Structural, mesomorphic, photoluminescence and thermoelectric studies of mononuclear and polymeric complexes of copper(ii) with 2-hexyldecanoato and 4,4′-bipyridine ligands

[Cu(R)₂(bpy)₂]·2RH (1) and [Cu₂(R)₄(bpy)]_x (2) were obtained from the reaction of [Cu₂(R)₄(RH)₂] (R = 2-hexyldecanoato) with 4,4′-bipyridine (bpy) using different work-up procedures. Complex 1 consisted of single crystals and its molecular structure showed a distorted octahedral Cu(ii) with two chelating bidentate R. Complex 2 has a similar structure as crystals of a linear-chain coordination polymer, [Cu₂(CH₃(CH₂)₅COO)₄(bpy)]_x (3). Both 1 and 2 were thermally stable (T_dec = 240°C for 1; 220°C for 2), have low melting temperatures (T_m = 52.3°C for 1; T_m = 48.3°C for 2) and exhibited temperature-dependent mesomorphisms. For 1, the mesomorphism arose from the change in the binding mode of R from chelating to bridging during isotropization, while for 2, the mesomorphism arose from the breaking of the Cu-N_bpy bonds. The mesophases for 2 were cubic columnar (Cub, P4₃32) and rectangular columnar (Col_R, space group p2gg) on cooling from 150°C, Cub and hexagonal columnar (Col_H) on cooling from 184°C (isotropic liquid phase), and Col_H and nematic columnar (Col_N) on cooling from 190°C. These complexes also exhibited photoluminescence upon charge transfer and d-d excitation. Finally, the Seebeck coefficient value (S_e) for 2 was -0.47 mV K^-1, identifying it as a potential thermoelectric material.