Structural, photophysical, and mesomorphic properties of luminescent platinum(II)-salen Schiff base complexes

A series of square planar platinum(II) salen complexes containing 4-substituted alkoxy chains of aromatic rings, [Pt((4-CnH2n+1O)2salen)] (n = 3 (1), 4 (2), 6 (3), 8 (4), 10 (5), 12 (6), 14 (7), 16 (8), and 18 (9)) has been prepared, and photophysical and mesomorphic properties have been investigated. Complexes 1–9 emitted intense phosphorescence from the mixed exited-state between the MLCT (MLCT = metal-to-ligand charge transfer) and LLCT (LLCT = ligand-to-ligand charge transfer) states both in dichloromethane solution and in solid states at room temperature. The red-shifted solid-state emission spectra from monomers in solution are caused by the supramolecular contact with one-dimensional stacking between neighboring chromophores (salen moieties) through the weak C–H⋯O type hydrogen bonding and van der Waals interactions in the solid state revealed by an X-ray crystallographic analysis for 2. Though 1–4 did not exhibit any mesophases (liquid–crystalline phases), 5–9 showed a lamello-columnar (ColL) mesophase with the high thermal stability, leading to the different phosphorescence spectra from both in solution and in solid states. This is affiliated with the appearance of the intermolecular d z 2 Pt ( II ) - d z 2 Pt ( II ) orbital interaction in a one-dimensional stacking distance with ca. 3.0 Å due to self-assemblies in the liquid crystal which gives rise to the MMLCT (MMLCT = metal–metal-to-ligand charge transfer) transition. This is supported by absorption and emission measurements in the wide range from room temperature in the solid state to the clearing points (around 280 °C). The relationship between molecular assemblies and photophysical properties is discussed.