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SOLVENT AND METHYL SUBSTITUENT EFFECT ON PHOTOTAUTOMERISM AND IONIZATION OF ALLOXAZINES
Author(s) -
Koziolowa Anna
Publication year - 1979
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.1751-1097.1979.tb07076.x
Subject(s) - chemistry , pyridine , substituent , steric effects , solvent , excited state , hydrogen bond , proton affinity , ground state , photochemistry , absorption (acoustics) , ionic bonding , maxima , medicinal chemistry , stereochemistry , protonation , organic chemistry , molecule , ion , atomic physics , physics , acoustics , art , performance art , art history
— The absorption and emission spectra of alloxazine and its methyl derivatives have been measured in solvents of different polarity. From roughly linear correlation between two long wavelength absorption maxima and the emission maxima positions vs solvent polarities. systematic deviations have been observed for alloxazines in pyridine, acetic acid and water. The deviations depend on the methylation site of alloxazine in a way suggesting formation of hydrogen bond in the ground state and phototautomerism in the excited state. As phototautomerism of alloxazines in the presence of pyridine and water may involve ionic equilibria, the pK a values of alloxazines have been measured (8.11–8.58) and the p K B * estimated from the Förster cycle (2.3–6.6 for N‐1 monoanion and 7.4–7.8 for N‐3 monoanion). The phototautomeric efficiency of alloxazines in pyridine, acetic acid and water depend on the acidity of the N‐1 proton and on the steric effects of methyl substituents, mainly at C‐9. The anionic forms of alloxazines differ from the phototautomeric species in emission maxima positions, radiative lifetimes and they are dissimilarly influenced by the methylation site and medium viscosity.