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Photochemistry of "Super"-Photoacids. Solvent Effects Kyril M. Solntsev and Dan Huppert

Summary: Photochemistry of "Super"-Photoacids. Solvent Effects
Kyril M. Solntsev and Dan Huppert
School of Chemistry, Raymond and BeVerly Sackler Faculty of Exact Sciences, Tel-AViV UniVersity,
Tel AViV 69978, Israel
Noam Agmon*
The Fritz Haber Research Center, Department of Physical Chemistry, The Hebrew UniVersity,
Jerusalem 91904, Israel
ReceiVed: January 19, 1999; In Final Form: May 11, 1999
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 occurs 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 solvents accounts for most of the shift in the excitation
spectra. This bond produces considerably larger shifts in the emission spectra, suggesting that it strengthens
in the excited state. In contrast, the hydrogen bond donated from protic solvents to the hydroxyl oxygen is
cleaved following photoexcitation. This bond (and 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 fluorescence data fit the solution of the Debye-Smoluchowski equation
for reversible geminate recombination in a field of force, provided that the difference in excited-state lifetimes
and contact quenching are taken into account. An extended theory of reversible geminate recombination
provides an accurate description of the asymptotic behavior in this case. The quenching processes correlate


Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem


Collections: Chemistry