R. Korenstein

Reversible photoisomerization and fluorescence of bithioxanthene: Temperature and spin-orbit perturbation effects

Abstract A detailed experimental study of the reversible photochemistry of bithioxanthene over a wide temperature range indicates the photoformation of two unstable isomers: E, absorbing at shorter wavelength, and P, absorbing at longer wavelengths. Both isomers revert thermally to the parent compound, A. E is formed through the triplet manifold while P is formed directly from the excited singlet state. The quantum yield for the A → E process is temperature dependent and is enhanced by molecular oxygen and by xenon. A low temperature proton n.m.r. analysis of the E and A forms indicates a structure for E which involves cis-trans isomerization about the 9-9′ central double bond and additional torsion about the four single bonds attached to it.

The reversible photochemistry of dixanthylidenes: Temperature, external spin-orbit perturbation effects and structure of the C and F photoisomers

Abstract An extensive photochemical study of dixanthylidenes shows the existence of three unstable isomers: B, F and C. The C isomer was shown by low temperature proton n.m.r. to be a photocyclization product at the 1 and 1′ positions. The structure of the F isomer was shown to involve both rotation of 180° about the central double bond and additional torsion about the four single bonds attached to it. The quantum yields for the A → B process are enhanced by external spin-orbit perturbers. The B isomer was shown to be formed from the triplet state. The formation of C, B and F are temperature controlled. Fluorescence quantum yields in these systems are strongly temperature dependent.

Large enhancement of photoisomerization quantum yields by external perturbing agents

Abstract Photoisomerization rates of dixanthylidenes and 10,10′-dimethyl biacridanes are enhanced up to thousand-fold by the external perturbers oxygen, alkyl halides, and also carbon disulphide, which is much more effective than ethyl iodide. Fluorescence quantum yields are lowered by these perturbers, and it is suggested that all of them act by enhancing the rates of intersystem crossing to the triplet state of the reactants, which then undergoes efficient isomerization.

Reversible photochemistry and photocyclization of 10H,10′H-bianthrylidene

The photochemistry of 10,H,10′H-bianthrylidene was extensively studied over a wide temperature range. Two photoisomers, C and E, are formed, both reverting thermally to the parent modification A. The C isomer is a cyclization product of the 4a,4b-dihydrophenanthrene type, whereas the E isomer results from cis–trans-isomerization about the central double bond together with additional torsions about the four single bonds of the 9,9′-ethylene system. The A, E, and C modifications are oxidized by atomic iodine. The dependence of the quantum yield of fluorescence on temperature indicates that intersystem crossing is an activated process above a certain critical temperature (ca.–130°) and non-activated at lower temperatures. The C modification is formed from the excited singlet and the E isomer from the triplet state.

Reversible photochemistry of 10,10′-dimethylbiacridan: internal and external heavy atom effects, and the structure of photoisomer F

An extensive study of the photochemistry of 10,10′-dimethyl-(3a), 1,1′,10,10′-tetramethyl-(3b), and of 2,2′,7,7′-tetrabromo-10,10′-dimethyl-biacridan (3c) revealed the formation of two light-stable photoisomers E and F, from the fundamental modification A. The photoisomerization quantum yields ϕA→E and ϕA→F are enhanced by oxygen, xenon, alkyl halides and also by carbon disulphide. Flash photolysis studies of both direct and sensitized photoisomerization A → E suggest that E is formed through the triplet manifold. The formation of the E isomer involves rotation by 180° about the 9,9′-double bond as well as additional rotation of the benzene rings about the single bonds attached to the central double bond. 1H N.m.r. spectra of photoisomer F measured at low temperatures, and geometry calculations by the CFF-π electron-Cl method suggest that compared with A the structure of F involves torsion about both the central double bond and about the single bonds attached to it.

Reversible photochemistry, photo-oxidation, and fluorescence of dixanthylidene: temperature and external spin-orbit perturbation effects

A detailed experimental study of the photochemistry of dixanthylidene over a wide temperature range indicates the existence of three labile photoisomers, B, C, and P, all of which revert thermally to A. Light-stable B, formed from the excited triplet of the fundamental modification A, is thermally stable below –140°. The structure of B involves torsion of ca. 50° about the central double bond. C And its possible precursor P are photolabile isomers formed from excited singlet A. They are photocyclisation products of the 4a,4b-dihydrophenanthrene type; C is photoconvertible into A. The thermal stability of P is much less than that of C. Helixanthen is obtained on thermal dehydrogenation of C with molecular oxygen or iodine. The quantum yield of A [graphic omitted] B drops with the temperature and is very smail at –140° but can be strongly enhanced (up to 220-fold) by the spin-orbit coupling perturbers molecular oxygen, carbon disulphide, and ethyl iodide.