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.

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