³MMLCT excited states of luminescent binuclear Pd^II complexes: excited state inner-sphere electron-transfer reactions...

Compared with Pt^II analogues that exhibit unique stimulus-induced switching luminescence properties and novel material applications, the properties and reactivity of the ³MMLCT excited state of Pd^II complexes in solutions are under-developed. Here, we prepared a series of binuclear cyclometalated Pd^II 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 photophysical properties have been examined by femtosecond time-resolved absorption spectroscopy, and the 1e oxidation products of binuclear Pd^II 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 Pd^II complex with a redox-active formamidinate bridging ligand reacts with benzyl bromide to immediately generate Pd^II-Pd^III-Br species upon light irradiation. Quenching and time-resolved absorption experiments show that the Pd^II-³MMLCT excited state reacts with alkyl bromides via an inner-sphere electron transfer pathway. These binuclear Pd^II complexes were examined as organic light-emitting diode (OLED) emitters and photocatalysts for C-C bond formation reactions.