ACID-BASE CHARACTERISTICS OF PHOTOCHROMISM.

The mechanisms of photochromic reactions are studied in regard to their acid-base characteristics. An intramolecular mechanism, consisting of a six-membered hydrogen transfer from the methylene group in p-nitrobenzyl structures to an o-carboxylate group, is elaborated by kinetic analysis of the flash photolysis intermediates from 2-4-nitrobenzylbenzoic acid and other additional model structures. Compounds synthesized for this purpose include 5-nitro-o-toluic acid and 4,4-dinitrobibenzyl-2,2-dicarboxylic acid. The spectra of similar short lived aci-anion intermediates are observed in the photodecarboxylation of o- and p-nitrophenylacetic acid derivatives in aqueous base. These are identified by comparison with similar spectra obtained from other reactions of the corresponding nitrotoluene compounds. A ketone model compound, 2-methyl-5-nitrobenzophenone, was synthesized and unexpectedly found to cause efficient oxidation of isopropyl alcohol to acetone, as well as showing several spectral intermediates. The trans to cis photoisomerization of N, N-dimethylindigo and N, N-dimethyl-5, 5, 7, 7-tetrabromoindigo is confirmed, and the cis spectra of these two dyes are calculated using photostationary state techniques. The reverse cis to trans reaction kinetics are studied with flash photolysis and found to be highly acid-catalyzed. The reaction rate is linearly dependent upon the hydrogen ion concentration. Optical saturation effects with helium-neon and with ruby lasers are described. Author