Synthesis, Characterization, Spectroscopic Properties, and Luminescence Quenching Mechanism of a Pt(II) Complex Decorated with a π-Conjugated TEMPO-Terpyridine Ligand System

A novel Pt(II)-based metallointercalator terpyridine complex linked with a 2,2,6,6-tetramethyl-1-piperidinyl N-oxide (TEMPO) derivative was prepared by a reaction between 4'-TEMPO-terpyridine (L) and a Pt(II) salt. This complex presented unusual luminescence quenching owing to the effect of the stable nitroxide radical. The crystal structure of[Pt(terpy-TEMPO)Cl]Cl·H2O·CH3OH (terpy=2,2':6',2"-terpyridine) was elucidated by X-ray crystallography. Additionally, the effect of TEMPO on the photophysical properties of[Pt(terpy-TEMPO)Cl] Cl·H2O·CH3OH was investigated by UV-Vis, fluorescence emission, and electron paramagnetic resonance (EPR) spectroscopy. Data from the absorption and luminescence properties (298 K) of the[Pt(terpy-TEMPO)Cl]+ complex indicated the presence of two groups of typical bands:an intense band B with distinct vibronic structures (270-350 nm, ε>104 dm3·mol-1·cm-1) and a less intense band A (370-450 nm, ε~103 dm3·mol-1·cm-1). These two bands are generally assigned to ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) excited states, respectively. Furthermore, efficient photoluminescent quenching behavior was observed in the emission spectra of this complex system. Quantum calculations of the molecular energy gaps and bands were performed by Gaussian 09 software. The calculated results verified that TEMPO greatly affects the energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Thus, the relationship between efficient photoquenching and molecular structure was theoretically interpreted. EPR results indicated that when TEMPO is attached to a macrocyclic terpyridine platinum complex, e.g.,[Pt(terpy)Cl]+, the terpyridine platinum complex does not affect the hyperfine coupling constant (A value) and g factor (g values) but the rotation and relaxation times of the TEMPO radical.