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When solutions of dianthrone, dixanthylen, xanthylideneanthrone, and their derivatives are irradiated a t temperatures between -70" and -150" with light of wave-length not exceeding 450 mp, reversible formation of coloured modifications takes place (photochromism) . Comparison of the visible and near ultra-violet absorption spectra of these coloured modifications with the spectra of the coloured forms obtained reversibly on heating solutions of some of the above compounds (thermochromism) shows that the colours produced by both processes are identical, and that lowtemperature irradiation may cause considerable or even complete conversion into the coloured form.Photochromism is also observed with non-thermochromic derivatives of the above compounds.From the temperature dependence of the rates of the spontaneous disappearance of colour, the critical increments and frequency factors of the thermal reversion to the colourless modification were estimated.The dielectric constant of solutions of xanthylideneanthrone in toluene at -78" increased considerably as a result of irradiation.The relation between photochromism and thermochromism is discussed, and tentative conclusions are drawn with regard to the mechanism of photochromism and the electronic and atomic configuration of the coloured and colourless modifications.THE reversible formation of colour on heating of solutions of dianthrone (dianthronylidene) (I), dixanthylen (dixanthylidene) (11), xanthylideneanthrone (111), and their derivatives is well-known (for refs. see E. D. Bergmann, " Isomerism and Isomerisation," Interscience Publishers Inc., New York, 1948 ; Grubb and Kistiakowsky, J . Amer. Chem. Soc., 1950, 72, 419). Theilacker, Kortum, and Friedheim (Ber., 1950, 83, 508) and Grubb and Kistiakowsky (Zoc. cit.) measured the molar extinction coefficient ( E ) of (I) and (11) at several temperatures, and concluded that thermochromism involves transition from a singlet to a diradical triplet state, the reverse transition requiring activation energies below 20 kcal./mole. Theilacker et d. also postulated a thermal equilibrium between two interconvertible modifications, and from a plot of log E-.(i.e., the value of E a t the peak of the absorption curve in the visible range) against the reciprocal of the absolute temperature * This extrapolation does not seem to be justified thermodynamically, but appears to give the correct order of magnitude for bX. of the pure coloured form.t The results there reported for dianthrone were obtained a t temperatures which were not sufficiently low, and should be revised in accordance with the present communication. * It was shown by Hirshberg at al . (Bull. Sac. chim., 1951, 18, 88) that anthrone and diphenylmethyleneanthrone and their derivatives mentioned in the preceding paragraph exhibit pronounced phosphorescence, whereas the colourless forms of dianthrone, 4 : 4'-disubstituted dianthrones, an! xanthylideneanthrone are practically non-phosphorescent. described in that communication is probably photochromism produced by stra...

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By M. D. COHEN, (the late) Y. HIRSHEERG, and G. M. J. SCHMIDT. All anils of salicylaldehydes investigated undergo colour changes whenThe In some solvents the change their solutions in rigid glasses are irradiated with ultraviolet light. effect is reversed when the solutions soften. can be reversed photochemically even while rigidity is maintained.Anils of aldehydes lacking an o-hydroxyl group show no photoactivity.PART VI * has discussed experimental work on the photoactivity of solid salicylideneanilines. It was concluded that the photo-behaviour of these compounds is topochemically controlled since photochromy and thermochromy, shown to be mutually exclusive properties in the solid state, may be found in different polymorphic forms of the same anil.It was further shown that the *' coloured " species obtained by the photochemical or thermal mechanisms resemble each other in their spectral properties : however, little information on their molecular structures could be derived from the spectra in solid films. The present and succeeding (Part VIII) Papers deal with the properties of salicylideneand hydroxynaphthylidene-anilines in solution and in rigid organic glasses. (Stobbe's experiments1 on anils incorporated in films of resins had suggested to him that photochromic properties are not maintained in the amorphous state; however, we felt that those experiments had been too crude in their choice of *' solvent " to be significant.) Our experiments were designed to test the photo-properties of the isolated anil molecules in an environment of lesser rigidity than that imposed by the crystalline lattice, and to obtain better spectral data than was possible with crystalline materials. The initial rigid-state experiments were conducted with solutions of salicylidene-mtoluidine in methyl cyclohexane-ligroin at liquid-air temperature. They showed that irradiation causes, in such solutions, a colour-change similar to that occurring in the crystal (yellow to red). Later experiments proved this to be a general effect occurring with most anils in a variety of rigid solvents. The photo-colour is stable as long as rigidity * Part VI, preceding paper.