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.

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.

Positional effects of the hydroxy substituent on the photochemical and photophysical behavior of 3- and 4-hydroxystilbene.

The photophysical and photochemical behavior of meta- and para-hydroxy- and methoxystilbene are reported and compared to the aminostilbenes. Absorption spectra of all four studied compounds in organic solution display a featureless, intense long-wavelength absorption band around 300 nm. In basic aqueous solution, meta-hydroxystilbene displays a less intense band with a long-wavelength shoulder, a consequence of configuration interaction from the O(-) substituent. Emission from the meta-substituted stilbenes is much stronger than from the para isomers, with an especially large effect for the hydroxystilbenes in solvents that are good hydrogen bond acceptors. Emission from the hydroxystilbenes is weak in aqueous solution, and even weaker in basic solution, a consequence of facile nonradiative decay. The excited state lifetimes of the meta isomers are longer than those with para substituents. Ground and excited state acidity constants are reported for the hydroxystilbenes. In the ground state, the para derivative is an order of magnitude more acidic than the meta isomer, however, its short excited state lifetime precludes excited state deprotonation. In contrast, 3-hydroxystilbene does exhibit deprotonation in its excited state.

Hydroxystilbene isomer-specific photoisomerization versus proton transfer.

The ground- and excited-state acidities of 3- and 4-hydroxystilbene have been investigated in aqueous solution. Approximate pKa* values determined from the Förster equation are similar for the two isomers; however, 3-hydroxystilbene behaves as a photoacid in water, whereas 4-hydroxystilbene does not. Singular value decomposition with self-modeling is used to obtain more accurate pKa* values for 3-hydroxystilbene. The different behavior of the isomeric hydroxystilbenes is attributed to differences in their excited-state C=C torsional barriers. 3-Hydroxystilbene has a high barrier and a long singlet lifetime, permitting proton transfer to compete with other singlet decay pathways, whereas 4-hydroxystilbene has a low barrier, and thus proton transfer cannot compete with C=C torsion.