Non-Stereogenic Dinuclear Ir(III) Complex with a Molecular Rack Design to Afford Efficient Thermally Enhanced Red...

Cyclometalated complexes containing two or more metal centers were recently shown to offer photophysical properties that are advantageous compared to their mononuclear analogues. Here we report the design, synthesis, and luminescent properties of a dinuclear Ir(III) complex formed by a ditopic N^{^}C^{^}N-N^{^}C^{^}N bridging ligand (L¹) with pyrimidine as a linking heterocycle. Two dianionic C^{^}N^{^}C terminal ligands were employed to achieve a charge-neutral and nonstereogenic dinuclear complex 5. This complex shows a highly efficient red emission with a maximum at λ_em = 642 nm as measured for a toluene solution. The decay time and emission quantum yield of the complex measured for the degassed sample are τ = 1.31 μs and Φ_PL = 80%, respectively, corresponding to the radiative rate of k_r = 6.11·10⁵ s^-1. This rate value is approximately fourfold faster than for the green-emitting mononuclear analogue 3. Cryogenic temperature measurements show that the three substrates of the lowest triplet state T₁ of 5 emit with decay times of τ(I) = 120 μs, τ(II) = 7 μs, and τ(III) = 1 μs that are much shorter compared to those of the mononuclear complex 3, which has values of τ(I) = 192 μs, τ(II) = 65.6 μs, and τ(III) = 3.6 μs. These data indicate that the spin-orbit coupling of state T₁ with the singlet states is much stronger in the case of complex 5, which results in a much higher T₁ → S₀ emission rate. Indeed, a computational analysis suggests that in the dinuclear complex 5 the T₁ state is spin-orbit coupled with twice the number of singlet states compared to that of mononuclear 3, which is a result of the electronic coupling of two coordination sites. The investigation of the temperature dependence of the emission rates of 3 and 5 shows that the room-temperature emission of both complexes is mainly contributed by a thermally populated excited state lying above the T₁ state. To the best of our knowledge, complexes 3 and 5 are the first examples of Ir(III) complexes that show photophysical behavior reminiscent of thermally activated delayed fluorescence (TADF).