Photophysical Properties Of Pt(Ii) Polypyridines With Five-Versus Six Membered Chelate Rings: Trade-Offs In Angle Strain

This report describes the synthesis and characterization of a series of eight [Pt(NNN)X](+) complexes where the tridentate NNN ligand is (2,2'-bipyrid-6-yl)(pyrid- N 2-yl)sulfide (btp) or methyl(2,2'-bipyrid-6-y1)(pyrid-2-yl)amine (bmap) and X is OMe, Cl, phenylethynyl (C2Ph), or cyclohexylethynyl (C2Cy). The expectation was that inserting a heteroatom into the backbone of 2,2':6',2"-terpyridine (trpy) would expand the overall intraligand bite angle, introduce ILCT character into the excited states, and improve the photophysical properties. Crystal structures of [Pt(bmap)C2Ph]+ and [Pt(btp) C1](+) reveal that atom insertion into the trpy backbone successfully expands the bite angle of the ligand by 8-10. However, the impact on the photophysics is minimal. Indeed, of the eight systems investigated, only the [Pt(bmap)C,Ph]+ and [Pt(btp) C2Ph](+) complexes display appreciable emission in fluid solution, and they exhibit shorter emission lifetimes than [Pt(trpy)C2Ph](+). One reason is that the bond angle preferences of platinum and the inserted heteroatom induce the six membered rings to deviate from planarity and adopt a boat -like conformation, impairing charge delocalization within the ligand. In addition, angle strain induces the donor atoms about platinum to assume a pseudotetrahedral arrangement, which offsets any benefit due to the increase in overall bite angle by promoting deactivation via d d excited states. The results reveal that, in order to improve the luminescence of a [Pt(NNN)X](+) system, one must take care to avoid trading one kind of angle strain for another.