Halide-Enhanced Spin-Orbit Coupling and the Phosphorescence Rate in Ir(III) Complexes

The spin-forbidden nature of phosphorescence in Ir(III) complexes is relaxed by the metal-induced effect of spin-orbit coupling (SOC). A further increase of the phosphorescence rate could potentially be achieved by introducing additional centers capable of further enhancing the SOC effect, such as metal-coordinated halides. Herein, we present a dinuclear Ir(III) complex IrI that contains two Ir(III)-iodide moieties. The complex shows intense phosphorescence with a quantum yield of Φ_PL(300 K) = 90% and a submicrosecond decay time of only τ(300 K) = 0.34 μs, as measured under ambient temperature for the degassed toluene solution. These values correspond to a top value T₁ → S₀ phosphorescence rate of k_r = 2.65 × 10⁶ s^-1. Investigations at cryogenic temperatures allowed us to determine the zero-field splitting (ZFS) of the emitting state T₁ ZFS(III-I) = 170 cm^-1 and unusually short individual decay times of T₁ substates: τ(I) = 6.4 μs, τ(II) = 7.6 μs, and τ(III) = 0.05 μs. This indicates a strong SOC of state T₁ with singlet states. Theoretical investigations suggest that the SOC of state T₁ with singlets is also contributed by halides. Strongly contributing to the higher occupied molecular orbitals of the complex (e.g., HOMO, HOMO - 1, and so forth), iodides work as important SOC centers that operate in tandem with metals. The examples of IrI and of earlier reported analogous complex IrCl reveal that the metal-coordinated halides can enhance the SOC of state T₁ with singlets and, consequently, the phosphorescence rate. A comparative study of IrI and IrCl shows that the share of halides in total contribution (halides plus metals) to the SOC of state T₁ with singlets increases strongly upon exchange of chlorides for iodides. The exchange also led to the decrease in values of ZFS of the T₁ state from ZFS(III-I) = 205 cm^-1 for IrCl to T₁ ZFS(III-I) = 170 cm^-1 for IrI. This results in a more efficient thermal population of the fastest emitting T₁ substate III, thus further enhancing the room-temperature phosphorescence rate.