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.

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.

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.