Spectroscopic evidence for the low-temperature charge-separated state of [Pd(dmit)2] salts

We have measured the temperature dependence of reflectivity spectra of a single crystal of (C2H5)2(CH3)2Sb[Pd(dmit)2]2 (dmit = 1,3-dithiol-2-thione-4,5-dithiolate), to elucidate the origin of its phase transition at 70 K. A remarkable splitting has been found for the bonding–antibonding excitation of the dimer, [Pd(dmit)2]2, in the low-temperature phase. This manifests that the phase transition is due to the charge separation, 2 dimer− → dimer0 + dimer2−. This result, supporting our quantum chemical model for a strongly dimerized two-orbital system, gives a clue to incorporate a new spin–charge–orbital interplay in a molecular assembly.