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Dresden 2011 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 49: Laseranwendungen: Laserspektroskopie

Q 49.2: Talk

Thursday, March 17, 2011, 10:45–11:00, SCH A215

pH dependence of the absorption and emission behaviour of lumiflavin in aqueous solutionAmit Tyagi and •Alfons Penzkofer — Fakultät für Physik, Universität Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany

The spectroscopic behaviour of lumiflavin (LF) in aqueous solutions of pH range -1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields and lifetimes are determined. The ionisation stage of ground-state LF changes from cationic (LFH2+) at low pH (pKc ≈ 0.38) via neutral (LFH) to anionic (LF) at high pH (pKa ≈ 10.8). The cationic, neutral, and anionic forms are identified by their different absorption spectra. LFH in neutral aqueous solution is reasonably fluorescent (fluorescence quantum yield φF = 0.29, fluorescence lifetime τF = 5.2 ns), while LF is weakly fluorescent (φF = 0.0042, τF = 90 ps), and LFH2+ is nearly non-fluorescent (φF ≈ 3.6×10−5, τF ≈ 0.4 ps).

In the ground state a pH dependent thermodynamic equilibration of cationic, neutral and anionic lumiflavin exists by reaction with H2O, H3O+ and OH. For lumiflavin in aqueous solution in the excited state no equilibrium distributions are reached between the cationic, neutral, and anionic forms. Some neutral excited lumiflavin transforms to the cationic ground-state form at low pH by intermolecular photo-induced proton transfer from H3O+ to LFH*. At high pH no photo-induced intermolecular proton transfer takes place.

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