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 (pK(a)(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 ((nu) over tilde (em)) and lifetimes (tau(em)) because of zero-magnetic-field splitting in the emissive excited triplet (T-1) states. The spin sublevel (Phi(n), n = 1-4) model in the T-1 state of the cluster explained very well the T-dependent emission characteristics ((nu) over tilde (em) and tau(em)), irrespective of the clusters studied. Furthermore, we revealed that the energy difference between the lowest-energy (Phi(1)) and energetically upper-lying third (Phi(3)) or fourth spin sublevels (Phi(4)), Delta E-13 or Delta E-14, respectively, correlated very well with pK(a)(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 pK(a)(L).