Photochemistry of "Super"-Photoacids. Solvent effects

We study steady-state and time-resolved fluorescence of 5-cyano-2-naphthol in various pure solvents. To some of these, excited-state proton transfer o ccurs within the excited-state lifetime of the chromophore. Solvatochromic shifts in the acid and anion bands are analyzed using the empirical Kamlet- Taft approach. The hydrogen-bond donated from the OH group to basic solvent s accounts for most of the shift in the excitation spectra. This bond produ ces considerably larger shifts in the emission spectra, suggesting that it: strengthens in the excited state. In contrast, the hydrogen bond donated f rom protic solvents to the hydroxyl oxygen is cleaved following photoexcita tion. This bond land not the change in dielectric constant) is responsible for the solvent-induced blue shift in anion fluorescence. Hence it must re- form simultaneously with the proton-transfer event. Our time-resolved fluor escence data fit the solution of the Debye-Smoluchowski equation for revers ible geminate recombination in a field of force, provided that the differen ce in excited-state lifetimes and contact quenching are taken into account. An extended theory of reversible geminate recombination provides an accura te description of the asymptotic behavior in this case. The quenching proce sses correlate with the solvent hydrogen-bond donation ability, implicating the involvement of hydrogen-bended pathways.