Core-Structure-Dependent Luminescence of Thiolato-Bridged Copper(I) Cluster Complexes

Thiolato-bridged dinuclear, tetranuclear, and hexanuclear Cu(I) complexes [Cu2(P∧S)2(PPh3)2], [Cu4(P∧S)4(CH3CN)2], and [Cu6(P∧S)6] (abbreviated as Cu2, Cu4, and Cu6, respectively, P∧S = 2-(diphenylphosphino)benzenethiolate, PPh3 = triphenylphosphine) were synthesized and characterized by elemental analyses and single-crystal X-ray diffraction measurements. These complexes had {Cu2S2}, {Cu4S4}, and {Cu6S6} cluster cores and exhibited strong luminescence at room temperature in solid states. Different luminescence properties were observed depending on the core structure. Cu2, Cu4, and Cu6 exhibited blue-green (λmax = 482 nm, Φem = 0.52), green (λmax = 526 nm, Φem = 0.19), and yellow (λmax = 553 nm, Φem = 0.49) luminescence, respectively, at 298 K in the solid state; among them, only Cu6 showed luminescence thermochromism. Different radiative rate constants at room temperature and 78 K derived from the emission lifetimes and quantum yields indicate that the luminescence from Cu2 and Cu4 at room temperature originated from thermally activated delayed fluorescence (TADF), whereas the luminescence at low temperatures was attributed to the phosphorescence. The temperature dependence of the emission lifetimes was successfully analyzed by the singlet–triplet two-state model with an energy difference (ΔES1-T1) of 547 and 775 cm–1 for Cu2 and Cu4, respectively. Based on the time-dependent density-functional theory calculations, the origin of the luminescence for Cu2 and Cu4 was attributed to the charge transfer from the cluster core to the ligand. Moreover, the small values of ΔES1-T1 for Cu2 and Cu4 were supported by the excited state calculations. On the other hand, the emission origin of Cu6 was attributed to the phosphorescence from the triplet cluster-centered (3CC) excited state in which the electron is located on a bonding in-phase orbital constructed from the 4s/4p orbitals of the Cu atoms because only Cu6 contains trigonal-planar Cu(I) ions in the cluster.