Developing Nanosecond Transient Absorption Measurement System for Detecting Radical Species

Abstract

Photochemical reactions begin when molecules are excited by absorbing light. The photophysical and photochemical processes of excited molecules generate several intermediates. Detection of the intermediates is necessary to mechanistic study of photochemical reactions. If the intermediates can emit fluorescence or phosphorescence, its lifetime can give the rate constant information, but if not, another method must be used. Transient Absorption (TA) is a method of measuring the absorption after excitation of substance. Since the absorption occurs in substances after excitation, TA is an appropriate method to detect the non-radiative intermediates. In this work, the TA setup was developed to detect the non-radioactive radical intermediates generated at larger than nanosecond time scale and two systems for generating radicals were studied. Pyrene-(CH2)12-O-(CH2)2-N, N-dimethylaniline (Py-12-O-2-DMA), following the radical ion pair (RIP) mechanism, generates exciplex and RIP through photo-induced electron transfer. Exciplex emission exhibits a magnetic field effect in polar solvents but not in non-polar solvents. The magnetic field effect of Exciplex is caused by charge recombination of RIP, a non-radiative product. In order to investigate the magnetic field effects of the RIP mechanism, it is necessary not only to observe Exciplex emission but also to study nonradiative products; such as RIP. In this work, magnetic field effect of the RIP mechanism was investigated by comparing the TA of Py-12-O-2-DMA with the photoluminescence lifetime. Advanced oxidation processes (AOP) are effective technologies to destroy organic contaminants in the aqueous phase. AOP create highly reactive radicals. These electrophilic radicals eventually mineralize refractory organic contaminants into CO2 and H2O. The UV / Free Chlorine (HOCl / OCl-) process photodegrade NaOCl to generate OH∙ and Reactive Chlorine Species (RCS, Cl∙, Cl2 ∙−, ClO∙, etc.). In previous studies, effects of OH∙ generated during photolysis in AOP system has been studied. However, the products and rate constant of RCS when reacted with organic contaminants have not been studied. In this study, Cl2 ∙− was generated by Laser Flash Photolysis of UV/S2O8 2−/Cl− system and second-order reaction rate constant with phenol was determined.