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 (ν̃em) 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 (ν̃em 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 in terms of the variation of the effective nuclear charge of the Mo metal center(s) in [{Mo6Br8}(RCOO)6]2– with that of pKa(L).