Crystal and Electronic Structures of Radical Salts of 2,7-Bis(methylthio)-1,6-dithiapyrene (MTDTPY) and 2,7-Bis(methylseleno)-1,6-dithiapyrene (MSDTPY)
Crystal and Electronic Structures of Radical Salts of 2,7-Bis(methylthio)-1,6-dithiapyrene (MTDTPY) and 2,7-Bis(methylseleno)-1,6-dithiapyrene (MSDTPY)
Crystal and Electronic Structures of Radical Salts of 2,7-Bis(methylthio)-1,6-dithiapyrene (MTDTPY) and 2,7-Bis(methylseleno)-1,6-dithiapyrene (MSDTPY)
Radical salts of 2,7-bis(methylthio)-1,6-dithiapyrene (MTDTPY) with PF6− and 2,7-bis(methylseleno)-1,6-dithiapyrene (MSDTPY) with AsF6− and [Au(CN)2]− were prepared using an electrochemical method. The crystal structures of these salts were determined with an X-ray method. MTDTPY–(PF6)0.67 (triclinic P\bar1) and MSDTPY–(AsF6)0.67 (triclinic P\bar1) form uniform segregated stacks of MTDTPY and MSDTPY molecules with interplanar distances of 3.40 and 3.59 Å, respectively. Short S–S contacts of 3.46–3.56 Å for MTDTPY–(PF6)0.67 and 3.54–3.61 Å for MSDTPY–(AsF6)0.67 were observed between the columns. The chains of disordered PF6− and AsF6− were found to be parallel to the stack axis. Analysis of the disorder showed a stoichiometric ratio of the DTPY derivatives and counter anions at 1:0.67; the formal charge of MTDTPY and MSDTPY was estimated at +0.67. MSDTPY–Au(CN)2 (monoclinic P2/c) forms a segregated stack of MSDTPY molecules with an interplanar distance of 3.40 Å. The optical reflection spectra of these crystals were measured with polarized light both parallel and perpendicular to the stack axis. In spite of the short intercolumnar S–S contacts, the spectra of MTDTPY–(PF6)0.67 and MSDTPY–(AsF6)0.67 showed a strong one-dimensional character. The optical conductivity spectra (σ(ω)) and the real part of the dielectric constant (ε′(ω)) along the stack axis were obtained by means of Kramers–Kronig transformation. Both the conductivity spectra and the dielectric constant substantially differ from the simple Drude model. These results were analyzed with a one-dimensional electron-localization model in a disordered system with the following optical parameters: ωp=14000 cm−1; τi=2.2×10−15s (τi−1=2400 cm−1) and εcore=3.0 for MTDTPY-(PF6)0.67 and ωp=10000 cm−1; τi=2.2×10−15 (τi−1=2400 cm−1) and εcore=3.5 for MSDTPY–(AsF6)0.67. From the spectra of MSDTPY–Au(CN)2, the on-site Coulomb repulsion U was estimated at 6000 cm−1.