High magnetic field and magnetic isotope effects on lifetimes of triplet biradicals consisting of two equivalent benzophenone ketyls linked by methylene chains

External magnetic field and magnetic isotope effects on the lifetimes of several isotope-labeled triplet biradicals (BPH.-O-(CH2)(n)-O-BPH., n=8, 10, and 12) composed of two equivalent benzophenone ketyl radicals (BPH(.)s) were studied by laser flash photolysis in magnetic fields up to 14 T. The biradical lifetimes increased with increasing a magnetic field below ca. 3 T and then slightly decreased at 4-14 T. Below 4 T, a large magnetic isotope effect on the biradical lifetimes was observed in C-13 isotope substitution at a benzylic carbon of BPH., whereas no magnetic isotope effect was observed above 4 T. The magnetic isotope effect in magnetic fields up to 4 T can be explained only by the spin-lattice relaxation mechanism due to the anisotropic hyperfine interaction of the biradicals.