Polarons and soliton pairs (bipolarons) in halogen-doped quasi-one-dimensional mixed-valence platinum complexes

The first experimental studies on halogen doping in quasi-one-dimensional, halogen-bridged, mixed-valence platinum complexes (HMPC) are presented. The electrical conductivity of the iodine compound increases over 7 orders of magnitude with iodine doping. The absorbance of the two optical-absorption bands near the midgap and the ESR intensity in dilute doped HMPC increase with the dopant concentration. In the dilute regime, halogen doping yields the same effects on optical absorption and ESR as does photoexcitation at an energy greater than or equal to the energy of the charge-transfer absorption band. The doping-induced states are located on single chains and have charge and spin. These results strongly suggest polaron formation by halogen doping. At high doping levels, a new doping-induced absorption appears at an energy of 1.55 eV at 77 K, and the ESR decreases in intensity as the doping proceeds. It appears that upon halogen doping, singly charged polarons are produced and the formation of confined charged soliton pairs (bipolarons) via the process $P^{+}$+$P^{+}$→$S^{+}$+$S^{+}$ (or $B^{2+}$) is favored.