Spectroscopic Behavior of Würster’s Blue Perchlorate and N-Ethylphenazyl Crystals

The electronic absorption spectra of Würster’s Blue perchlorate and N-ethylphenazyl in solid were measured at various temperatures ranging from 48°K to 300°K. In both cases, strong bands due to the charge-transfer between the half occupied orbitals of respective component radicals were observed in near infrared region. This assignment of these bands was supported by the polarized absorption measurement carried out with the Würster’s blue perchlorate single crystal. The charge-transfer band of solid Würster’s blue perchlorate was found to increase its intensity with decreasing temperatures; in particular, the increasing tendency became predominant below the phase transition point (186°K). The similar tendency was also observed for solid N-ethylphenazyl. The temperature dependence of the band was found to be closely related with the population of triplet excitons determined from the measurement of magnetic susceptibility or paramagnetic resonance. The separation J between the ground singlet state and the thermally accessible triplet state was determined from the analysis of the temperature dependence of the charge-transfer band to be 235 cm−1 and 320 cm−1 for Würster’s blue perchlorate and N-ethylphenazyl, respectively.