Luminescent Pd(II) Complexes with Tridentate −C∧N∧N Aryl-pyridine-(benzo)thiazole Ligands

Abstract

Cyclometalated Pd(II) complexes generally show inferior luminescence properties compared with their Pt(II) analogues. The established approach employing tridentate (Formula presented.) cyclometalating ligands has allowed us to create a series of square planar Pd(II) complexes [Pd((Formula presented.))X] from their protoligands H (Formula presented.) (2-(6-phenylpyridin-2-yl)thiazoles and -benzothiazoles; coligands X=Cl, Br, I) with extensive variations at the Carene group (phenyl, naphthyl, fluorenyl), the central Npyridine (pyridine, 4-phenylpyridine, 3,5-di-tert-butyl-4-phenylpyridine), and the peripheral Nthiazole (thiazole, benzothiazole) to probe for structural factors that might enhance efficient luminescence. Long-wavelength bands at 400–500 nm were assigned to transitions into mixed ligand-centred/metal-to-ligand charge transfer (MLCT) states based on time-dependent (TD)DFT calculations. The MLCT contributions are rather low, in agreement with relatively long lifetimes and high photoluminescence quantum yields of up to 0.79 recorded in frozen glassy solvent matrices at 77 K along with emission bands showing pronounced vibrational progressions and peaking at about 520 nm. No photoluminescence was observed at 298 K in solution. Variation of the (Formula presented.) ligand allowed to shift the experimental absorption energies from about 2.4 to 2.7 eV, in good agreement with the electrochemical band gaps (2.58 to 2.81 eV). The theoretical absorption and emission spectra excellently reproduced the experimental trends.