K.A. Muszkat

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.

Electronic overlap population as reactivity measure. The photodimerization of acenaphthylene

Abstract An improved representation of the photodimerization reactivity of acenaphthylene is provided by an electronic overlap population analysis of excited molecule-pairs.

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.

Effects of electrostatic interactions on the photocyclization reactivity of diaza-1,2-diarylethylenes

Abstract Electrostatic repulsion of nonbonded nitrogen atoms is recognized as the responsible factor for the diminished photocyclization reactivity of those initial conformations of diaza-1,2-diarylethylenes having short NN distances.

Quasiline electronic spectrum as evidence forplanarity in single-bond conformers of 2-naphthylethylenes

Abstract Quasiline fluorescence is obtained at 4 K from almost planar s -conformers of trans-1,2-diaryl ethylenes I and II embedded in n -hexane polycrystalline matrices, while the other non-planar s -conformers show the usual band-type emission.

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.

Structure, spectra, photochemistry, and thermal reactions of the 4a,4b-dihydrophenanthrenes

Ultraviolet irradiation of solutions of stilbene and its derivatives gives mixtures of their respective cis and trans isomers, together with, in many cases, 4a,4b-dihydrophenanthrene, or its corresponding derivatives. The extent of the photocyclisation depends strongly on the wavelength of the light used for irradiation. In the photostationary state in stilbene, the maximum photocyclisation (21%) is attained by irradiation at 280 mµ. In diphenylcyclopentene, 68% of the starting compound undergoes cyclisation into cyclopentenodihydrophenanthrene. The absorption spectra of the cyclic products are all similar with three main bands, at approximately 460, 310, and 240 mµ. The number, position, and intensity of these bands were correctly predicted by Simpsons exiton method. The n.m.r. spectrum and the molecular weight of hexamethyldihydrophenanthrene, formed from hexamethylstilbene, give unambiguous proof for the structure suggested for 4a,4b-dihydrophenanthrene and its derivatives. The photocyclisation is a completely reversible reaction, since the parent cis-stilbene, or its respective derivative, are reformed either thermally or by irradiation with light in the visible region. The photochemical ring-formation and ring-fission, as well as the thermal ring-fission, were investigated in detail, in particular in the system diphenylcyclopentene ⇌ cyclopentenodihydrophenanthrene, which is not complicated by cis–trans interconversion. The quantum yields of photochemical ring-fission were found to be independent of the temperature, and retained their high value (about 0·6) even in a hydrocarbon glass at –180°. The yield of photocyclisation [0·4 for (diphenylcyclopentene → cyclopentenodihydrophenanthrene) at –20°] was found to decrease sharply on cooling, with a concurrent increase in the quantum yield of the fluorescence of diphenylcyclopentene. The results can be explained satisfactorily by assuming that the only temperature-dependent reaction of the first excited singlet state of diphenylcyclopentene is the photocyclisation, with an activation energy of about 3 kcal./mole. No photosensitisation was observed for either ring-formation, or ring-fission. The kinetic parameters of the first-order thermal ring-opening reaction of 4a,4b-dihydrophenanthrene, cyclopentenodihydrophenanthrene, and hexamethyldihydrophenanthrene were determined. 4a,4b-Dihydrophenanthrene and cyclopentenodihydrophenanthrene are oxidised by oxygen in the dark at rates which increase with increases in the concentration of oxygen and the compound being oxidised. The products are hydrogen peroxide and phenanthrene, or its analogue. Oxidation is inhibited by aniline and by 2,6-di-t-butylcresol. It is suggested that the radical formed by hydrogen abstraction from dihydrophenanthrene, and the HOO· radical, are intermediates in this oxidation. At very low oxygen concentrations a photo-oxidation of either cyclopentenodihydrophenanthrene or, directly of diphenylcyclopentene, to give the corresponding phenanthrene derivative, was also observed. The system diphenylcyclopentene ⇌ cyclopentenodihydrophenanthrene represents the simplest photochromic system described hitherto.

Emissions of sterically hindered stilbene derivatives and related compounds. Part IV. Large conformational differences between ground and excited states of sterically hindered stilbenes: implications regarding stokes shifts and viscosity or temperature dependence of fluorescence yields

Large effects of molecular overcrowding on fluorescence spectra (Stokes shifts) and quantum yields were observed in a series of stibene derivatives. The measured Stokes shifts depend strongly on the extent of molecular over-crowding. The fluorescence quantum yields increase sharply with increasing viscosity and decreasing temperature. As the extent of overcrowding increases, the temperature range over which fluorescence is observed is shifted downwards, and the importance of viscosity increases. This reflects a corresponding complementary variation of the radiationless processes competing with fluorescence. CFF–SCF–PI calculations of the ground and first excited singlet state geometries of the trans- and cis-isomers of stilbene and of αα′-dimethylstilbene chosen as model molecules indicate that excitation results in large changes in the ring–α and α–α′ torsion angles of the relaxed sterically hindered molecules, and therfore in substantial relaxation energies. Good agreement is obtained between observed and calculated Stokes shifts. The viscosity dependence of fluorescence quantum yields is correlated with the calculated Franck–Condon factors for the radiationless transitions competing with fluorescence.

The thermodynamic equilibrium between cis- and trans-isomers in stilbene and some derivatives

The thermodynamic cis⇌trans equilibrium in stilbene and its derivatives can be established at room temperature only by the use of catalysts such as atomic iodine. The latter has now been used to determine the above equilibrium in a number of cases. The fraction of cis-isomer at equilibrium is ca. 0·002 in stilbene and the para-substituted derivatives investigated, but increases to 0·07 in αβ-difluorostilbene and 2,4,6-trimethylstilbene, and to 0·20 in α-methylstilbene. These values were used to calculate the respective differences in free energy. Catalysed isomerization, in particular if the catalyst is a product of irradiation, may, under certain conditions, occur concurrently with direct or sensitized photoisomerization, and distort the results of the latter reactions. It is therefore important to distinguish between true photoisomerization, involving the excited states of the molecules undergoing isomerization, and photocatalysed isomerization, involving only the ground states of these molecules.

Reversible hydrogen photoabstraction by nitroaromatic compounds. A chemically induced nuclear polarization study

Proton nuclear polarization is observed in the photoreaction of aromatic vitro-compounds with 2,6-di-t-butyl-phenols. The polarization is due to a reversible abstraction of the phenol hydroxy-hydrogen by the excited triplet nitro-compound. The occurrence of polarization depends on the triplet energy of the vitro-compound and on the O–H bond dissociation energy of the phenol. Observation of polarization forms the basis for the estimation of the O–H bond dissociation energy of ArNO2H· radicals.