128. Low-temperature photochromism and its relation to thermochromism

Abstract

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...

Photosensitive Cholesteric Copolymers with Spiropyran-Containing Side Groups: Novel Materials for Optical Data Recording

“…[2±4] Carbon nanotubes, which are macromolecular carbon structures, could fill this gap because of their nanosize dimensions, fibrous and inherently porous structure, and chemical stability. [5,6] These structures are rolled up graphene sheets (cylinders) collected in the form of ropes (singlewalled nanotube ropes) or concentric Russian-dolls (multiwalled nanotubes). Nanotubes are mechanically and chemically robust and have unique electronic and transport properties depending on the way the carbon atoms are arranged in their cylindrical hexagonal honeycomb lattices.…”

“…Carbon nanotubes were prepared by the well-established electric-arc method. [5] Macroscopically discernable whiskers present in the arc deposit (which consist of aligned microbundles) were separated and these nanotube whiskers, of typical dimensions 0.15 cm long and 0.028 cm diameter, were used as electrodes by attaching them to the tips of copper wires with conductive paint. The copper wire was then reinforced in a plastic rod for rigidity.…”

Dianthrone photochromism 1950-1970

“…results for the C isomer could fit structure IV, but for B the results are at least ambiguous. (7) The photochromism of dixanthylene 31 , II, which has recently received renewed attention 37 • 38 , is at least superficially similar tothat ofDA, although it takes place with much lower quantum yields 38 . This would indicate that the carbonyl group is not directly involved in the formation of B.…”

Polarity‐Dependent Isomerization of an Unsymmetrical Overcrowded Ethylene Promoted by Zwitterionic Contribution in the Twisted Isomer

“…[2][3][4][5][6][7][8][9][10] In responset oe xternal stimuli, such as photoirradiation, thermals timulation and mechanicalg rinding, 1 shows an isomerization between two isomers:t he yellow-colored folded isomer and the green-colored twisted isomer.This structural change reflects the nature of the overcrowdede thylene. In this study,a nu nsymmetrical bianthrone (2), consisting of the electron-withdrawinga nthrone and electron-donating acridane, have been synthesized and shown to exhibit as olvent-polarity-dependent isomerization reaction between the folded and twisted isomers.…”