Light-Emitting Cyclopalladated Complexes of 6-Phenyl-2,2‘-bipyridines with Hydrogen-Bonding Functionality

The tridentate ligand 4-carboxy-6-phenyl-2,2‘-bipyridine (HL1) was used to prepare the cyclometalated Pd(II) complex [Pd(L1)Cl] (1). Similar ligands were effective in the formation of [Pd(L2)Cl] (2) and [Pd(L3)Cl] (3) (where HL2 = 4-carboxyphenyl-6-phenyl-2,2‘-bipyridine and HL3 = 4-hydroxyphenyl-6-phenyl-2,2‘-bipyridine). The crystal structure of [Pd(L1)Cl]·H2O has been determined by X-ray diffraction, revealing a variegated array of H-bonding motifs and π-stacking. The [Pd(L1)Cl] molecules are associated in dimers with an intradimer Pd···Pd distance of 3.27 Å, while dimers are separated by interdimer Pd···Pd contacts of 5.41 Å. All complexes 13 exhibited intense luminescence at 77 K, with lifetimes in the 10−200 μs range. The luminescence is assigned to metal-perturbed triplet ligand-centered (LC) excited states for 1 and 3 (partially mixed with an upper-lying metal-to-ligand charge transfer (MLCT) level) and to an excimeric excited state for 2. Room-temperature solid-state luminescence was also revealed by 1 (emission energy and lifetime of 650 nm and 1 μs, respectively), which has been assigned to oligomeric species. Differences in the photophysical properties of 13 are addressed with reference to the chemical structure and electronic properties of their polypyridine ligands.