A Different Perspective on Tuning the Photophysical and Photochemical Properties: The Influence of Constitutional Isomers in Group 6 Carbonyl Complexes with Pyridyl-Mesoionic Carbenes

Exploring novel and existing design principles to tune the photochemical and photophysical properties of transitionmetal complexes is an important goal in contemporary research. Here, we highlight the influence of constitutional isomers of pyridyl-1,2,3-triazolylidene mesoionic carbene (MIC) ligands on the photophysical and photochemical properties of the corresponding tetracarbonyl group 6 metal complexes (M = Cr, Mo, W). All new complexes [M(C-C)] presented herein incorporate a C-C linked pyridyl-MIC ligand and were fully characterized by X-ray diffraction analysis, elemental analysis, and H-1 NMR and IR spectroscopy. Detailed photophysical investigations reveal a single emission in the VIS region, which extends into the NIR with lifetimes of up to 3.5 mu s in the solid state at lower temperatures. The quantum yields were determined for all three complexes, and, in particular, the W-0 complex shows an unusually high quantum yield of 29% compared to the values of 0.02% obtained for the [M(C-N)] isomers investigated in earlier works. Beyond this, the investigated W-0 complex also exhibits an emission at 717 nm in a fluid solution. The combination of luminescence and FTIR-step scan spectroscopy with theoretical calculations reveals an emissive (MLCT)-M-3 state. Irradiation of the presented complexes leads to a clean cleavage of one axial CO ligand. A metastable 16 VE species with a vacant axial coordination site was detected in the solid state at low temperatures. In solution, the respective solvato complexes are formed. A dark reverse reaction is observed, as previously described for the [M(C-N)] analogues. The increased electron density induced by the C-C linked pyridyl-MIC ligand leads to an increased kinetic rate constant for the reformation of the starting species and is also reflected in the lower photodissociation quantum yields.