Ultrafast Excited-State Proton Transfer from Cyano-Substituted 2-Naphthols

Huppert, Dan; and, Laren M. Tolbert; Linares-Samaniego, Sandra

doi:10.1021/jp970506c

Cited by 79 publications

(112 citation statements)

References 22 publications

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“…Thus a systematic investigation of solvent effects on these reactions becomes feasible. The present work extends our previous reports concerning the steady-state 15 and time-resolved 14 fluorescence of 5CN2OH in various solvents. The extremely high photoacidity of cyanonaphthols results in large solvatochromic shifts, as revealed by their steady-state fluorescence spectra.…”

Section: Introductionsupporting

confidence: 90%

“…Previous studies suggest that, with an increase of the acidity in the series of monocyanonaphthol isomers, k d increases and its isotope effect decreases. 14,19 This is in line with structure-reactivity correlations applied to protontransfer reactions. [30][31][32][33][34] The upper limit for k d may reflect the required solvent organization time.…”

Section: Introductionsupporting

confidence: 75%

“…14,18,19 It may be depicted schematically by the following scheme In this mechanism, the initial state is that of a vibrationally relaxed, electronically excited ROH molecule (denoted by R*OH). It may dissociate to produce a "contact" geminate pair with a rate coefficient k d .…”

Section: Introductionmentioning

confidence: 99%

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Photochemistry of “Super”-Photoacids. Solvent Effects

1999

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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.

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Section: Introductionsupporting

confidence: 90%

Section: Introductionsupporting

confidence: 75%

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Photochemistry of “Super”-Photoacids. Solvent Effects

1999

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“…26 But 7-HQ(C*) should rather be compared to photoacids bearing an electron-withdrawing substituent such as the cyano-2-naphthols synthesized by Tolbert; 27 (k 1 ) 0 for 7-HQ(C*) is also larger than the deprotonation rate constants of the 6-, 7-, and 8-cyano-2-naphthols in water and close to that of 5-cyano-2-naphthols (7 × 10 10 s -1 , the error given by the authors in the determination of this value being ∼20%). 28 The remarkable tendency of 7-HQ(C*) to undergo photoinduced proton dissociation is thus brought to the fore. Let us now compare the 6-and 7-hydroxyquinolinium ions.…”

Section: Discussionmentioning

confidence: 99%

Photoinduced Coupled Proton and Electron Transfers. 2. 7-Hydroxyquinolinium Ion

et al. 1999

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The hydroxyl group of 7-hydroxyquinoline in acidic media (i.e., of 7-hydroxyquinolinium cation, 7-HQ(C)) undergoes photoinduced deprotonation even in strongly acidic media. It occurs from dilute perchloric acid solutions up to 8 mol L -1 concentration, which is evidence of an outstanding photoacidity. Excited-state proton ejection was shown to be reversible at acid concentrations greater than 1 mol L -1 . Below this value, the proton ejection was not balanced by the proton recombination. Concerning the rates of the excited-state reactions, (i) the proton ejection rate constants k 1 increased upon acid dilution, attaining at high dilution the limiting value of (k 1 ) 0 ) 5.5 × 10 10 s -1 . k 1 was shown to vary with the water activity according to k 1 ) (k 1 ) 0 (a H 2 O ) 4 . The number 4 is then a key number for the ability of water to accept a proton from 7-HQ(C*). (ii) The proton recombination rate constant varies from 3 × 10 9 to 4 × 10 10 s -1 when the acid concentration ranges from 1 to 5 mol L -1 . This observation can tentatively be ascribed to a nondiffusional recombination process occurring with the ejected proton in concentrated acid solutions. The results as a whole are consistent with the coupling of an intramolecular electron transfer with the proton transfer, due to the withdrawing effect of the quinolinium function gNH + on the -O -group generated by the deprotonation. The photoproduct is then most likely to be the ketonic tautomeric form of 7-hydroxyquinoline rather than a zwitterion. Thus, the excited-state behavior is fully accounted for by the photoinduced synergy of the two functional groups.

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“…Proton transfer dynamics of photoacids to the solvent have thus, being reversible in nature, been modelled using the Debye-von Smoluchowski equation for diffusion-assisted reaction dynamics in a large body of experimental work on HPTS [84][85][86][87] and naphthols [88][89][90][91][92], with additional studies on the temperature dependence [93][94][95][96][97][98], and the pressure dependence [99][100][101], as well as the effects of special media such as reverse micelles [102] or chiral environments [103]. Moreover, results modelled with the Debye-von Smoluchowski approach have also been reported for proton acceptors triggered by optical excitation (photobases) [104,105], and for molecular compounds with both photoacid and photobase functionalities, such as 10-hydroxycamptothecin [106] and coumarin 4 [107].…”

Section: Proton Recombination and Acid-base Neutralizationmentioning

confidence: 99%