Light-emitting cyclopalladated complexes of 6-phenyl-2,2′-bipyridines with hydrogen-bonding functionality

The tridentate ligand 4-carboxy-6-phenyt-2,2'-bipyridine (HL1) was used to prepare the cyclometalated Pd(II) complex [Pd(L1)C1] (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)-H-. has been determined by X-ray diffraction, revealing a variegated array of H-bonding motifs and pi-stacking. The [Pd(L1)Cl] molecules are associated in dimers with an intradimer (PdPd)-Pd-... distance of 3.27 Angstrom, while dimers are separated by interdimer (PdPd)-Pd-... contacts of 5.41 Angstrom. All complexes 1-3 exhibited intense luminescence at 77 K, with lifetimes in the 10-200 mus 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 mn and 1 mus, respectively), which has been assigned to oligomeric species. Differences in the photophysical properties of 1-3 are addressed with reference to the chemical structure and electronic properties of their polypyridine ligands.