A possible explanation for the photochromism of two 4-alkenyl-sydnones

Abstract

Blue reversible photochromism of 3-phenyl-4-(3-methyl-1-buten-2-yl)sydnone IV and 3-phenyl-4-(1-cyclohexen-1-yl)sydnone V may be interpreted by an electrocyclic transformation in the sydnone ring leading to the N-nitrosoaminoketene system. Both sydnones are photochromic in frozen solutions. In the solid state only sydnone V is photochromic; stabilization of the coloured form is probably due to the intramolecular hydrogen bond between the carbonyl oxygen of the opened sydnone skeleton and hydrogen of the cyclohexene substituent. By a comparison between the experimentally recorded UV—VIS spectra and the respective theoretical spectra, obtained by the semiempirical MO INDO method, the validity of the ring opening model for sydnone photochromism is discussed. The net charge values in the ground and excited states suggest that the idea of the sydnone ring opening by cleavage of the O(1)C(5) bond could be justifiable.