Pulsed EPR and ENDOR Studies of Photochemical Reaction Dynamics of Transition Metal Complexes

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The pulsed EPR spectroscopy has higher time resolution and sensitivity than CW-time resolved EPR spectroscopy. We have attempted to apply such a powerful spectroscopy to study of transient processes of photochemical reactions of metal complexes as the following.1. The home made electron spin echo and spin echo ENDOR instrument was modified on the following points so as to enable study of photochemical reactions. (1) Synchronous operation of laser light and microwave pulses and data accumulation. (2) Preparation of bridged loop gap resonator to get higher time resolution and sensitivity. (3) Preparation of highly stable microwave oscillator system. (4) Microwave pulse generator with four channels of different phases which enables measurements of FT-EPR spectra.2. Efficiency of the prepared pulsed EPR spectrometer was compared with that of the CW-time resolved EPR spectrometer and it was confirmed that the pulse method has much higher time resolution and sensitivity, but it has still some problems to apply it for studies of reaction systems which produces paramagnetic intermediate species containing spin centers in the transition metal ions.3. By using the spectrometer, the following photochemical reactions have been investigated. (1) Photoinduced electron transfer reactions between zinc porphyrins and quinones. (2) Photoreduction reaction of bis(diethyldithiocarbamato)copper(II) and its related complexes. (3) Hyperfine coupling interactions in the photoexcited triplet state of porphyrins by the time resolved ENDOR method.