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Dynamics of Excited State Proton Transfer in Nitro Substituted 10-Hydroxybenzo[h]quinolines

Abstract

The ground state tautomerism and excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo[h]quinoline (HBQ) and its nitro derivatives, 7-nitrobenzo[h]quinolin-10-ol (2) and 7,9-dinitrobenzo[h]quinolin-10-ol (3), has been studied in acetonitrile by steady state as well as time dependent spectroscopy and quantum-chemical calculations. In addition to the enol form absorbance in the range 360-390 nm, the absorption spectra of 2 and 3 exhibit a red shifted band at ~450 nm. Chemometric data processing, based on individual bands decomposition, allowed to estimate the position of the ground state enol-keto tautomeric equilibrium (ΔG values of 1.03 and 0.62 kcal/mol respectively for 2 and 3). The fluorescence stems from the enol form even if the keto form is optically excited as proven by the shape of the excitation spectra indicating that ESIPT takes place. The Stokes shift of the substituted compounds is substantially lower compared to HBQ, which follows from the fact that the substitution occurs in the formal cyclohexa-2,4-dienone moiety and leads to decrease of the HOMO level of the keto tautomer. The pump-probe experiments show that in the nitro substituted HBQs 2 and 3 ESIPT happens with a time constant of 0.89 ps and 0.68 ps, respectively. In both cases a mixture of the enol and proton transfer form is optically excited. The enol form exhibits then the ESIPT and subsequently both fractions take the same relaxation path. We propose that in 2 and 3 the ESIPT path exhibits a potential energy barrier resulting in an incoherent rate governed process while in HBQ the ESIPT proceeds as a ballistic wavepacket motion along a path without significant barrier. The theoretical calculations (M06-2X/TZVP) confirm the existence of a barrier in the ground and excited state as result of the substitution.

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Publication details

The article was received on 04 Jul 2017, accepted on 12 Sep 2017 and first published on 13 Sep 2017


Article type: Paper
DOI: 10.1039/C7CP04476C
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    Dynamics of Excited State Proton Transfer in Nitro Substituted 10-Hydroxybenzo[h]quinolines

    H. Marciniak, S. Hristova, V. Deneva, F. S. Kamounah, P. E. Hansen, S. Lochbrunner and L. Antonov, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP04476C

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