Push–pull effects and emission from ternary complexes of platinum(ii), substituted terpyridines, and the strong-field cyanide ion

Abstract

As part of an effort to develop new lumaphors involving late transition metal ions, this report describes the synthesis and characterization of the first platinum(II) derivatives containing 2,2′:6′,2″-terpyridine (trpy) and cyanide as co-ligands. According to existing models, including cyanide in the coordination sphere should raise the energies and minimize the influence of short-lived d–d excited states that otherwise compromise the excited-state lifetime. Both [Pt(trpy)(CN)]⁺ and the 4′-cyano-2,2′:6′,2″-terpyridine analogue [Pt(CN–T)(CN)]⁺ are emissive in dichloromethane solution, but the signals are weak. Part of the problem is that the d–π* charge-transfer excited states also rise in energy, so that the emission actually originates from a ³π−π* state with a relatively low radiative rate constant. However, another member of the series, the 4′-dimethylamino-2,2′:6′,2″-terpyridine (dma–T) derivative [Pt(dma–T)(CN)]⁺, proves to be a very promising platform with an emission quantum yield of ϕ = 0.26 and an excited-state lifetime of τ = 22 µs in room-temperature, deoxygenated dichloromethane solution. In the dma–T complex the electron-rich dimethylamino substituent provides the basis for an emissive, but largely ligand-based, charge-transfer excited state. The orbital parentage is such that the photoluminescence persists in donating solvents like dimethylformamide, which ordinarily quenches d–π* excited states in complexes of this type.