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]

Abstract

Abstract 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 3 B 2 −stabilized below the exciton band) with LC and MLCT character, respectively. In the Pt compound, the 3 B 2 is split by spin‐orbit coupling into three states. The states B and A , which determine the emission properties, are separated by Δ v ∼ 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.