Photochromic dibenzobarrlenes: long-lived triplet biradical intermediates.

Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 +/- 2 s (Ea for decoloration is 14.5 +/- 0.7 kcal mol-1 and log A is 8.92 +/- 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 +/- 0.2 x 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular delta-H abstraction, to yield the triplet biradical intermediate (3)2. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The delta-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical (3)2, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical (3)2 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical (3)2 in the photochromism of appropriately substituted dibenzobarrelenes.

Solvent dependent photocyclization and photophysics of some 2-ethynylbiphenyls.

The spectroscopic properties and photochemical behavior of molecules having 2-ethynylbiphenyl or 2-phenyldiphenylacetylene structures are reported. These molecules undergo photocyclization reactions to yield phenanthrene and dihydrophenanthrene products via putative isophenanthrene (cyclic allene) intermediates. Phenanthrene formation from the isophenanthrene intermediates does not occur via a unimolecular sigmatropic hydrogen shift, but rather by protonation or hydrogen abstraction mechanisms involving the solvent. Cyclization efficiencies are much lower than is the case for previously-investigated 2-vinylbiphenyl systems. The 2-phenyldiphenylacetylenes have high fluorescence quantum yields and long singlet lifetimes when compared to diphenylacetylene. The 2-ethynylbiphenyls decay via a combination of fluorescence and intersystem crossing.

Ring-closing photoisomerization of some 2,6-diarylstyrenes.

The structure, spectroscopy and photochemical reactions of three symmetrical 2,6-diarylstyrenes (aryl = phenyl, 2-furyl and 2-thiophenyl) have been investigated. The ground-state structures are highly nonplanar, having large aryl-phenyl and vinyl-phenyl dihedral angles. All three diarylstyrenes have broad UV absorption bands attributed to allowed, delocalized pi,pi* (highest occupied molecular orbit-lowest unoccupied molecular orbit) transitions. The 2-furylstyrene is weakly fluorescent, with a large Stokes shift attributed to a change in geometry from the nonplanar ground state to a more planar singlet state. Irradiation in fluid solution results in efficient conversion of the diarylstyrenes to cyclized 9,10-dihydrophenanthrene and 4,5-dihydronaphthofuran or thiophene products, thus extending the scope of the 2-vinylbiphenyl photocyclization reaction to heterocyclic analogs. Irradiation at low temperatures in glassy media permits observation of the UV absorption spectra of the unstable primary photoproducts. Upon warming of the glass, these intermediates undergo rapid hydrogen migration to form the stable dihydroarene products.

Competitive 1,2- and 1,5-hydrogen shifts following 2-vinylbiphenyl photocyclization.

[reaction: see text] The photocyclization of 2-vinylbiphenyl and its derivatives has been proposed to occur via a two-step mechanism: photocyclization to form an unstable 8a,9-dihydro-phenanthrene intermediate, followed by exothermic unimolecular isomerization to a 9,10-dihydrophenanthrene. The mechanism of the hydrogen shift process has been investigated using deuterated derivatives of 2-isopropenylbiphenyl and 2,6-diphenylstyrene. 1H NMR analysis of the photoproducts indicates that the thermally allowed 1,5-hydrogen or deuterium shift is a minor product-forming pathway and that an unusual double 1,2-hydrogen or deuterium shift is the major product-forming pathway. The potential energy surface for photocyclization and hydrogen shift processes has been explored computationally. The calculated barrier for the 1,5-shift is predicted to be significantly lower than that for the 1,2-shift. Alternative mechanisms for the occurrence of 1,2-hydrogen or deuterium migration are presented.

Photocyclization of a conformationally constrained 2-vinylbiphenyl.

The synthesis and photochemical behavior of a rigid analog of syn-2-vinylbiphenyl are reported. This analog undergoes quantitative conversion to a tetrahydrobenzanthracene derivative upon irradiation in fluid solution. Product formation is proposed to occur via photocyclization to yield an unstable intermediate followed by an intramolecular thermal hydrogen shift. The intermediate can be observed upon irradiation at low temperature either in a rigid glass or in liquid propane solution. The temperature dependence of its decay is indicative of a tunneling mechanism for the hydrogen shift process.

Photocyclization of 2-vinyldiphenylacetylenes and behavior of the isonaphthalene intermediates.

The conformation, electronic structure, spectroscopy, and unimolecular photoisomerization of 2-vinyldiphenylacetylene and two derivatives have been investigated. 2-Vinyldiphenylacetylene exists predominantly in a planar anti conformation. Introduction of an alpha-methyl substituent results in increased phenyl-vinyl dihedral angles for both syn and anti conformers, whereas a cyclic analog is constrained to a syn conformation with a large phenyl-vinyl dihedral angle. All three molecules undergo photocyclization to yield unstable cyclic allene (isonaphthalene) intermediates which undergo further reactions leading to stable products. Both the photocyclization process and behavior of the allene intermediate are dependent upon ground state conformation. The photophysical behavior of the 2-vinyl derivative, namely its short singlet lifetime and low fluorescence quantum yield, is similar to that of diphenylacetylene. It also has a low quantum yield for photocyclization. The 2-isopropenyl derivative and conformationally locked cyclic analog have relatively long singlet lifetimes and large quantum yields for fluorescence and cyclization. The difference in excited state behavior of the planar 2-vinylacetylene and its non-planar analogs is attributed to the effect of the phenyl-vinyl dihedral angle on the barriers for activated decay of the linear singlet state. However, the behavior of the 2-isopropenyl derivative does not appear to be dependent upon ground state conformation (synvs.anti). The cyclic allene intermediates undergo sequential protonation-deprotonation in methanol solution to yield stable products. The 2-vinyl derivative yields only the fully aromatized 2-phenylnaphthalene. However, the 2-isopropenyl and cyclic derivatives yield mixtures of fully and partially aromatized products. Preferential formation of the partially aromatized products is attributed to a stereoelectronic effect on the deprotonation step. In diethyl ether solution only the fully aromatized product is formed via a free radical mechanism.

Photoisomerisation of dibenzobarrelenes--a facile route to polycyclic synthons.

Triplet state mediated di-pi-methane rearrangements of dibenzobarrelenes give a variety of interesting synthons, formed as primary and secondary photoproducts. These synthons could find use for the synthesis of complex synthetic targets. This tutorial review highlights the photoisomerisation of some bridgehead substituted dibenzobarrelenes and the products derived from them. Selected examples of photoisomerisations proceeding through a tri-pi-methane pathway are also included.

Ultrafast proton transfer dynamics of hydroxystilbene photoacids.

The effects of 4-cyano and 3-cyano substituents on the spectroscopic properties and photoacidity of 3- and 4-hydroxystilbene have been investigated. In nonpolar solvents, the 3-hydroxycyanostilbenes have much longer singlet lifetimes and larger fluorescence quantum yields than do the 4-hydroxycyanostilbenes. The longer lifetimes of 3-hydroxystilbene and its cyano derivatives are attributed to a "meta effect" on the stilbene torsional barrier, similar to that previously observed for the aminostilbenes. The cyano substituent causes a marked increase in both ground state and excited-state acidity of the hydroxystilbenes in aqueous solution. The dynamics of excited-state proton transfer in methanol-water solution have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. Assignment of the transient absorption spectra is facilitated by comparison to the spectra of the corresponding potassium salts of the conjugate bases and the methyl ethers, which do not undergo excited-state proton transfer. The 4-cyanohydroxystilbenes undergo excited-state proton transfer with rate constants of 5 x 10(11) s(-1). These rate constants are comparable to the fastest that have been reported to date for a hydroxyaromatic photoacid and approach the theoretical limit for water-mediated proton transfer. The isotope effect for proton transfer in deuterated methanol-water is 1.3 +/- 0.2, similar to the isotope effect for the dielectric response of water. The barrier for excited state double bond torsion of the conjugate bases is small for 4-cyano-4-hydroxystilbene but large for 4-cyano-3-hydroxystilbene. Thus the "meta effect" is observed for the singlet states of both the neutral and conjugate base.

Viscosity-dependent diastereoselectivity and product selectivity in the photodenitrogenation of a spirocyclopropane-substituted, DBH-type azoalkane.

In the photodenitrogenation of the spirocyclopropane-substituted azoalkane 1 in alcoholic solvents of various viscosity (from 0.6 to 89.2 cP) to the diastereomeric housanes 2 (major product) and bicyclo[3.2.0]heptenes 3 (minor product), the same (within experimental error) viscosity dependence is observed in the diastereoselectivity of the housane 2 formation as well as in the product selectivity (2/3). These viscosity effects corroborate the intermediacy of the diazenyl diradical 1DZ as a common branching point for the housane 2 and bicycloheptene 3 formation. In contrast, the diastereoselectivity of the bicycloheptene 3 generation is independent of viscosity, which implies that a nitrogen-free, symmetrical 1,4 diradical serves as precursor to the rearrangement products. The free-volume model of viscosity is employed to rationalize the product selectivity as well as the diastereoselectivity.