Charge-bistable Pd(iii)/Pd(ii,iv) coordination polymers: phase transitions and their applications to optical properties

Abstract

Quasi-one-dimensional halogen-bridged Pt, Pd, and Ni complexes (MX chains) are a family of materials with potential charge bistability: M(II)–M(IV) mixed valence charge-density-wave (CDW) and M(III) averaged valence Mott–Hubbard (MH) states can form. However, until recently, no examples of MX chains showing charge bistability have been reported. Charge-bistable Pd MX chains, [Pd(en)₂Br](Y1-C_n)₂·H₂O (en = 1,2-diaminoethane; Y1-C_n = dialkyl 2-sulfosuccinate; n = the number of carbon atoms in alkyl chains) and [Pd(en)₂Br](Y2-C_n)₂·H₂O (Y2-C_n = dialkyl α-sulfomalonate), both of which showed phase transitions between Pd(III) MH and Pd(II,IV) CDW states, were prepared by utilizing chemical pressure. In addition, [Pd(en)₂Br](Y1-C_n)₂·H₂O underwent pressure- and photo-induced MH–CDW phase transitions. [Pd(en)₂Br](Y2-C_n)₂·H₂O has a higher transition temperature, i.e., higher stability of the MH state, than [Pd(en)₂Br](Y1-C_n)₂·H₂O does due to the difference in the crystal packing. The recent progress will lead to new pressure sensing and optical switching devices.