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 that contains two Ir(III)-iodide moieties. The complex shows intense phosphorescence with a quantum yield of Φ(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 T → S phosphorescence rate of = 2.65 × 10 s. Investigations at cryogenic temperatures allowed us to determine the zero-field splitting (ZFS) of the emitting state T ZFS(III-I) = 170 cm and unusually short individual decay times of T substates: τ(I) = 6.4 μs, τ(II) = 7.6 μs, and τ(III) = 0.05 μs. This indicates a strong SOC of state T with singlet states. Theoretical investigations suggest that the SOC of state T 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 and of earlier reported analogous complex reveal that the metal-coordinated halides can enhance the SOC of state T with singlets and, consequently, the phosphorescence rate. A comparative study of and shows that the share of halides in total contribution (halides plus metals) to the SOC of state T with singlets increases strongly upon exchange of chlorides for iodides. The exchange also led to the decrease in values of ZFS of the T state from ZFS(III-I) = 205 cm for to T ZFS(III-I) = 170 cm for . This results in a more efficient thermal population of the fastest emitting T substate III, thus further enhancing the room-temperature phosphorescence rate.