Terminal Ligand (L) Effects on Zero-Magnetic-Field Splitting in the Excited Triplet States of [{Mo6Br8}L6]2– (L = Aromatic Carboxylates)

We report the emission properties of the octahedral hexamolybdenum(II) bromide-core ({Mo6Br8}4+) clusters having a series of terminal aromatic carboxylate ligands (RCOO), [{Mo6Br8}(RCOO)6]2–, in solution and crystalline phases. The acid dissociation constant of RCOOH (pKa(L)) was shown to govern the redox and emission properties of the clusters. Temperature (T)-controlled emission experiments (3–300 K) demonstrated that the clusters showed large T-dependent emission energies (νem) and lifetimes (τem) because of zero-magnetic-field splitting in the emissive excited triplet (T1) states. The spin sublevel (Φn, n = 1–4) model in the T1 state of the cluster explained very well the T-dependent emission characteristics (νem and τem), irrespective of the clusters studied. Furthermore, we revealed that the energy difference between the lowest-energy (Φ1) and energetically upper-lying third (Φ3) or fourth spin sublevels (Φ4), ΔE13 or ΔE14, respectively, correlated very well with pKa(L). The results are discussed ...