Heteronuclear double resonance study of pentafluorobenzene

The electronic absorption spectrum of solid Würster's blue perchlorate in the low temperature phase was measured at various temperatures, special attention being paid to the first π* ← π transition band of the monomer at ～ 600 mμ. The isosbestic points were clearly found for the spectral curves measured at several different temperatures, showing the coexistence of two transition bands in this wavelength region. By analysing the temperature dependence of the absorption intensity and by comparing the result with E.S.R. data, these two bands were interpreted as the singlet-singlet and triplet-triplet absorption bands originating from the ground singlet state and the thermally accessible triplet state, respectively. A similar result was obtained for the 260 mμ band. The present results can be interpreted by the dimer model for the low temperature phase of Würster's blue perchlorate but are inconsistent with the disproportionation mechanism for the phase transition.

The singlet-singlet and triplet-triplet transition bands were separately obtained for each of the 600 mμ and 260 mμ bands from analysis of the temperature dependence; the singlet-triplet separation in the corresponding excited states is 310±80 cm^-1 and 530±200 cm^-1, respectively, with the triplet state at lower energy.