Dianthrone photochromism 1950-1970

Abstract

Photachromie 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 ofboth stationary and flash methods, in a wide range oftemperatures 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' positionsundergo 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 Bis produced. However, at still higher temperatures, and only in compounds methylated in the 1 and 1' positions, a second coloured modification Cis 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 ~ 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 temperature 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 analogaus to 4a,4b-dihydrophenanthrene 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 tobe 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, pro bably connected with steric relaxat In memoriam Yehuda Hirshberg, 1903Hirshberg, -1960 t This is Part VI in the series 'Photochromism in Dianthrone and Related Compounds '. Part of the material described here has been published in two preliminary not...

Energy Profiles for Ring Formation‐ and Ring Opening‐Processes in the cis‐Stilbene‐4a,4b‐Dihydrophenanthrene System. An example of the feasibility of a process forbidden by the rules of orbital symmetry conservation

“…Similar thermal behaviour is observed for the C cyclic form (XI, R = CH,) of 1, l', 3,3'-tetramethyL9,9'-bianthro- . Similar activation energies were measured for XI, R = H, and for its 3,3'-dimethyl derivative XI, R = CH, [9]. The reason for the apparent discrepancy between the experimental finding and the forbidden character for this process according to the rules of orbital symmetry conservation will be discussed subsequently.…”

Polymorphism Versus Thermochromism: Interrelation of Color and Conformation in Overcrowded Bistricyclic Aromatic Enes

“…[26] The thermochromic, photochromic, and piezochromic B forms are identical. [8,[28][29][30][31] The reversible formation of color raises the question of the nature of the colored species. [2,27,32] In fact, the enigma of thermochromism in BAEs has played an important role in the development of the theory of organic chromophores.…”

“…[2,31,56] The photocyclization of BAEs is analogous to that of (Z)-stilbene to give 4a,4b-dihydrophenanthrene. [8,57] Substitution with bulky substituents at the 1-and 1'-positions prevents thermochromism. [2,35,53,58,59] Because substituents in the fjord region would cause prohibitive strain in planar conformations, this has been interpreted as indicating a planar thermochromic B form.…”

“…The structures of the heteromerous fluorenylidene-xanthenes may be compared to those of the corresponding homomerous BAEs bifluorenylidene (7) and dixanthylene (8). The bright red bifluorenylidene has a twisted conformation with w = 32.08 and C9=C9' = 1.36 (b modification), [65] which is 88 less twisted and 0.036 shorter than the twisted fluorenylidene-xanthenes.…”

Thermally Reversible Photoisomerizations

“…They have been suggested as systems for temporary storage of information ("memory models") [28]. as systems for the temporary storage of energy [29], as subjects for practice in handling of transients [30], and for use in reproduction processes [31], see [32] and further literature cited there.…”