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A study on electron spin resonance (ESR) spectroscopic determination of exchange interactions in organic oligoradicals is given. When the intramolecular exchange coupling J between unpaired electron spins in nitroxide-based oligoradicals falls within the order of 10 Oe (1 mK or 10 -3 cm -~ for g = 2), which is on the same order as the hyperfine coupling A of magnetic nuclei such as nitrogen atoms of nitroxide radicals, the magnitude of J can be determined from the hyperfine splitting pattern of ESR spectra in solutions. This range of the exchange coupling J is not detectable in conventional magnetic susceptibility measurements. We demonstrate an application of hyperfine ESR spectroscopy as a probe for the exchange coupling to a series of organic oligoradicals, which the authors have recently developed as building blocks for molecule-based magnetic materials.

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