Terminal Ligand (L) Effects on Zero-Magnetic-Field Splitting in the Excited Triplet States of [{Mo 6 Br 8 }L 6 ] 2- (L = Aromatic Carboxylates)

We report the emission properties of the octahedral hexamolybdenum(II) bromide-core ({Mo Br } ) clusters having a series of terminal aromatic carboxylate ligands (RCOO), [{Mo Br }(RCOO) ] , in solution and crystalline phases. The acid dissociation constant of RCOOH (p K (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 (ν̃ ) and lifetimes (τ ) because of zero-magnetic-field splitting in the emissive excited triplet (T ) states. The spin sublevel (Φ , n = 1-4) model in the T state of the cluster explained very well the T-dependent emission characteristics (ν̃ and τ ), irrespective of the clusters studied. Furthermore, we revealed that the energy difference between the lowest-energy (Φ ) and energetically upper-lying third (Φ ) or fourth spin sublevels (Φ ), Δ E or Δ E , respectively, correlated very well with p K ( L). The results are discussed in terms of the variation of the effective nuclear charge of the Mo metal center(s) in [{Mo Br }(RCOO) ] with that of p K (L).