Spin and charge fluctuation occurring in valence tautomerism and cooperative effects in the solid-state

Abstract

The character of low-spin and high-spin redox isomer conversion of [Co(3,6-di- t-butyl-1,2-benzoquinone) 2(2,2′-bipyridine)] in the solid-tate was investigated by magnetic susceptibility, solid-state high-resolution 13C NMR, and dielectric measurements. We found anomalously large entropy change for relatively sharp transition between the low-spin and high-spin state, when we assumed a random equilibrium between the low-spin and high-spin isomers. A model of domain formation by a cooperative effect for the isomer conversion accounted for the sharp transition with a typical entropy change. It was suggested that three or four molecules form a domain in average. Solid-state high-resolution 13C NMR spectrum revealed the conversion process in a microscopic point of view. Two differently reduced states of 3,6-di- t-butyl-1,2-benzoquinone, i.e. semiquinoate (SQ) and catecholate (Cat), are averaged by rapid electron exchange even in the low-spin state. The exchange rate is much larger than the NMR time scale. Since fluctuation of molecular dipole moment is expected for an electron hopping between SQ and Cat, temperature dependence of dielectric constant was measured at different frequencies between 1 and 100 kHz. The result showed no evidence of freezing of the fluctuation above 10 K. Almost the same result was obtained for the stable crystalline phase of [Co(3,5-di- t-butyl-1,2-benzoquinone) 2(2,2′-bipyridine)], which showed a gradual isomer conversion around 300 K as a random equilibrium process.