OBSERVATION OF QUANTUM COHERENCE OF PARAMAGNETIC SPECIES BY MEANS OF SUDDEN MAGNETIC FIELD JUMP

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This project aims at the observation and deep investigation of the spin coherent phenomenon of radical pairs formed transiently by photochemical method. We employed two different methods under the existence of radical pairs : 1) sudden jumping of the external magnetic field, 2) quick switching of the electromagnetic field (microwave field) under the conditions of the electron spin resonance. The radical pairs are prepared using a laser pulse irradiation to several solutions. The systems we employed were a) polymethylene-linked biradical, b) radical-ion pairs formed by photoinduced electron transfer reaction, c) micellar systems that include transiently formed radical pairs. Method 1) was applied to the the system b) and it showed the controlled reaction by the magnetic field jump, but we realized the difficulty of the theoretical handling of low magnetic field phenomenon. This study is still under investigation. Method 2) provided many new discoveries about the spin dynamics of radical pair and radical-ion pair systems. First of all, we succeeded in observing the different spin quantum beats of several radical pair systems. The beat data showed the double tequency signal due to simultaneous excitation of the component radicals of radical pairs by microwave field. The decay of the beat signal was proved to be due to the triplet-triplet dephasing phenomenon probably induced by anisotropic dipole-dipole interaction in the radical pair. Another important finding is the appearance of a broadened spectral pattern under the strong microwave field, that was also rationalized by the spin dephasing effect. This project apparently demonstrated that the switching of the external field and/or the electromagnetic alternating field can control the chemical reaction pathways and it also provides a lot of important information about the spin and chemical dynamics of photochemical systems.