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HYDROGEN BONDING AND PHOTOTAUTOMERISM OF 3‐METHYLLUMICHROME
Author(s) -
Szafran Magdalena M.,
Koziot Jacek,
Heelis Paul F.
Publication year - 1990
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.1751-1097.1990.tb04191.x
Subject(s) - trifluoroacetic acid , chemistry , protonation , acetic acid , hydrogen bond , hydrogen , fluorescence , dichloroethane , aqueous solution , emission spectrum , excited state , spectral line , photochemistry , inorganic chemistry , analytical chemistry (journal) , molecule , organic chemistry , catalysis , ion , atomic physics , physics , quantum mechanics , astronomy
— The spectral changes of 3‐methyllumichrome in 1,2‐dichloroethane upon addition of hex‐afluoroisopropanol, trifluoroacetic acid and acetic acid as hydrogen donor agents, have been defined. In the presence of 0.8 M hexafluoroisopropanol, the absorption and fluorescence emission spectra of 3‐methyllumichrome in 1,2‐dichloroethane acquire features nearly identical to those observed in aqueous solution. Therefore, hexafluoroisopropanol can be considered as a model compound for studies on hydrogen bonding of alloxazine derivatives in nonpolar media. The observed spectral changes upon addition of trifluoroacetic acid are caused mainly by protonation. The presence of hydrogen bonded and protonated forms of 3‐methyllumichrome has been checked by means of excitation spectra. The addition of acetic acid causes minor hydrogen bonding effects but gives rise to the phototautomery with an efficiency of one order of magnitude higher in 1,2‐dichloroethane than previously observed in methanol or dioxane. The isoalloxazinic emission band appears at about 495 nm and shifts bathochromically to 540 nm upon increasing acetic acid concentration, probably due to the opening of the primary cyclic structure between 3‐methyllumichrome and acetic acid. The fluorescence emission spectra can be resolved using lognormal approximation into three bands, one ascribed to the alloxazinic form (maximum at about 440 nm) and the others to isoalloxazinic species (maxima at about 495 and 540 nm). The excitation spectra confirm the presence of two excited alloxazinic forms, one solvated by 1,2‐dichloroethane with emission at 435 nm and T ˜1 ns, and the second bonded with acetic acid, bathochromically shifted with T ˜5 ns and able to phototautomerize. The sum of excitation spectra of both forms in suitable proportion reconstructs the absorption spectra of 3‐methyllumichrome in 1,2‐dichloroethane and acetic acid.