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 1−3 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 1−3 are addressed with reference to the chemical structure and
electronic properties of their polypyridine ligands.