Effect of nitro substituents on the photochromism of some spiro[indoline-naphthopyrans] under laser excitation

Abstract

Nanosecond laser flash photolysis has been used to study the photochromic reaction of three spiro(indoline-naphthopyrans) substituted by one or two nitro groups on the two moieties of the molecule: either on the naphthopyran ring (compound I), or on the indoline ring (compound II), and the third compound ( III) with a nitro group on both rings; the unsubstituted compound ( IV) was also studied for comparison. In the present work it was found for the three nitro-substituted compounds that the photomerocyanine (the opened all-trans isomer of the spironaphthopyran) is formed partially via a triplet pathway and also to a less extent via an excited singlet pathway, according to the position of the nitro group. In contrast, the photomerocyanine of the unsubstituted compound is formed only via an excited singlet state. The quantum yields of photomerocyanine formation were determined in toluene solution: substitution by NO 2 on the naphthopyran moiety leads to a very high quantum yield (∼0.9) and proceeds essentially via the triplet pathway, while for the other two compounds ( II and III) the yield obtained is around 0.5 and both triplet and singlet pathways are involved; in the unsubstituted compound in which only the singlet pathway is present, the coloration yield is much lower (∼0.2). The results show that the triplet channel induced by the presence of the nitro group is much more efficient than the singlet one for CO bond rupture in the indolinospironaphthopyrans. Moreover semi-empirical calculations were performed using the AM1 method; the theoretical results show the importance of the S 1—T 1 energy gap in the closed form for coloration efficiency.