Ligand effects on the photophysical properties of N,N′-diisopropylbenzimidazolylidene-protected C-centered hexagold(I) clusters

A gold(I) cluster with formula of [(C)(Au-BIiPr)₆](BF₄)₂ (1, BIiPr = N,N′-diisopropylbenzimidazolylidene) was synthesized and characterized. It has a molecular structure almost identical to previously reported [(C)(Au-IiPr)₆](BF₄)₂ (2, IiPr = N,N′-diisopropylimidazolylidene) and [(C)(Au-PPh₃)₆](BF₄)₂ (3), except for the ligands. It is worth noting that the crystal of 1 shows intense green emission under photoexcitation (λ_em^max = 509 nm), which significantly blue-shifted compared to the yellow-emissive 2 (λ_em^max = 567 nm) and yellowish-green-emissive 3 (λ_em^max = 525 nm). Moreover, UV–vis spectra of 1, 2 and 3 show a similar absorption profile around 340 nm. However, the ε values at absorption maxima in this region were calculated to be 9.5 × 10⁴, 5.2 × 10⁴ and 2.1 × 10⁴ M⁻¹ cm⁻¹ for 1, 2 and 3, respectively. The results of time-dependent density functional theory (TD-DFT) calculation indicate that the ligand BIiPr promotes the metal-to-ligand charge transfer (MLCT) process dominant in the electronic structure of 1 and thereby alters the energy gaps. Meanwhile, another underlying origin of these ligand effects on the photophysical properties is likely to be their different capabilities to stabilize the excited states of clusters.