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.