pH switch for OH-photoacidity in 5-amino-2-naphthol and 8-amino-2-naphthol

Abstract

Photoactive charge transfer compounds are of strong interest for their potential applications in material, chemical, and biological science and their abilities to elucidate fundamental charge transfer mechanisms. Aminonaphthols, photoacids with both oxygen (OH) and nitrogen-based (NH2) protonation sites, have been reported to undergo simultaneous excited-state proton transfer (ESPT) in water upon excitation. In this paper, the ESPT mechanism for zwitterion formation in 8-amino-2-naphthol (8N2OH) and 5-amino-2-naphthol (5N2OH) was examined using a combination of time-resolved emission spectroscopy and time-dependent density functional theory (TD-DFT) calculations. The measurements prompted a re-assignment of the zwitterion state in the steady-state emission spectra; analysis of the time-correlated single-photon counting emission data showed that the zwitterion was formed only from excitation of protonated 5N2OH and 8N2OH such that ESPT occurred only at the single hydroxyl group. The protonation state of the amino group dramatically altered the photoacidity of OH, such that the pH behaved as an on/off switch for photoacidity. In the protonated state (NH3+), the pKa*(OH) values of 5N2OH and 8N2OH were both 1.1 ± 0.2, while in the deprotonated state (NH2), the two pKa*(OH) values were similar to the ground state proton acidity, pKa(OH) = 9.5 ± 0.2. The switching of the photoacidity was investigated using TD-DFT calculations and the linear free energy Hammett relation. The latter was shown to not describe the excited state data over the broad pH range.

Biological evaluation and spectral characterization of a novel tetracenomycin X congener

“…All the differences, observed in high pH region were found to be fully reversible. The pattern of pH dependence is similar to that observed for naphtol derivatives due to OH-proton acidity (pKa(OH) ~ 9.5), leading to the formation of naphtolate anion at pH>9 [24]. The reversible nature of TcmX UV absorbance changes suggests the stability of the compound under basic conditions.…”

Divergent Hammett Plots of the Ground- and Excited-State Proton Transfer Reactions of 7-Substituted-2-Naphthol Compounds

“…It is unclear whether the electronic effects of the functional groups will thus be equally weighted. Previous spectral studies on the photochemistry of aminonaphthol isomers differed on whether the protonated or deprotonated species (i.e., cation or neutral in Figure ) leads to zwitterion formation in the excited state. − Our time-correlated single-photon counting (TCSPC) emission measurements on 5N2OH and 8N2OH confirmed that the excited cation is the precursor to the zwitterion . 7N2OH is still assumed to undergo zwitterion formation from the neutral state, i.e., undergo dual excited-state proton transfer (ESPT) at both functional sites, but there have been no dynamical studies to support this assignment.…”

“…For example, 1-hydroxypyrene exhibits increased photoacidity in water upon substitution by sulfonate versus sulfonamide EWG: p K a * of 1.3 for HPTS vs 0.7 for 8-hydroxy-hexamethylpyrene-1,3,6-trisulfonamide; mono- and disubstituted 5-cyano-2-naphthol (5CN2OH) and 5,8-dicyano-2-naphthol (DCN2OH) have boosted p K a * of −0.75 and −4.5 compared to 2OH, p K a * = 2.8, ,, such that proton donation can be extended beyond water to select organic solvents. Reversible protonation of functional groups provides additional flexibility in tuning the photoacidity, as in 6-carboxyl-2-naphthol (6COOH2OH), the protonated and deprotonated carboxyl species have p K a *­(OH) of 1.4 vs 2.5, respectively, , while in 8-amino-2-naphthol (8N2OH), the corresponding p K a * vary more greatly, p K a *­(OH) = 1.2 vs 9.5 . In contrast, there have been few studies exploring the use of electron-donating groups (EDG) to control photoacidity.…”

“…The small size and broad electronic range of aminonaphthols also make them ideal compounds to test the application of both computational and experimental predictors of photoacidity. Our group has recently reported on the photochemistry of isomers 5-amino-2-naphthol (5N2OH) and 8-amino-2-naphthol (8N2OH); substitution at the distal α-C sites (C5/C8) enhanced the EWG/EDG effect such that the protonation state of the amino group acted as an on–off switch for photoacidity . 7-Amino-2-naphthol (7N2OH) differs, in that the functional groups are placed symmetrically on the naphthalene framework with respect to the short axis.…”

Confinement Catalysis of Non‐covalently Functionalized Carbon Nanotube in Ascorbic Acid Sensing

“…8 N2OH has the structure close to 1N2OH. In the literature 8 N2OH is reported to exhibit neutral, cation, and Zwiter ion forms [46] in water. In case of the MWCNT-1N2OH, we have observed the red shift due to the π-π interaction of 1N2OH with MWCNT, and this could be partly due to formation of polaron in the π stack of 1N2OH on MWCNT due to the electron donor-acceptor (EDA) interaction [47][48][49] which induces the formation of localized electronic states within the bandgap of the MWCNT-1N2OH system.…”