Luminescence of neutral and ionic gold(I) complexes containing pyrazole or pyrazolate-type ligands

A series of pyrazole (Hpz) and pyrazolate (pz⁻) Au(I) complexes of types Au(Hpz^2R(n))(PPh₃)]⁺ (I), Au(Hpz^2R(n))₂]⁺ (II), Au(μ-pz^R(n))]₃ (III), Au(pz^R(n)/2R(n))(PPh₃)] (IV), AuCl(Hpz^R(n)/2R(n))] (V) and (PPh₃)Au(μ-pz^R(n))Au(PPh₃)]⁺ (VI), R(n) and 2R(n) represent C₆H₄OC_nH_2n+1 substituents at the 3- or 3- and 5-positions of the heterocyclic ring, respectively, have been shown to be luminescent in the solid state at 77 K, independently of the presence or not of inter-metallic Au-Au interactions. The emission spectra of all complexes consist of structured bands in the region 395-500 nm, attributed to ligand-to-metal charge transfer (LMCT) transitions involving the Hpz or pz⁻ ligands, the pattern of bands of compounds being related with the molecular structure and/or the nature of the ligands. The thermal behaviour of several complexes of the types III, IV and V containing long-chain substituents (n ? 12) was examined by polarising light optical microscopy (POM). The derivative AuCl(Hpz^R(12))] was proved to have liquid crystal properties exhibiting a mesophase SmA but the remaining complexes were not liquid crystal materials. This complex is one of the scarce examples of Au(I) derivatives exhibiting both liquid crystal and luminescent properties.