Photochemistry and photochromism: I. A comprehensive spectroscopic investigation of the spiropyrans. II. Survey of the hydrazones and a general mechanism of photochromism
The absorption transitions for the colorless forms of a series of spiropyrans, at both room temperature and 77[degrees]K, are presented. In addition, spectral data are given for chromenes and an indoline which are treated as models for the appropriate halves of the spiropyrans, Analyses of the effect of substitution on the electronic transitions of the model compounds and the benzospiropyrans show that the absorption transitions of the colorless forms are essentially localized on a particular half of the molecule. Similarly, the absorption transitions of the dibenzospiropyrans are shown to have a very large amount of "chromene character" but do not appear to be localized on a particular half of the molecule. The interaction between the orthogonal halves of the molecule is thus apparently greater in the dibenzospiropyrans than in the benzospiropyrans. In addition, a vibrational analysis of the chromene transitions of g-naphthospiropyran is given and shown to be similar to that observed for one of the model compounds, dimethylbenzo-chromene. Assignments of the nature of the absorption transitions; i.e., whether of [pi]<â€” n or [pi]<â€”[pi] character, are made for a number of the molecules considered. The emission and excitation spectra of the colorless forms of the benzospiropyrans at 77[degrees]K as well as spectral data for some chromenes, an indoline, and a substituted aniline, which were used as models for the appropriate halves of the benzospiropyran, is also presented. Comparison of the spectral data of the model compounds with that of the benzospiropyrans indicates that the emissions are localized or originate from a particular half of the molecule. With the exception of the 5,6'-dinitro derivative, these emissions are shown to originate from the chromene portion of the molecule. Consideration of the excitation spectra determined for the dibenzospiropyrans indicates that the observed emissions originate from trace amounts of impurities and any intrinsic emission must be very weak. Assignments of the nature of the emissions; i.e., whether fluorescence or phosphorescence and of n, [pi]* or [pi], [pi]* character, are made for a number of the molecules considered. Calculations of the relative quantum yields of emission are shown for the 6'-nitrobenzospiropyran and [beta]-naphthospiropyran and intramolecular energy transfer between the indoline and chromene halves of the molecule is shown to occur. In addition, a vibronic effect in photochemistry as shown try Becker, et, al. for the chromenes is found for these two molecules and the ratio of [raised k]IC/k[lowered PC] for two vibrations of the g-naphthospiropyran is given. Application of a general mechanism for photochromism of organic molecules involving valence isomerization in a six-membered ring to the photochromism of the substituted hydrazones of aldehydes and ketones is considered. The photochromism of cinnamaldehyde phenylhydrazone in the solid state and in solution is shown to be identical and thus an intramolecular phenomenon. Further, the photochromism of cinnamaldehyde phenylhydrazone is shown to be only 50 per cent reversible. This brief survey of the photochromism of hydrazones indicates that further work involving more hydrazones and model systems needs to be done in order to elucidate the mechanism and prove the applicability of the general mechanism of photochromism to hydrazones.