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Absorption and Emission Spectroscopic Characterization of Lumichrome in Aqueous Solutions †
Author(s) -
Tyagi Amit,
Penzkofer Alfons
Publication year - 2010
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2010.00836.x
Subject(s) - chemistry , excited state , tautomer , aqueous solution , fluorescence , photochemistry , proton , absorption (acoustics) , ground state , absorption spectroscopy , analytical chemistry (journal) , stereochemistry , organic chemistry , atomic physics , physics , quantum mechanics , acoustics
The spectroscopic behavior of lumichrome (7,8‐dimethyl‐alloxazine, LC) in aqueous solutions in a pH range from −1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields, and lifetimes are determined. The ionization stage of ground‐state LC changes with rising pH from the cationic form (LCH 2 + ) to the neutral form (LCH) with a mid‐point pH of p K c ≈ −0.53, and to the anionic form (LC − ) with a mid‐point pH of p K a ≈ 12.5. Above pH 7 a partial ground‐state tautomerization of LCH to 7,8‐dimethyl‐isoalloxazine (IAH) occurs by N1–N10 intra‐molecular proton transfer. For pH > p K a ≈ 12.5 LCH and IAH change to the anionic forms LC − and IA − , and above pH 14 LC − tautomerizes completely to IA − . In the excited state some neutral lumichrome (LCH*) converts to cationic lumichrome (LCH 2 + ) at low pH by proton transfer from H 3 O + to LCH*. No photoinduced excited‐state tautomerization of lumichrome was observed. LCH for pH > 3 and IAH are reasonably fluorescent. The fluorescence efficiencies of LC − and IA − are lower than those of LCH and IAH. The fluorescence of LCH 2 + is strongly quenched likely by intra‐molecular diabatic charge transfer and excited‐state relaxation by potential surface touching with the ground state.