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PHOTOTAUTOMERISM OF LUMICHROME IN METHANOL‐ACETIC ACID MIXTURES. A STEADY‐STATE AND TIME‐RESOLVED FLUORESCENCE STUDY
Author(s) -
Koziolowa A.,
Visser A. J. W. G.,
Koziol J.
Publication year - 1988
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.1751-1097.1988.tb02779.x
Subject(s) - chemistry , fluorescence , excited state , acetic acid , photochemistry , hydrogen bond , quenching (fluorescence) , methanol , substituent , steric effects , reaction rate constant , proton , analytical chemistry (journal) , kinetics , molecule , stereochemistry , organic chemistry , atomic physics , physics , quantum mechanics
— Fluorescence lifetimes of 3‐methyllumichrome dual fluorescence at 440 nm (alloxazinic) and 520 nm (isoalloxazinic) and of 1,3‐dimethyllumichrome, which is unable to phototautomerize, at the same wavelengths have been measured in methanol‐acetic acid mixtures. The fluorescence decays of both lumichromes studied are exponential and the phototautomeric fluorescence of 3‐methyllumichrome is created within approximately 50 ps. From the initial values of about 0.9 ns (alloxazinic) and 6.4 ns (isoalloxazinic) in 5% acetic acid the lifetimes shorten considerably with increasing acid concentration reaching 0.2 ns for 1,3‐dimethyllumichrome and 80 ps for the alloxazinic and 2.4 ns for the isoalloxazinic form of 3‐methyllumichrome in pure acid. Static and dynamic fluorescence quenching constants were estimated. The differences in the quenching rate and equilibrium constants of both lumichromes are interpreted in terms of different equilibria of hydrogen bond formation in ground and excited state as influenced by steric effects of the methyl substituent at the N‐l position in 1,3‐dimethyl. The hydrogen bonding atN–10 of 3‐methyllumichrome with acetic acid is a prerequisite for additional hydrogen bonding at N‐l enabling excited state proton transfer.