 ratio of TC and TT-isomers according to ¹⁹F and ¹H NMR investigations. Electrocyclization of TT isomers back to the closed form is efficiently achieved upon visible irradiation whereas it has been found to be a particularly unfavoured thermal process. These simple 2H-chromenes could act as molecular photoswitch at ambient temperature.     

New benzopyranocarbazoles: synthesis and photochromic behaviour

The synthesis of three new benzopyranocarbazoles (=[indole]naphthopyrans) from hydroxybenzo[a]carbazoles is described. The photochromic properties of these novel compounds were investigated under flash photolysis and continuous irradiation. Compared to known [indole]benzopyrans these new compounds showed a significant bathochromic shift in the spectra of the open forms, an increase in colourabilities and slower ring closure kinetics. The photochromic behaviour of compound 4 has been further investigated. Continuous near-UV irradiation led to the formation of one photoisomer (TC) that was subsequently partially converted, to the other (TT). Thermal reversion of the preirradiated system to the original form was only partial and followed a monoexponential decay involving the back-conversion of the TC-isomer to the uncoloured closed form (CF). The thermally stable TT-isomer could only be photobleached with visible light. This process was shown to proceed through a fast photoconversion TT→TC followed by the thermal path TC→CF. Thermal relaxation of the activated system was also studied at various temperatures.