Dr. Raanan Carmieli discusses the use of electron paramagnetic resonance (EPR) as a powerful technique for the study of transition metals, organic radical ions and transient radical-pairs in photoexcited systems.
Electron paramagnetic resonance (EPR) is a powerful technique for the study of systems with paramagnetic centers such as transition metals, organic radical ions and is among the most important tools for studying transient radical-pairs in photoexcited systems. Time-resolved EPR (TR EPR) methods have been extensively used to monitor the primary photochemistry in RCs of purple photosynthetic bacteria and photosystems I and II of higher plants and in artificial analogs. In these experiments EPR signals of the transient charge separated states, of D●+ A●-, are observed.
This increase in understanding redox processes and various complex reactions in all kind of chemical and biochemical systems associated with paramagnetic intermediates has been strongly promoted by the application of in-situ spectroelectrochemical techniques. Due to its high sensitivity towards paramagnetic species, EPR can provide key information about radical species generated or consumed during electrode reaction. Thus, EPR complement electrochemical data from other techniques by directly identifying radical species, confirming reaction mechanisms and revealing more subtle interaction for example, between the radical and its environment. As such, EPR has not only provided a wealth of information to electrochemist but EPR spectroscopist have also found that in-situ electrochemical EPR is a feasible option to standard optical study of electrochemistry.
Currently, most studies of the paramagnetic intermediates in electron transfer reactions are done by generating them chemically ex-situ, followed by a transfer to the spectroscopic cell. In most cases those paramagnetic intermediates are not long lived and need an oxygen free environment, in addition chemical generation of these radical ion intermediates is not accurate as electrochemical one and can also generate by-products as well. All this makes the current working procedure complicated and a bit tricky for generating the studied samples and transferring them from the synthesis lab to the spectrometer.
Associate Staff Scientist
Weizmann Institute of Science