pH effect on the efficiency of the photodeactivation pathways of naphazoline: a combined steady state and time resolved study

The effect of pH on the spectroscopic behavior of naphazoline [NP, 2-(1-nap hthylmethyl)imidazoline] has been examined in aqueous solution by combining steady state and time resolved spectroscopic experiments. The photodeactiv ation pathways are modulated by the pH changes and can be rationalized on t he basis of the chemical structure of the compound. The naphthalene-localiz ed singlet formed upon light excitation is partially deactivated via a reve rsible energy transfer process involving the imidazoline moiety when NP is present in its cationic form. This process leads to an imidazoline-assisted intersystem crossing being responsible for the population of the lowest na phthalene-localized triplet state. In the case of the neutral form, the imidazoline ring behaves as an efficie nt electron donor, which is able to quench the excited naphthalene aia a ph otoinduced electron transfer. Such a process is responsible for the decreas e in the naphthalene-localized triplet generation as well as for the reduct ion of drug photodegradation.