Revisiting ligand-to-ligand charge transfer phosphorescence emission from zinc(II) diimine bis-thiolate complexes: it is actually thermally activated delayed fluorescence

In this work we revisit and re-evaluate the photophysical behavior of the prototypical complex [Zn(SC6H4-4-R)2(phen)] as the most in-depth studied type of Zn(II)-based triplet state emitters. Previous reports suggest population of ligand-to-ligand charge transfer (LLCT) states via phenanthroline localized ππ* states, with an energy barrier between the 3ππ* and the bk;1/3LLCT states requiring thermal activation. Besides very weak prompt fluorescence, the dominant radiative mechanism was attributed to phosphorescence. Our photophysical studies, including temperature-dependent quantum yield determination and time-resolved luminescence measurements, reveal a high radiative rate constant kr=3.5×105 s−1 at room temperature and suggest thermally activated luminescence as the major emission path. High-level DFT/MRCI calculations confirm this assignment and provide deeper insight into the excited-state kinetics, including rate constants for the (reverse) intersystem crossing processes. Thus, our study demonstrates that further optimization of the photophysical properties of this type of Zn(II) triplet exciton emitter bears great potential for future application in devices

Forty Years after the Discovery of Its Nucleolytic Activity: [Cu(phen)(2)](2+) Shows Unattended DNA Cleavage Activity upon Fluorination

[Cu(phen)(2)](2+) (phen=1,10-phenanthroline) is the first and still one of the most efficient artificial nucleases. In general, when the phen ligand is modified, the nucleolytic activity of its Cu-II complex is significantly reduced. This is most likely due to higher steric bulk of such ligands and thus lower affinity to DNA. Cu-II complexes with phen ligands having fluorinated substituents (F, CF3, SF5, SCF3) surprisingly showed excellent DNA cleavage activity-in contrast to the unsubstituted [Cu(phen)(2)](2+)-in the absence of the otherwise required classical, bioabundant external reducing agents like thiols or ascorbate. This nucleolytic activity correlates well with the half-wave potentials E-1/2 of the complexes. Cancer cell studies show cytotoxic effects of all complexes with fluorinated ligands in the low mu m range, whereas they were less toxic towards healthy cells (fibroblasts)

An Air- and Moisture-stable Zinc(II) Carbene Dithiolate Dimer Showing Fast TADF and Dexter Energy Transfer Catalysis

A dimeric ZnII carbene complex featuring bridging and chelating benzene-1,2-dithiolate ligands is highly stable towards air and water. The donor-Zn-acceptor structure leads to visible light emission in the solid state, solution and polymer matrices with max between 577-657 nm and, for zinc(II) complexes, unusually high radiative rate constants for triplet exciton decay of up to kr = 1.5105 s-1 at room temperature. Variable temperature and DFT/MRCI studies show that a small energy gap between the 1/3LL/LMCT states of only 79 meV is responsible for efficient TADF. Time-resolved luminescence and transient absorption studies confirm the occurrence of long-lived, dominantly ligand-to-ligand charge transfer excited states in solution, allowing for application in Dexter energy transfer photocatalysis