EFFECT OF NITRO SUBSTITUENTS ON THE PHOTOCHROMISM OF SOME SPIRO[INDOLINE-NAPHTHOPYRANS] UNDER LASER EXCITATION

Nanosecond laser flash photolysis has been used to study the photochro mic reaction of three spiro(indoline-naphthopyrans) substituted by one or two nitro groups on the two moieties of the molecule: either on th e 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 pr esent work it was found for the three nitro-substituted compounds that the photomerocyanine (the opened all-trans isomer of the spironaphtho pyran) is formed partially via a triplet pathway and also to a less ex tent via an excited singlet pathway, according to the position of the nitro group. In contrast, the photomerocyanine of the unsubstituted co mpound is formed only via an excited singlet state. The quantum yields of photomerocyanine formation were determined in toluene solution: su bstitution by NO2 on the naphthopyran moiety leads to a very high quan tum yield (similar to 0.9) and proceeds essentially via the triplet pa thway, while for the other two compounds (II and III) the yield obtain ed is around 0.5 and both triplet and singlet pathways are involved; i n the unsubstituted compound in which only the singlet pathway is pres ent, the coloration yield is much lower (similar to 0.2). The results show that the triplet channel induced by the presence of the nitro gro up 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.