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Issue 2, 2012
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Excited-state dynamics of phenol–pyridinium biaryl

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Abstract

The excited-state dynamics of a donor–acceptor phenol–pyridinium biaryl cation was investigated in various solvents by femtosecond transient absorption spectroscopy and temperature dependent steady-state emission measurements. After excitation to a near-planar Franck–Condon delocalized excited S1(DE) state with mesomeric character, three fast relaxation processes are well resolved: solvation, intramolecular rearrangement leading to a twisted charge-shift (CSh) S1 state with localized character, and excited-state proton transfer (ESPT) to the solvent leading to the phenoxidepyridinium zwitterion. The proton transfer kinetics depends on the proton accepting character of the solvent whereas the interring torsional kinetics depends on the solvent polarity and viscosity. In nitriles, ESPT does not occur and interring twisting arises with no significant intrinsic barrier, but still slower than solvation. The CSh state is notably fluorescent. In alcohols and water, ESPT is faster than the solvation and DE → CSh relaxation processes and yields the zwitterion hot ground state, which strongly quenches the fluorescence. In THF, solvation and interring twisting occur first, leading to the fully relaxed, weakly fluorescent CSh state, followed by slow ESPT towards the zwitterion. At low temperature (77 K), the large viscous barrier of the solvent inhibits the torsional relaxation but ESPT still arises to some extent. Strong emission from the DE geometry and planar zwitterion is thus observed. Finally, quantum chemical calculations were performed on the ground and excited state of model phenol–pyridinium and phenoxide–pyridinium compounds. Strong S1 state energy stabilization is predicted upon twisting in both cases, consistent with a fast relaxation towards the perpendicular geometry. A substantial S0–S1 energy gap is still present for the twisted cationic species, which can explain the long-lived emission of the CSh state in nitriles. A quite different situation arises with the zwitterion for which the S0–S1 energy gap predicted at the twisted geometry is very small. This suggests a close-lying conical intersection and can account for the strong fluorescence quenching observed in solvents where the zwitterion is produced by ESPT.

Graphical abstract: Excited-state dynamics of phenol–pyridinium biaryl

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Publication details

The article was received on 06 Sep 2011, accepted on 19 Oct 2011 and first published on 10 Nov 2011


Article type: Paper
DOI: 10.1039/C1CP22829C
Citation: Phys. Chem. Chem. Phys., 2012,14, 562-574
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    Excited-state dynamics of phenol–pyridinium biaryl

    J. Malval, H. Chaumeil, W. Rettig, V. Kharlanov, V. Diemer, E. Ay, F. Morlet-Savary and O. Poizat, Phys. Chem. Chem. Phys., 2012, 14, 562
    DOI: 10.1039/C1CP22829C

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