Recently, heteroleptic Ir(III) catalysts open a new era in the field of organic chemistry as antenna catalysts which can be used for dual-catalytic reactions (DCR) under visible light. It had been predicted that the iridium catalyst becomes a single-electron oxidant upon light irradiation and electron transfer (ET) between Ir and co-catalyst (e.g., Ni and Co catalysts) is a key process to initiate DCR: the long-lived excited state of Ir intemediates transfers an electron to a co-catalyst which eventually enables cross-coupling among reagents. Interestingly, development of dual catalytic systems mainly leans to trial-and-error approaches, and only a little is known about their photophysical properties and intermolecular reactions (IMRs). The lack of knowledge inspired us to investigate oxidation and geometry changes during DCR as a function of time in order to elucidate IMRs. We chose the simplest model system containing [Ir{dF(CF3)ppy}2(dtbpy)]PF6 (1) and (4,4′-dtbbpy)NiCl2 (2) and monitored their oxidation and geometry changes using time-resolved xray absorption spectroscopy (TR-XAS) when pump is 400 nm and xray probe energy is near Ir L-III and Ni K-edge. 1 absorbs 400 nm light and forms a long lived 3MLCT state leading to the oxidation status change from Ir(III) to Ir(IV). The 3MLCT state shows the presence of two decaying pathways presumably resulted from intramolecular energy transfer (EnT) among triplet states or thermal relaxation of the state. The presence of 2 shortens these two lifetime components of 3MLCT of 1, and no reduction to Ir(II) is monitored from photoexcited Ir(III) in the presence of 2. Moreover, we revealed that EnT between 1 and 2 occurs rather than ET and the EnT does not induce oxidation status change of 2 but induces geometry change from square planar to octahedral due to solvent ligation. The lifetime of energetically excited 2 is firstly monitored to ~ 20 µs which is a substantially long enough for further ET processes to organic reagents.

Probing energy transfer-mediated Ir/Ni dual catalysis upon visible light excitation using time-resolved X-ray absorption spectroscopy

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.