Pyridine as axial ligand on the [Mo₆Cl₈]⁴⁺ core switches off luminescence

TDDFT calculations including scalar, spin-orbit and solvent effects were carried out on [Mo₆Cl₈Cl₆]^2-, and [Mo₆Cl₈(py)₆]⁴⁺ clusters. The calculations indicate that the contribution of the axial pyridine ligands in the active lowest unoccupied molecular orbitals which are involved in the higher intensity transitions in the [Mo₆Cl₈(py)₆]⁴⁺ cluster are significant (∼31%). This situation differs from all the luminescent [M₆X₈L₆] clusters, where most of the closely spaced excited states are located in the [M₆X₈]^q+ core. Thus, the pyridine axial ligands switch off the cluster luminescence.