Zwitterion formation by 2,3-aminonaphthol in its excited state

Ellis, David W.; Rogers, L. B.

doi:10.1016/0371-1951(62)80042-3

Cited by 10 publications

(5 citation statements)

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“…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.…”

Section: Introductionmentioning

confidence: 59%

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

et al. 2019

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The rational design of photoacids requires accessible predictive models of the electronic effect of functional groups on chemical templates of interest. Here, the effect of substituents on the photoacidity and excited-state proton transfer (PT) pathways of prototype 2-naphthol (2OH) at the symmetric C7 position was investigated through photochemical and computational studies of 7-amino-2-naphthol (7N2OH) and 7-methoxy-2-naphthol (7OMe2OH). Time-resolved emission experiments of 7N2OH revealed that the presence of an electron-withdrawing versus electron-donating group (EWG vs EDG, NH3 + vs NH2) led to a drastic decline in photoacidity: pK a* = 1.1 ± 0.2 vs 9.6 ± 0.2. Time-dependent density functional theory calculations with explicit water molecules confirmed that the excited neutral state (x = NH2) is greatly stabilized by water, with equation-of-motion coupled cluster singles and doubles calculations supporting potential mixing between the La and Lb states. Similar suppression of photoacidity, however, was not observed for 7OMe2OH with EDG OCH3, pK a* = 2.7 ± 0.1. Hammett plots of the ground- and excited-state PT reactions of substituted 7-x-2OH compounds (x = CN, NH3 +, H, CH3, OCH3, OH, and NH2) vs Hammett parameters σp showed breaks in the linearity between the EDG and EWG regions: ρ ∼ 0 vs 1.14 and ρ* ∼ 0 vs 3.86. The divergent acidic behavior most likely arises from different mixing mechanisms of the lowest Lb state with the La and possible Bb states upon substitution of naphthalene in water.

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Section: Introductionmentioning

confidence: 59%

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

et al. 2019

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“…Like carboxy substituted 2-naphthol, amino substituted 2-naphthols are also interesting photoacids that have two different protonable/deprotonable substituents, OH/O – and NH 3 + /NH 2 , on the same naphthalene framework. − Early studies with some aminonaphthol derivatives had reported formation of their zwitterions in the excited state, which was thought to be due to intramolecular proton transfer from OH to the NH 2 group. , In recent years, Takematsu and co-workers have carried out more detailed studies and concluded that zwitterion formation in aminonaphthols is not due to proton migration from one site to another in the excited neutral molecules, but rather due to establishment of cation ↔ zwitterion prototropic equilibrium in the excited state because of drastic reduction in p K a * of the OH group upon protonation of the NH 2 substituent. − The dramatic alteration in photoacidity of the OH group when the protonation state of amino group is changed from NH 2 to NH 3 + (p K a * decreases from about 9.5 to ∼1) was consistent with previous studies by Pines et al on carboxynaphthols . However, in the case of aminonaphthols, considerable divergence from linear Hammett relations was observed for the ground and excited state acidity constants. , There were also noticeable differences among different positionally substituted aminonaphthols that raised interesting questions about whether ESPT in these bifunctional molecules should be treated within the purview of a naphthol framework or a naphthylamine framework.…”

Section: Introductionmentioning

confidence: 99%

“…28−31 Early studies with some aminonaphthol derivatives had reported formation of their zwitterions in the excited state, which was thought to be due to intramolecular proton transfer from OH to the NH 2 group. 28,29 In recent years, Takematsu and co-workers have carried out more detailed studies and concluded that zwitterion formation in aminonaphthols is not due to proton migration from one site to another in the excited neutral molecules, but rather due to establishment of cation ↔ zwitterion prototropic equilibrium in the excited state because of drastic reduction in pK a * of the OH group upon protonation of the NH 2 substituent. 32−34 The dramatic alteration in photoacidity of the OH group when the protonation state of amino group is changed from NH 2 to NH 3 + (pK a * decreases from about 9.5 to ∼1) was consistent with previous studies by Pines et al on carboxynaphthols.…”

Section: Introductionmentioning

confidence: 99%

Dual Excited State Proton Transfer Pathways in the Bifunctional Photoacid 6-Amino-2-naphtol

Jagushte,

Sadhukhan,

Upadhyaya

et al. 2023

J. Phys. Chem. B

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This study investigates the photoacidity and excited state proton transfer (ESPT) pathways of a bifunctional molecule, 6amino-2-naphthol (6N2OH), using absorption, steady-state fluorescence, time-resolved fluorescence, and theoretical calculations. 6N2OH attains four different prototropic forms in the excited state (cation, neutral, anion, or zwitterion) depending on pH of the solution. Interestingly, ESPT at the OH site of the molecule can be controlled by the protonation state of the amino substituent. Conversion of the electron donating NH 2 group to the electron withdrawing NH 3+ group brings about a reduction of more than 7 pK a units for the deprotonation of OH in the excited state. Further, the position of the NH 2 substituent on the naphthalene framework is found to play an important role in dictating the ESPT pathways of aminonaphthols. Unlike most aminonaphthol derivatives that undergo ESPT only at the OH site, akin to substituted naphthols, 6N2OH undergoes ESPT at both OH and NH 3 + sites, indicating its similarity to substituted naphthols and substituted naphthylamines. ESPT at the NH 3 + site resulting in cation ↔ neutral equilibrium of 6N2OH in the excited state is well-corroborated by comparative studies with another reference photoacid, 6-amino-2-methoxynaphthalene (6N2M). Correlation of the acidity constants of 6N2OH with the σ p parameters according to the Hammett model reveals that while 6N2OH can be treated either as naphthol or as naphthylamine in the ground state, the structure−function correlation cannot be extrapolated directly in the excited state, thus highlighting the rich and complex photophysics of bifunctional photoacids.

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“…Excited state behavior of naphthol and its derivatives have shown great sensitivity towards surrounding media [23][24][25][26][27][28][29][30]. Substitution of electron withdrawing group at different positions results in remarkable changes in its prototopic behaviours [31][32][33][34].The case of electron donating amino (-NH2) group present at different positions has been studied theoretically as well as experimentally under various pH conditions [35][36][37].…”

Section: Introductionmentioning

confidence: 99%

Photophysical Investigation of 8-amino 2-Naphthol in Different Micellar Environment

Gahlaut

Pandey²,

Arora³

et al. 2019

Preprint

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*8-amino 2-naphthol (8A2NP) is an interesting molecule containing both proton acceptor (-NH<sub>2</sub>) and proton donating (-OH) groups. Our earlier investigation on the photophysics / photochemistry of 8A2NP reveals that its prototropic behavior is sensitive to the solvent polarity and pH. Interesting photophysical properties of 8A2NP in homogeneous environment prompting towards the study of the said probe in in different surfactants<b>.</b> In the present work, 8A2NP has been studied in cationic (CTAB), anionic (SDS) and non-ionic (TX-100) surfactants and changes in the excited state deactivation are explored in the mentioned surfactants

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Zwitterion formation by 2,3-aminonaphthol in its excited state

Cited by 10 publications

References 0 publications

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

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

Dual Excited State Proton Transfer Pathways in the Bifunctional Photoacid 6-Amino-2-naphtol

Photophysical Investigation of 8-amino 2-Naphthol in Different Micellar Environment

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