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Article

Ground- and Excited-State Double Proton Transfer in Lumichrome/Acetic Acid System: Theoretical and Experimental Approach

Supporting Info

Ewa Sikorska,^† Igor Khmelinskii,^‡ Marcin Hoffmann,^§ Isabel F. Machado, Luis F. V. Ferreira, Krzysztof Dobek,^# Jerzy Karolczak,^# Alina Krawczyk,^§ Małgorzata Insińska-Rak,^§ and Marek Sikorski*^§

Faculty of Commodity Science, Pozna University of Economics, al. Niepodlegloci 10, 60-967 Pozna, Poland, Universidade do Algarve, DQB, FCT, Campus de Gambelas, Faro 8005-139, Portugal, Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Pozna, Poland, BioInfoBank Institute, Limanowskiego 24A, 60-744 Pozna, Poland, Centro de Qumica-Fsica Molecular, Complexo Interdisciplinar, Instituto Superior Tcnico, 1049-001 Lisbon, Portugal, Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Pozna, Poland, and Center for Ultrafast Laser Spectroscopy, A. Mickiewicz University, Umultowska 85, 61-614 Pozna, Poland

J. Phys. Chem. A, 2005, 109 (51), pp 11707–11714

DOI: 10.1021/jp053951d

Publication Date (Web): December 6, 2005

†

Faculty of Commodity Science, Poznań University of Economics.

‡

Universidade do Algarve, DQB, FCT.

Faculty of Chemistry, A. Mickiewicz University.

BioInfoBank Institute.

Centro de Química-Física Molecular, Complexo Interdisciplinar, Instituto Superior Técnico.

Faculty of Physics, A. Mickiewicz University.

Center for Ultrafast Laser Spectroscopy, A. Mickiewicz University.

Corresponding author. Fax: +48 61 8658008. Telephone: +48 61 8291309. E-Mail: sikorski@amu.edu.pl.

Abstract

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 N(1) 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 × 10⁸ dm³ mol^-1 s^-1 in ethanolic solutions of lumichrome with added acetic acid.

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