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pH Effects on the Spectroscopic and Photochemical Behavior of Enoxacin: A Steady‐State and Time‐Resolved Study
Author(s) -
Sortino S.,
Guidi G. De,
Giuffrida S.,
Monti S.,
Velardita A.
Publication year - 1998
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.1751-1097.1998.tb05182.x
Subject(s) - chemistry , quantum yield , protonation , photochemistry , flash photolysis , heterolysis , zwitterion , photodissociation , aqueous solution , dissociation (chemistry) , fluorescence , kinetics , reaction rate constant , molecule , organic chemistry , ion , physics , quantum mechanics , catalysis
The spectroscopic and photochemical behavior of Enoxacin (ENX), 1‐ethyl‐6‐fluoro‐1,4‐dihydro‐4‐oxo‐7‐(1‐piperazinyl)‐1,8‐naphthyridine‐3‐carboxylic acid, has been investigated in aqueous solutions between pH 3.5 and pH 12. The absorption and emission properties of ENX are strongly affected by pH. The fluorescence quantum yield, 4 x 10 −3 at pH 3.5, increases by a factor of two on going to neutral pH while a strong reduction is observed at alkaline pH. The photodegradation quantum yield also depends on pH, being maximum in neutral conditions ( ca 0.04). Nanosecond flash photolysis experiments confirm that the yield of absorbing transients is maximum at neutral pH while it decreases to zero at acid and alkaline pH. These results indicate that both the dissociation of the carboxylic group and the protonation of the piperazinyl residues are key steps for the formation of the photochemically active form of ENX. Loss of F − by heterolytic cleavage of the C–F bond is proposed to occur from the triplet state of the zwitterion with formation of a carbocation. A path for the evolution of this intermediate to the final product is also proposed.