A 195Pt NMR study on zero-magnetic-field splitting and the phosphorescence properties in the excited triplet states of cyclometalated platinum(ii) complexes.

We report the factors governing zero-magnetic-field splitting (zfs) in the lowest-energy electronically excited triplet (T1) states of cyclometalated platinum(ii) complexes, whose zfs energies between the lowest- (φ1) and highest-energy spin-sublevels (φ3) in the T1 states (ΔEzfs) are already known: [Pt(bhq)(dpm)] (1Pt), [Pt(thpy)(acac)] (2Pt), [Pt(ppy)(acac)] (3Pt), cis-[Pt(thpy)2 (4Pt), and cis-[Pt(ppy)2] (5Pt), where bhq, dpm, thpy, acac, and ppy are benzo[h]quinoline, dipivaloylmethane, 2-(2-thienyl)pyridine, acetylacetone, and 2-phenylpyridine, respectively. As one of the important findings, we show the relationship between the ΔEzfs and 195Pt NMR chemical shifts of the five Pt(ii) complexes. The implications of the ΔEzfs values on the emission properties of the Pt(ii) complexes in acetonitrile at 293 K are also discussed. In particular, we demonstrate that the radiative rate constants of the Pt(ii) complexes correlate with both ΔEzfs and 195Pt NMR chemical shifts.

Synthesis, Structures, and Photoluminescent Properties of Tricyanidonitridorhenium(V) Complexes with Bipyridine-Type Ligands.

Tricyanidonitridorhenium(V) complexes with 2,2'-bipyridine (bpy) derivatives in which the 4 and 4' positions were substituted by X, [ReN(CN)3(X2bpy)]- (X = NMe2, NH2, OMe, Me, Cl, and Br), were newly synthesized and characterized. The structures of the new complexes were determined by single-crystal X-ray analysis. UV-vis spectra of the complexes in dimethyl sulfoxide (DMSO) showed that the peak maximum wavelengths of rhenium-to-π* bpy-type-ligand charge transfer were in the range of 474-542 nm. Cyclic voltammograms in n-(C4H9)4NPF6-DMSO showed one-electron oxidation and reduction waves corresponding to the Re(VI/V) and X2bpy0/- processes, respectively. The new complexes and [ReN(CN)3bpy]- showed photoluminescence in the crystalline phase at 295 and 80 K and in DMSO at 295 K. The origin of the emission in DMSO was attributed to the triplet nature of the rhenium-to-π* bpy-type-ligand charge-transfer transition. Density functional theory calculations showed that the highest occupied and lowest unoccupied molecular orbitals were primarily localized on the dxy orbital of the rhenium and π* orbitals of the bpy-type ligand, respectively.

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 (p Ka(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 (Φ n, 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), Δ E13 or Δ E14, respectively, correlated very well with p Ka( 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 p Ka(L).

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

The temperature ( T) dependences of the emissions from the tetra- n-butylammonium salts of [{Mo6X8}Y6]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}Y6]2-, [{Mo6Br8}Y6]2-, and [{Mo6I8}I6]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}Br6]2- and [{Mo6I8}Cl6]2- exhibited the emission properties different from those of other [{Mo6X8}Y6]2- clusters. The photophysical behavior of these clusters was explained by the excited triplet state spin-sublevel ( Φn, n = 1-4) model irrespective of the nature of X and Y. The zero-magnetic-field splitting energies between the lowest energy (Φ1) and the higher energy spin sublevels (Φ4 or Φ3) caused by the first- or second-order spin-orbit coupling, Δ E14 or Δ E13, were evaluated to be 620-870 or 50-99 cm-1, respectively. We found the linear correlation between the Δ E14 or Δ E13 value and the fourth power of the atomic number ( Z) of the inner halide X: Δ E14 or Δ E13 vs { Z(X)}4 (correlation coefficient: cc = ∼ 0.999). Furthermore, we also found the correlation between Δ E14 or Δ E13 and the 95Mo 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.

A study on the redox, spectroscopic, and photophysical characteristics of a series of octahedral hexamolybdenum(ii) clusters: [{Mo6X8}Y6]2- (X, Y = Cl, Br, or I).

We report a systematic study on the redox, spectroscopic, and photophysical properties of a series of [{Mo6X8}Y6]2- (X, Y = Cl, Br, or I. 1-9). All of the [{Mo6X8}Y6]2- clusters show intense and long-lived phosphorescence in both CH3CN and crystalline phases at 298 K. We found that the emission quantum yields (Φem) of 1-9 increase in the sequences X = Cl < Br < I and Y = I < Br < Cl for given Y and X, respectively. The emission lifetimes (τem) of the clusters also increase in the sequence Y = I < Br < Cl for given {Mo6X8}4+-core clusters. The present data demonstrate that arbitrary combinations of X and Y in [{Mo6X8}Y6]2- could tune τem and Φem in the ranges of 85-300 µs and 0.09-0.47, respectively. Both capping (X) and terminal ligand (Y) effects on the photophysical properties of the clusters are discussed on the basis of the energy gap (i.e., emission energy) dependence of the nonradiative decay rate constant.

Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo6X8}(n-C3F7COO)6]2- (X = Cl, Br, or I).

Temperature (T)-dependent emission from [{Mo6X8}(n-C3F7COO)6]2- (X = Cl (1), Br (2), and I (3)) in optically transparent polyethylene glycol dimethacrylate matrices were studied in 3 K < T < 300 K to elucidate the spectroscopic and photophysical properties of the clusters, in special reference to zero-magnetic-field splitting (zfs) in the lowest-energy excited triplet states (T1) of the clusters. The cluster complexes 1 and 2 showed the T-dependent emission characteristics similar to those of [{Mo6Cl8}Cl6]2-, while 3 exhibited emission properties different completely from those of 1 and 2. Such T-dependent emission characteristics of 1, 2, and 3 were explained successfully by the excited triplet state spin-sublevel (Φn, n = 1-4) model. The zfs energies between the lowest-energy (Φ1) and highest-energy (Φ4) spin sublevels, ΔE14, resulted by the first-order spin-orbit coupling, were evaluated to be 650, 720, and 1000 cm-1 for 1, 2, and 3, respectively. The emission spectra of 1, 2, and 3 in CH3CN at 298 K were reproduced very well by the ΔE14 values and the population percentages of Φn at 300 K. We also report that the ΔE14 values of the clusters correlate linearly with the fourth power of the atomic number (Z) of X: ΔE14 ∝ {Z(X)}4.

Quasi-One-Step Six-Electron Electrochemical Reduction of an Octahedral Hexanuclear Molybdenum(II) Cluster.

We report for the first time quasi-one-step six-electron electrochemical reduction of a new hexanuclear molybdenum(II) bromide cluster having terminal 3,5-dinitrobenzoate ligands: [Mo6Br8(DNBA)6]2-. The electrochemical responses of the cluster were studied based on cyclic (CV), differential pulse, and normal pulse voltammetries, together with the analytical simulations of the CV and spectroelectrochemistry. CV simulations have revealed that the electrochemical reaction of the cluster proceeds in an EEEEEE scheme, and the potential differences between the two adjacent reduction steps are in the range of 15-30 mV. These potential differences indicate quite smooth and quasi-one-step six-electron reduction of the cluster.

Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8}(4+) Core Cluster Complexes by Terminal Carboxylate Ligands.

The reactions between the tetra-n-butylammonium salt of [{Mo6I8}I6](2-) and silver carboxylates RCOOAg (R = CH3 (1), C(CH3)3 (2), α-C4H3O (3), C6H5 (4), α-C10H7 (5), or C2F5 (6)) in CH2Cl2 afforded new carboxylate complexes [{Mo6I8}(RCOO)6](2-). The complexes were characterized by X-ray single-crystal diffraction and elemental analysis, cyclic/differential pulse voltammetry, and IR, NMR, and UV-visible spectroscopies. The emission properties of the complexes 1-6, and those of the earlier reported complexes with R = CF3 (7) and n-C3F7 (8), were studied both in acetonitrile solution and in the solid state. In deaerated CH3CN at 298 K, all of the complexes 1-8 exhibit intense and long-lived emission with the quantum yield and lifetime being 0.48-0.73 and 283-359 µs, respectively. The oxidation (Eox)/reduction (Ered) potentials of the complexes correlate linearly with the pKa value of the terminal carboxylate ligands L = RCOO (pKa(L)). Reflecting the pKa(L) dependences of Eox/Ered, the emission energy (νem) of the complexes was also shown to correlate with pKa(L). The present study successfully demonstrates synthetic tuning of the redox, spectroscopic, and photophysical characteristics of a {Mo6I8}(4+)-based cluster complex with pKa(L).