Photochromic anils: spectroscopy and mechanisms of photo- and thermal reactions

The photochromism and thermochromism of several salicylideneanilines and related compounds have been investigated. At room temperature or low temperature most salicylideneanilines exist almost exclusively in the enol form in solutions of hydrocarbon solvents. At low temperature in solutions of hydrogen bonding solvents or solutions in which strongly hydrogen bonding species are present, some of the anil molecules exist as the cis-keto tautomer. In the cases of the nitrosalicylideneanilines and 2-hydroxynaphthylideneaniline some cis-keto is produced at low temperatures even in solutions of hydrocarbon solvents. This phenomenon is attributed to the enhanced stability of the cis-keto tautomer. In rigid solutions at low temperature excitation of the enol form of most salicylideneanilines produces a colored species via a cis-keto-like transition state. It is found that increased viscosity decreases the yield of the photo-product by increasing the potential, energy barrier between the transition state and the photo-colored species. This photo-colored species has a characteristic long wavelength absorption band which is different from either the absorption of the enol or the cis-keto. This species is identified as a non-equilibrium form of the cis-keto in which the C=C bond is twisted and the internal hydrogen bond broken in order to relieve the strain which the nitrogen acquires in the transition state. Warming the solution to decrease its viscosity results in the immediate conversion of this photo-colored species to the cis-keto. Further warming converts this cis-keto to enol. Under the proper conditions of temperature and viscosity the photo-colored species may be eradicated by irradiation into its absorption band. This process converts the photo-colored species to the enol through the transition state. No cis keto is produced. Nitrosalicylideneanilines are not photochromic; 2-hydroxynaphthylideneaniline is photochromic, but in this case irradiation produces only more cis keto tautomer. This photo-produced cis keto may be eradicated by irradiation into the cis keto absorption band.