Intermolecular interactions and aggregation of fac -Tris(2- phenylpyridinato- C ², N)iridium(III) in nonpolar solvents

The intermolecular interaction and aggregation of the neutral complex fac-tris(2-phenylpyridinato-C²,N)iridium(III) (fac-Ir(ppy) ₃) in solution was investigated. Intermolecular interactions were found to effectively decrease the luminescence lifetime via self-quenching with increasing fac-Ir(ppy)₃ concentrations. A Stern-Volmer plot for quenching in acetonitrile was linear, due to bimolecular self-quenching, but curved in toluene as the result of excimer formation. ¹H NMR spectra demonstrated a monomer-aggregate equilibrium which resulted in spectral shifts depending on solvent polarity. X-ray crystallography provided structural information concerning the aggregate, which is based on a tetramer consisting of two Δ-fac-Ir(ppy)₃-Λ-fac-Ir(ppy)₃ pairs. Offset π-π stacking of ppy ligands and electrostatic dipole-dipole interactions between complex molecules play an important role in the formation of these molecular pairs.