Ultrafast Excited-State Proton-Transfer Reaction of 1-Naphthol-3,6-Disulfonate and Several 5-Substituted 1-Naphthol Derivatives

Abstract

The 1-naphthol molecule has been the subject of intense research activity for the past 60 years due to its complex behavior as a photoacid upon optical excitation. We have utilized femtosecond mid-infrared spectroscopy and time-resolved fluorescence spectroscopy to investigate the excited-state proton-transfer reaction of 1-naphthol-3,6-disulfonate (1N-3,6diS) and several 5-substituted 1-naphthol derivatives. The proton dissociation rate constant of 1N-3,6-diS was found to be about 3 times faster and the pKa* about 2 pKa units more acidic than the values previously reported in the literature. A Marcus (free-energy) plot of excited-state proton dissociation rate constants vs the excited-state equilibrium constant of the photoacids, Ka*, was constructed using the C-5 series of 1-naphthol derivatives. The newly measured values for the ESPT rate constant and pKa* of 1N-3,6diS was found to fit well with the Marcus correlation. We discuss our findings in the context of the photoacidity phenomenon in general, and the photoacidity of 1-naphthol and its derivatives in particular.

Bifunctional Photoacids: Remote Protonation Affecting Chemical Reactivity

“…Similarly, for the 1-naphthol family of the 5-C substituents, we have found a ρ value for the equilibrium constant about twice larger in the excited state than in the ground electronic state of the same set of photoacids. 19 One may quantitatively discuss the effect of remote protonation on the acidity of the main functional group of OH photoacids by considering three of the most abundant protonatable side groups: OH/O − , COOH/COO − , and NH 3 + / NH 2 (Table 3). Each of the six possible functional groups belonging to the three acid/base pairs may undergo a reversible protonation/deprotonation process, which will cause a significant change in the acidity of the main functional group of a bifunctional photoacid.…”

“…Also shown in the correlation are the kinetic rate constants for the pyranol derivatives 59 and 1-naphthol derivatives (1N5X). 19 All reaction rates were taken at room temperature. The solid line is the Marcus BEBO equation, eq 4.…”

Correlating Photoacidity to Hydrogen-Bond Structure by Using the Local O–H Stretching Probe in Hydrogen-Bonded Complexes of Aromatic Alcohols

“…Ultrafast UV-pump mid-IR-probe measurements on 2-naphthol were performed as previously described. 26 Computational Methods. All calculations were performed using density functional theory at the B3LYP level of theory 55−58 with the TZVP basis set 59 as implemented in Gaussian09.…”

“…23,24 For instance, free-energy reactivity correlations of photoacids have been reported showing the close connection between changes in pK a values and proton transfer rates. 25,26 Moreover, using femtosecond mid-infrared spectroscopy on photoacids, it has been possible to obtain unprecedented dynamical detail on the possible reaction pathways in aqueous acid−base neutralization reactions. 27 −29 Despite the richness in kinetic details revealed by studies of aqueous proton transfer pathways, a microscopic characterization of the hydrogen bond reaction coordinate associated with photoacidity remains a topic of intense research.…”

Wide-dynamic-range kinetic investigations of deep proton tunnelling in proteins

“…We note that the excited state PT rate is much faster than observed for ground state tunneling, presumably due to the different H-bonding arrangement, the vibrational heating from the absorbed photon, and the excited state photo-acid structure 41 , which has a weaker chromophore O-H bond and contracted donor-acceptor oxygen distances 26 . An interesting conjecture for the excited state process involves diffusive proton movement along an alternative wire and out into solution 42 .…”