Spectroscopic Studies of Cyclometallated Pd(II) and Pt(II) Complexes: Optical absorption and emission of single crystals of [Pd(thpy)2] and [Pt(thpy)2]

The polarized optical absorption and emission (spectra, decay times) of single crystals of [Pd(thpy)₂] and [Pt(thpy)₂] (thpy ≡ C(3′)-deprotonated form of 2-(2-thienyl)pyridine) at temperatures 1.9 K ? T ? 80 K are reported. The emission of [Pt(thpy)₂] can be influenced strongly by applied magnetic fields (0 ? H ? 6 T). Depending on the central ions Pd and Pt, the lowest excited electronic states of the single complexes are ligand-centered (LC) states and metal-to-ligand charge transfer (MLCT) states, respectively. This difference leads to distinctly dissimilar properties of the emission of both compounds. The experimental data show that the emission of single crystals of [Pd(thpy)₂] and [Pt(thpy)₂] at T ? 30 K originates from several types of traps (defect states of symmetry ³B₂?stabilized below the exciton band) with LC and MLCT character, respectively. In the Pt compound, the ³B₂ is split by spin-orbit coupling into three states. The states B and A, which determine the emission properties, are separated by Δv ～ 13 cm^?1. Both states can mix under the influence of an applied magnetic field yielding an increase of the emission intensity by a factor of ～ 1.5 at H = 6 T.