Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo6X8}Y6]2-(X, Y = CI, Br, I)

The temperature (T) dependences of the emissions from the tetra-n-butylammonium salts of [{Mo6X8}Y-6](2-) (X, Y = Cl, Br, and I) in optically transparent polyethylene glycol dimethacrylate matrixes were studied in the T range of 3-300 K. [{Mo6Cl8}Y-6](2-), [{Mo6Br8}Y-6](2-), and [{Mo6I8}I-6](2-) showed the T-dependent emission characteristics similar to those of other hexanuclear Mo(II), Re(III), and W(II) clusters reported previously, while [{Mo6I8}Br-6](2-) and [{Mo6I8}Cl-6](2-) exhibited the emission properties different from those of other [{Mo6X8}Y-6](2-) clusters. The photo physical behavior of these clusters was explained by the excited triplet state spin-sublevel (Phi(N), wn = 1-4) model irrespective of the nature of X and Y. The zero-magnetic-field splitting energies between the lowest energy (Phi(1)) and the higher energy spin sublevels (Phi(4) or Phi(3)) caused by the first- or second-order spin-orbit coupling, Delta E-14 or Delta E-13, were evaluated to be 620-870 or 50-99 cm(-1), respectively. We found the linear correlation between the Delta E-14 or Delta E-13 value and the fourth power of the atomic number (Z) of the inner halide X: Delta E-14 or Delta E-13 vs {Z(X)}(4) (correlation coefficient: cc = similar to 0.999). Furthermore, we also found the correlation between Delta E-14 or Delta E-13 and the Mo-95 NMR chemical shift of the cluster. These findings gave very important insight into the spin-orbit coupling and zero-magnetic-field splitting in the excited triplet states of transition metal complexes.