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Issue 33, 2018
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pH switch for OH-photoacidity in 5-amino-2-naphthol and 8-amino-2-naphthol

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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.

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

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The article was received on 22 Jun 2018, accepted on 01 Aug 2018 and first published on 01 Aug 2018

Article type: Paper
DOI: 10.1039/C8CP03984D
Citation: Phys. Chem. Chem. Phys., 2018,20, 21325-21333
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    pH switch for OH-photoacidity in 5-amino-2-naphthol and 8-amino-2-naphthol

    M. S. Groves, K. J. Nelson, R. C. Nelson and K. Takematsu, Phys. Chem. Chem. Phys., 2018, 20, 21325
    DOI: 10.1039/C8CP03984D

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