(3)MMLCT excited states of luminescent binuclear Pd(II) complexes: excited state inner-sphere electron-transfer reactions and application.

Compared with Pt(II) analogues that exhibit unique stimulus-induced switching luminescence properties and novel material applications, the properties and reactivity of the (3)MMLCT 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 à and luminescent (3)MMLCT excited states in solutions at 298 K (emission quantum yield and radiative decay rate constant up to 0.70 and 2 × 10(5) 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 ((3)dd) 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)-(3)MMLCT 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.