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Notes on the Fluorescence and the Nature of the Photocoloured Form in Photochromic Azomethines

Knyazhansky M. I.

Polish Journal of Chemistry

2008

Vol. 82, nr 4

795-806

The analysis of the results obtained by recent investigations of photochromic azomethines (anils), including the author's data, shows that the path way for the photo-coloured product generation proceeds consecutively via (i) the S1 excited state of the NH structure (S1 NHflu), arising from the excited state intramolecular proton transfer OH→NH and responsible for the fluorescence with the anomalous Stokes shift (ASSflu); (ii) the ASSflu quenching TICT structure (S1 state) with the twisted "keto" ring (alfa = 90°) in the trans isomer (with respect to the C-N bond, beta = 0°); (iii) the unstable trans twisted post1TICT structure (alfa = 90°, beta = 0°) in the S 0 state, which is the immediate precursor of (iv) low-stability post2TICT cis structure twisted in the S0 state (alfa = 90°, beta asymptotic to 180°). The latter is responsible for the photochromism of anils and stabilized by steric interactions and rigid media.