Intersystem crossing (ISC) rates of transition-metal complexes are determined by the complex interplay of a molecule’s electronic and structural dynamics. To broaden our understanding of these key factors, we investigate the case of the prototypical d8–d8 dimetal complex [Pt­(ppy)­(μ- t Bu2pz)]2 using broad-band transient absorption anisotropy in combination with ultrafast fluorescence up-conversion and ab initio calculations. We find that, upon excitation of the molecule’s metal–metal-to-ligand charge-transfer transition, ISC occurs in hundreds of femtoseconds from the lowest excited singlet state S1 to the triplet state T2, from where the energy relaxes to the lowest energy triplet state T1. ISC to the T2 state, rather than T1, is further rationalized through supporting arguments. Observed vibrational coherences along the Pt–Pt mode are attributed to the formation of nuclear wavepackets on the ground and excited electronic states that dephase prior to ISC because of the structural flexibility of the complex. Beyond demonstrating the relationship between the energy relaxation and structural dynamics of [Pt­(ppy)­(μ- t Bu2pz)]2, our results provide new insights into the photoinduced dynamics of d8–d8 dimetal complexes more generally.

Excitation energy transfer inducing molecular switching was studied in a prototypical dyad consisting of a benzimidazole fluorophore and a naphthopyran molecular switch.

We report on the ultrafast photodynamics of an iron(II) complex with a photoisomerizable pentadentate azo-tetrapyridylamino ligand after irradiation with ultraviolet light. The results of femtosecond transient electronic absorption spectroscopy performed on the low-spin (LS) form of the title complex show that initial excitation of the ππ* state of the azopyridine unit in the ligand at λ pump = 312 nm is followed by an ultrafast intersystem crossing (ISC) that leads to the formation of a metal-centered (MC) 5 T state, in competition with the intended photoswitching of the azopyridine unit. Additional measurements carried out upon excitation of the singlet metal-to-ligand charge-transfer ( 1 MLCT) transition at λ pump = 455 nm suggest that this energy transfer occurs via an MLCT state. The resulting high-spin (HS) 5 T state of the complex is metastable and recovers to the LS ground state with a time constant of ∼3 ns. The implications of these observations on the ligand-driven light-induced spin change concept are discussed.

Multi-catalytic reaction modes have attracted widespread attention in synthetic chemistry. The merger of nickel catalysis with photoredox catalysis has offered a powerful platform for synthesis of molecules with attractive properties. Nonetheless, the conceptual development of nickel-catalysed, sensitized electron transfer is of pivotal relevance, but is still greatly limited. Here we describe the development of a radical cross-thioesterification process by nickel-catalysed sensitized electron transfer. The strategy can produce diverse methyl thioesters, which are not only found in natural products, materials and pharmaceuticals but also are widespread precursors in synthetic chemistry and biological processes. This catalytic mode features high chemoselectivity, good functional group tolerance and excellent scalability. Perhaps more important was the finding that various drugs and amino acids were successfully functionalized in this system. Experimental studies, nanosecond transient spectroscopic analysis, and density functional theory calculations reveal that the merger of photocatalytic electron transfer, energy transfer and nickel catalysis plays an essential role in this radical thioesterification reaction.