Photochemistry of “Super”-Photoacids. Solvent Effects

Abstract

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 protontransfer 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 with the solvent hydrogen-bond donation ability, implicating the involvement of hydrogen-bonded pathways.

Charge Transfer in Photoacids Observed by Stark Spectroscopy

“…7 Photoacids have been studied in both gas and condensed phases, with benzene and naphthalene derivatives examined most often in the gas phase, [8][9][10][11] and naphthalene and pyrene derivatives studied more extensively in condensed phases. [12][13][14][15][16][17] However, there are many other types of aromatic molecules that also display an excited-state shift in pK a , including hydroxystilbenes, 18 triarylamines, 19 and the chromophore of Green Fluorescent Protein (GFP). 20, 21 The work presented below will focus specifically on the pyrene derivatives shown in Figure 1.…”

Investigation of contrasting hydrogen bonding pattern of 3-(phenylamino)-cyclohexen-1-one with solvents in the ground and excited states

“…The s, a and b are the coefficient of the corresponding solvent parameters. The Kamlet-Taft analysis has been reported to be useful in predicting the contribution of different solvent parameters [39][40][41][42][43][44]. The choice of cyclohexane as a reference for this investigation can be justified as it is a unique medium that has p ⁄ , a and b values of zero.…”

Design and Implementation of “Super” Photoacids

“…A Kamlet-Taft approach [31] allows separation of general (polar) and specific (H-bonding) solvation responsible for ground-and excited-state stabilization of super photoacids that have large excited-state dipole moments in the excited state. Using this method, we [8,32] and others [33] have determined several types of hydrogen bonds of super photoacids with solvents (HS) and estimated their relative strengths. In amphoteric solvents, such as water and alcohols, two types of hydrogen bonds exist for neutral super photoacids (Fig.…”