Ground- and excited-state double proton transfer in lumichrome/acetic acid system: theoretical and experimental approach.

Authors:
Ewa Sikorska
Ewa Sikorska
Poznań University of Economics
Poland
Dr. Igor Khmelinskii, PhD, Prof. Agregado
Dr. Igor Khmelinskii, PhD, Prof. Agregado
University of the Algarve
PhD, Prof. Agregado
physcal chemistry; spectroscopy; climate science
Faro, Algarve | Portugal
Marcin Hoffmann
Marcin Hoffmann
A. Mickiewicz University
Krzysztof Dobek
Krzysztof Dobek
Adam Mickiewicz University
Poland
Jerzy Karolczak
Jerzy Karolczak
Adam Mickiewicz University
Poland
Alina Krawczyk
Alina Krawczyk
Poznań University of Economics
Poland

J Phys Chem A 2005 Dec;109(51):11707-14

Faculty of Commodity Science, Poznań University of Economics, al. Niepodleglości 10, 60-967 Poznań, Poland.

Experimental time-resolved spectral and photon counting kinetic results confirm formation of an isoalloxazinic excited state via excited-state double proton transfer (ESDPT) catalyzed by a carboxylic acid molecule that forms a hydrogen-bond complex with the parent alloxazine molecule. This isoalloxazinic tautomer manifests itself as a distinct long-lived emissive species formed only in such alloxazine derivatives that were not substituted at the N1 nitrogen atom, being a product of the excited-state reaction occurring from the alloxazinic excited state. Theoretical calculations support the idea that the ESDPT occurs by the concerted mechanism. The calculated activation barrier in the excited state is much lower than the same barrier in the ground state and even disappears for the HOMO-1 to LUMO excitation, which explains the fact that the reaction takes place in the excited-state only. The reaction rate estimated from the emission kinetics is ca. 1.4 x 10(8) dm3 mol(-1) s(-1) in ethanolic solutions of lumichrome with added acetic acid.

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December 2005
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