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-2-yl)­sulfide (btp) or methyl­(2,2′-bipyrid-6-yl)­(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)C 2 Ph]+ and [Pt(btp)Cl]+ 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 2 Ph]+ and [Pt(btp)C 2 Ph]+ 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.