3MMLCT excited states of luminescent binuclear PdII complexes: excited state inner-sphere electron-transfer reactions and application

Abstract

Compared with PtII analogues that exhibit unique stimulus-induced switching luminescence properties and novel material applications, the properties and reactivity of the 3MMLCT excited state of PdII complexes in solutions are under-developed. Here, we prepared a series of binuclear cyclometalated PdII complexes with short intramolecular Pd-Pd distances of 2.79–2.89 Å and luminescent 3MMLCT excited states in solutions at 298 K (emission quantum yield and radiative decay rate constant up to 0.70 and 2×105 s-1, respectively). Their photo-physical properties have been examined by femtosecond time-resolved absorption spectroscopy, and the 1e oxidation products of binuclear PdII complexes have been studied by electron paramagnetic resonance spectroscopy and computational studies. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations show that changing the C-deprotonated aryl pyridine (C^N) ligand to the strong σ-donor aryl N-heterocyclic carbene (C^C*) ligand significantly increases the energy level of the metal centered (3dd) excited state. The binuclear PdII complex with a redox-active formamidinate bridging ligand reacts with benzyl bromide to immediately generate PdII-PdIII-Br species upon light irradiation. Quenching and time-resolved absorption experiments show that the PdII-3MMLCT excited state reacts with alkyl bromides via an inner-sphere electron transfer pathway. These binuclear PdII complexes were examined as organic light-emitting diode (OLED) emitters and photocatalysts for C-C bond formation reactions.