Femtosecond transient absorption spectroscopy study of the early events of norfloxacin in aqueous solutions with varying pH values.

The photophysics and photochemistry of norfloxacin (NF) have been investigated in aqueous solutions of different pH using femtosecond transient absorption spectroscopy (fs-TA). Resonance Raman spectroscopic experiments on NF have also been conducted in aqueous solutions of different pH to characterize the vibrational and structural information on the initial forms of NF. The experimental results in combination with density functional theory calculations of the key intermediates help us to elucidate the early events for NF after photoexcitation in aqueous solutions with varying pH values. The fs-TA results indicate that NF mainly underwent photophysical processes on the early delay time scale (before 3 ns), and no photochemical reactions occurred on this time scale. Specifically, after the irradiation of NF, the molecule reaches a higher excited singlet Sn and then decays to the lowest-lying excited singlet state S1 followed by intersystem crossing to transform into the lowest-lying triplet state T1 with a high efficiency, with an exception that there is a lower efficiency observed in basic aqueous solution due to the generation of an intramolecular electron transfer as an additional pathway to waste energy.