Enhancement of the Luminescent Efficiency in Carbene-Au^(I)-Aryl Complexes by the Restriction of Renner–Teller Distortion and Bond Rotation

A series of (carbene)­Au^(I)(aryl) complexes are reported. The nature of the lowest excited state in these complexes changes character from metal-to-ligand charge transfer (MLCT) to interligand charge transfer (ICT) with increasing electron-donating strength of the aryl ligand. Complexes that have the MLCT lowest excited state undergo a Renner–Teller bending distortion upon excitation. Such a distortion leads to a large rate of nonradiative decay, on the order of 10⁸ s^–1. Renner–Teller-based nonradiative decay does not occur in chromophores with an ICT emissive state. Introducing a julolidine moiety and ortho-methyl substituents to the aryl group makes the molecule rigid and hinders the rotation along the Au–C_aryl-coordinate bond. Consequently, the nonradiative decay rates of these ICT emitters are decreased and become lower than the radiative decay rate constants (k_r = 10⁵ s^–1). Thus, high-luminescent efficiencies (Φ_PL = 0.61 and 0.77) along with short lifetimes (τ < 2 μs) are obtained for yellow and green emitters, respectively. Thermally assisted delayed fluorescence behavior is observed, owing to the small exchange energy (ΔE_ST < 1600 cm^–1) in these emitters.