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Q: Fachverband Quantenoptik und Photonik
Q 49: Laseranwendungen: Laserspektroskopie
Q 49.2: Vortrag
Donnerstag, 17. März 2011, 10:45–11:00, SCH A215
pH dependence of the absorption and emission behaviour of lumiflavin in aqueous solution — Amit 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.