Kinetics of the rearrangement of the solvation shell of an excited fluorescent probe 4″-dimethylaminochalcone

• Published in: Biophysics (Oxford) 2007-02, Vol.52 (1), p.8-12
• Main Authors: Dobretsov, G. E., Gularyan, S. K., Sarkisov, O. M., Gostev, F. E., Petrukhin, A. N., Titov, A. A., Svetlichny, V. Yu, Syrejshchikova, T. I.
• Format: Article
• Language: English
• Subjects: Biophysics; Fluorescence; Kinetics; Molecular biology; Solvents
• ISSN: 0006-3509; 1555-6654
• DOI: 10.1134/S0006350907010022

A study was made of the processes associated with the quenching of 4''-dimethylaminochalcone (DMAC) fluorescence by proton-donor solvent (1-butanol). The kinetics of deactivation of the DMAC excited state was assessed by transient absorption spectra with a time resolution about 50 fs and by fluorescence decay with 30-ps resolution. The following sequence of events could thus be envisaged: (i) the DMAC molecule in the ground state (prior to excitation) makes a hydrogen bond with an alcohol molecule; (ii) absorption of a light quantum causes a corresponding increase of the DMAC dipole moment; the H-bond is retained; (iii) the solvation shell formed by alcohol dipoles is reorganized in response to the raise of the DMAC dipole moment, with an energy expenditure about 24 kJ/mol and a time constant about 40 ps; the initial H-bond is still retained; (iv) processes leading to fluorescence quenching occur with an effective time constant of nearly 200 ps. Since quenching is far slower than solvate rearrangement, one can suppose that it is not a direct consequence of shell relaxation or prior H-bonding. Thus, DMAC fluorescence quenching may involve different processes observed with other aromatic molecules: H-bond rearrangement from a nonquenching to a more 'efficient' conformation, charge transfer between the excited molecule and alcohol, or solvent-induced out-of-plane twist of the DMAC amino group.[PUBLICATION ABSTRACT]