Dianthrone photochromism 195<m970tt

Photochromic phenomena are very briefly surveyed, with special emphasis on reversible photocyclizations on which many photochromic phenomena are based. The chromic behaviour of dianthrone. DA. and its derivatives is reviewed. It is suggested that the coloured modification B is common to all cases observed and that it is identical with the thermochromic coloured form. A detailed study of the reversible and irreversible photoreactions, thermal reactions, and emission properties of DA and its derivatives is reported, based on the use of both stationary and flash methods, in a wide range of temperatures and solvents. All compounds are fluorescent and phosphorescent, and their triplet—triplet absorption spectra and triplet decay were observed at sufficiently low temperatures. With increasing temperature the triplet decays mainly via its conversion into B, in some cases via a precursor D. Both triplet - D and D - B are strictly viscosity-controlled reactions. At still higher temperatures those derivatives not substituted in the 1 and 1’ positions undergo irreversible photoreactions in competition with the formation of B. B itself is not affected by irradiation, apart from the low yield thermally-reversible formation of a free radical. Electron spin resonance measurements confirm the absence of any correlation between photochromism and e.s.r. signals. In the derivatives substituted in the 1 and 1’ positions, only B is produced. However, at still higher temperatures, and only in compounds methylated in the 1 and 1’ positions, a second coloured modification C is formed in competition with B. The ratio [C]/[B] is strongly dependent on the nature of the solvent, the temperature, and the wavelength of the photoactive light. The latter effect is due to the reverse photoreaction C ft A, which is characteristic for the C form. This property of C was used in order to determine its absolute absorption spectrum in various solvents. The quantum yields of photocoloration are around 0.6 and decline to practically zero at very high viscosities, while the quantum yield of photoerasure of C is about 0.05 but goes down with the terne tem-peratur* only to a limited extent. Low-temperature n.m.r. and i.r. measurements of B and C were carried out. The tempting proposal that B has the cyclic structure analogous to 4a cannot be upheld in view of the thermal and photochemical stability of B towards oxygen. It may hold for the C isomer, and could explain the n.m.r. results obtained for C. The cyclic structure should be an intermediate in the photo-oxidation of DA to helianthrone, but has as yet to be detected. The structure of B thus remains open for discussion. The fluorescence of substituted dianthrones is blue-shifted on cooling, again in what seems to be a pure viscosity effect, probably connected with steric relaxation phenomena in the system excited solute—solvent. The quantum yields of fluorescence pass through a maximum again by a mechanism connected with the above relaxation.