K-band EPR of photoinduced midgap states in an M-X chain complex [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$${)}_4$, (en)=ethylenediamine

The K-band electron paramagnetic resonance due to the photoinduced midgap state has been measured in an M-X chain complex [Pt(en${)}_2$ ][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$${)}_4$,(en)=ethylenediamine, at 77 K. The angular variations of the spectral line shape and the intensity show that the photoinduced unpaired electron is localized on the segment Pt-Cl-Pt of the M-X chain bonds. The ${\mathit{p}}_{\mathrm{\sigma }}$ orbit of the intervening ${\mathrm{Cl}}^{\mathrm{-}}$ ion is found to share 30–40 % of the density of the unpaired electron, based on the following hyperfine constants of the constituent ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ and ${}_{}{}^{35,37}{}_{}{}^{}\mathrm{Cl}$ nuclei: ${\mathit{A}}_{\mathrm{\parallel }}^{\mathrm{Pt}}$=378×${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$ and ${\mathit{A}}_{\mathrm{\parallel }}^{\mathrm{Cl}}$=55×${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$ for the quantization direction parallel to the chain axis; ${\mathit{A}}_{\mathrm{\perp }}^{\mathrm{Pt}}$=375×${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$ and ${\mathit{A}}_{\mathrm{\perp }}^{\mathrm{Cl}}$<5×${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$ for the direction perpendicular to the chain axis. The present result supports the notion that the photoinduced state is the spin soliton of two-site-type propagating along the M-X chain bonds. The possibility of the polaron is clearly excluded.