The effect of high magnetic fields (< 14 T) on the lifetime of a triplet biradical generated by a photoinduced intramolecular electron transfer reaction of the title compound has been studied in water at room temperature by using pulse-magnet laser flash photolysis apparatus. On increasing the magnetic field from zero to ca. 1 T, the lifetime of the biradical increases steeply, and then decreases gradually in higher magnetic fields. The lifetime at ca. 13 T is about 30% of that at 1 T. This drastic reversal of the effect is qualitatively interpreted by a spin-lattice relaxation (SLR) mechanism. The SLR induced by the anisotropic Zeeman interaction is responsible for the decrease in lifetime in the higher magnetic fields. The mechanism is discussed in detail based on model calculations.
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