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

Thiolato-bridged dinuclear, tetranuclear, and hexanuclear Cu(I) complexes [Cu-2(P boolean AND S)(2)(PPh3)(2)], [Cu-4(P boolean AND S)(4)(CH3CN)(2)], and [Cu-6(P boolean AND S)(6)] (abbreviated as Cu-2, Cu-4, and Cu-6, respectively, P boolean AND 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. Cu-2, Cu-4, and Cu-6 exhibited blue-green (lambda(max) = 482 nm, Phi(em) = 0.52), green (lambda(max) = 526 nm, Phi(em) = 0.19), and yellow (lambda(max) = 553 nm, Phi(em) = 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 Cu-2 and Cu-4 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 singlettriplet two-state model with an energy difference (Delta ES1-T1) of 547 and 775 cm(-1) for Cu-2 and Cu-4, respectively. Based on the time-dependent density-functional theory calculations, the origin of the luminescence for Cu-2 and Cu-4 was attributed to the charge transfer from the cluster core to the ligand. Moreover, the small values of Delta ES1-T1 for Cu-2 and Cu-4 were supported by the excited state calculations. On the other hand, the emission origin of Cu-6 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.