A luminescent Pt(ii) complex with a terpyridine-like ligand involving a six-membered chelate ring

Abstract

The ligand 2-(8′-quinolinyl)-1,10-phenanthroline (1) was prepared in 79% yield by the Friedländer condensation of 8-amino-7-quinolinecarbaldehyde and 8-acetylquinoline. The complex [Pt(1)Cl]⁺ was prepared and compared with the isomeric 2-(2′-quinolinyl)-1,10-phenanthroline (2) complex. An X-ray analysis indicated that the six-membered chelate ring in the tridentate complex resulted in a relief of angle strain as well as some non-planarity in the bound ligand 1. The control system for photophysical studies is [Pt(3)Cl]⁺ where 3 denotes 2-(2′-pyridyl)-1,10-phenanthroline. Relative to the complex of 3, in dichloromethane solution [Pt(1)Cl]⁺ exhibits noticeably higher energy charge-transfer absorption but slightly lower energy emission. The gap between the onset of absorption and emission is larger because the emission from [Pt(1)Cl]⁺ originates from a triplet excited state with substantial intra-ligand character. At room temperature in deoxygenated dichloromethane, [Pt(1)Cl]⁺ has an excited-state lifetime of 310 ns vs. 230 ns for [Pt(3)Cl]⁺. Within the series, [Pt(1)Cl]⁺ also exhibits the largest activation barrier for thermally induced quenching at 2730 cm⁻¹ in fluid dichloromethane solution. However, the barrier is only about 50% larger than that found for [Pt(3)Cl]⁺. There is reduced ring strain in [Pt(1)Cl]⁺, but inter-ligand steric interactions weaken the ligand field.