Optical properties and Raman spectra of the quasi-one-dimensional gold complexes Au$X_2$dibenzylsulfide ($X_2$=${\mathrm{Cl}}_2$,ClBr,${\mathrm{Br}}_2$)

The compounds Au$X_2$dibenzylsulfide ($X_2$=${\mathrm{Cl}}_2$,ClBr,${\mathrm{Br}}_2$) are quasi-one-dimensional (1D) halogen-bridged mixed-valence gold complexes with a sequence of -Au(I)-X-Au(III)-X- along the chains. These semiconducting compounds have been studied by absorption, reflectance, luminescence, and Raman measurements. The optical properties and Raman spectra are explained by a charge-density wave with a large Peierls gap in the Peierls-Hubbard insulator model. The Au compounds differ from related nearly ideal 1D Pt complexes by a pairing of chains. Hence, two types of charge-transfer excitons polarized parallel and perpendicular to the main chain are observed that both relax to the same self-trapped state.