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Absorption, emission, and NMR studies indicate that: (1) Formaldehyde exists as a monomer (form A) in nonpolar solvents to a temperature of approximately −95°C, (2) at −95°C in nonpolar solvents, formation of a formaldehyde polymer (form C) begins which is thermally reversible at temperatures > −90°C. The form C also exists at room temperature in water, dimethyl sulfoxide, dimethylformamide, and tetrahydrofuran. The formaldehyde-formaldehyde interaction is principally of a dipole-dipole type. (3) Formaldehyde can act as a hydrogen-bonding donor or acceptor in a complex with solvent molecules or silica gel (form B). In aromatic hydrocarbons, formaldehyde acts as a donor whereas in 2-methyltetrahydrofuran, chloroform, and silica gel it acts as an acceptor. Also, in 2-methyltetrahydrofuran, forms A and B exist in addition to C. Monomer formaldehyde exhibits a fluorescence from the lowest Sn,π* state in nonpolar solvents to −95°C. Both the forms B and C exhibit emission. In addition to the classification of the forms of formaldehyde, it is possible to classify the solvents into four main groups based on the rate of formation of paraformaldehyde or the fact that formaldehyde essentially totally reacts (strong hydrogen-bond donors or molecules with active α-hydrogen atoms).

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...

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