Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

Diisocyanide ligands with a m-terphenyl backbone provide access to Mo-0 complexes exhibiting the same type of metal-to-ligand charge transfer (MLCT) luminescence as the well-known class of isoelectronic Ru-II polypyridines. The luminescence quantum yields and lifetimes of the homoleptic tris(diisocyanide) Mo-0 complexes depend strongly on whether methyl- or tert-butyl substituents are placed in alpha-position to the isocyanide groups. The bulkier tert-butyl substituents lead to a molecular structure in which the three individual diisocyanides ligated to one Mo-0 center are interlocked more strongly into one another than the ligands with the sterically less demanding methyl substituents. This rigidification limits the distortion of the complex in the emissive excited-state, causing a decrease of the nonradiative relaxation rate by one order of magnitude. Compared to Ru-II polypyridines, the molecular distortions in the luminescent (MLCT)-M-3 state relative to the electronic ground state seem to be smaller in the Mo-0 complexes, presumably due to delocalization of the MLCT-excited electron over greater portions of the ligands. Temperature-dependent studies indicate that thermally activated nonradiative relaxation via metal-centered excited states is more significant in these homoleptic Mo-0 tris(diisocyanide) complexes than in [Ru(2,2 '-bipyridine)(3)](2+).