Triplet Metal-to-ligand Charge-transfer State Research Articles

Specific Role of Water in Radiationless Transition from the Triplet MLCT States of Tris(polypyridine) Complexes of Osmium(II)

The solvent effect on the radiationless transition rates from the excited triplet MLCT (metal-to-ligand charge-transfer) states of tris(bipyridine)osmium(II) (Os(bpy)32+; bpy = 2,2‘-bipyridine) and tris(phenanthroline)osmium(II) (Os(phen)32+; phen = 1,10-phenanthroline) was investigated in water−methanol mixed solvent and its deuterated analogues at various temperatures. Similar measurements were carried out in H2O−D2O and H2O−dioxane mixed solvents. A significant isotope effect of the solvent on the rate constant of the radiationless relaxation to the ground state was observed in water, while only a small one was recognized in methanol. The radiationless transition rate in the H2O−D2O mixture depends linearly on the mole fraction of D2O, while that in the H2O−methanol mixture shows no linear dependence on the mole fraction of methanol. Those solvent effects of the radiationless transition are described as the energy transfer from the metal complex to the water molecule included in the Os(II) complex. It was suggested that the observed radiationless transition rates were strongly affected by the microscopic environment around the Os(II) complex from the results in the mixed solvents.

A CASSCF study of the relaxation effects in the lowest 3MLCT excited state of HMn(CO) 3(dab)

The search for minimum energy structures based on the gradient method of optimization and on CASSCF reference wavefunctions is reported for the electronic ground state and the lowest triplet MLCT (metal-to-ligand charge transfer) excited state of HMn(CO) 3(dab; dab = 1,4-diaza-1,3-butadiene). A comparison between the structures obtained on the basis of this correlated method with those arising from gradient SCF calculations points to the importance of correlation effects in structure determination. The metal-carbonyl bond lengths are shortened by around 10% when going from the single-determinant approach to the multiconfigurational treatment due to a better description of the π back bonding interaction. A large CASSCF active space including the σ metal-hydrogen bonding orbital and its antibonding counterpart σ ∗ gives a good description of the hydrogen trans effect in this molecule. The deformation of the molecule in the lowest triplet MLCT excited state is characterized by an elongation of the order of 18% of the metal-CO axial bond, a small shortening (less than 2%) of the metal-hydrogen bond and a bending of the hydrogen towards the equatorial carbonyls. The metal-CO equatorial bonds are only affected slightly when going from the electronic ground state to the lowest triplet MLCT state.

Synthesis, Structure, and Spectroscopic Properties of Ortho-Metalated Platinum(II) Complexes

The ortho-metalated Pt(II) complexes Pt(ppy)(CO)Cl (1), Pt(ptpy)(CO)Cl (2), and Pt(ppy)(Hppy)Cl (3) (where ppy and ptpy are respectively the ortho-C-deprotonated forms of 2-phenylpyridine and 2-p-tolylpyridine and Hppy is 2-phenylpyridine) have been prepared. The CO ligand is coordinated trans to the nitrogen atom of the ortho-metalated ligand and exerts a strong trans effect resulting in a relatively long Pt-N bond [2.114(19) {angstrom}]. This structure shows both the bidentate ppy ligand and the monodentate Hppy with the nitrogens of these ligands trans to each other. The UV/vis electronic absorption spectra of 1-3 have intense bands in the near-UV region ({approximately}375 nm) which have been assigned as metal to ligand charge transfer (MLCT) transitions, and higher energy bands were assigned as ligand-centered transitions. Each complex exhibits relatively long-lived structured emissions in the solid state at ambient temperature and at 77 K and 77 K glassy toluene solutions. These emissions are proposed to originate from triplet MLCT states. Notably, in solution both the lifetime and spectrum of 2 proved to be a function of the concentration, a phenomenon interpreted in terms of the propensity of square planar d{sup 8} complexes to oligomerize. In contrast, the more sterically hindered complex 3 displayed no such tendency towardmore » oligomerization.« less

Improvement of the photochemical reactivity of [Cu(dmp)P2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline; P = tertiary phosphine) by substituting triphenylphosphine with larger phosphines: application to photoreduction of methylviologen

Cu(I) complexes containing heteroaromatic ligands such as 2,2'-bipyridine (bpy) or 1,10-phenanthroline exhibit a metal-to-ligand charge-transfer (MLCT) band in the visible to near-ultraviolet spectral region and have a triplet MLCT state as the lowest energy excited state like (Ru(bpy)3)S which has been applied extensively to the photochemical reduction of water to hydrogen gas. Nevertheless, application of those Cu(I) complexes to the photochemical reduction of water has not been carried out, excepting only one report of Sauvage et al. Recently, they reported that Cu(I) complexes containing a heteroaromatic ligand, (Cu(NN)(PPh3)2) (NN = 2,9-dimethyl-1,10-phenanthroline (dmp) or 4,4',6,6'-tetramethyl-2,2'-bipyridine), found application as a sensitizer in photocatalytic reduction of methylviologen (MVS ) upon near-UV-light irradiation. Unfortunately, however, the quantum yield for MVS reduction (phi/sub MV /) is lower than that of (Ru(bpy)3)S . In this work, they have succeeded in improving the photoreducing activity of (Cu(dmp)(PPh3)S) by using such bulky and/or donating tertiary phosphines as cyclohexyldiphenylphosphine, PCyPh2, and tris(p-methoxyphenyl)phosphine, P(p-C6H4OMe)3, instead of PPh3. Another emphasis presented here lies in investigating the phosphine ligand effect on the photoreactivity of Cu(I) complexes, because such investigation has not been carried out yet to their knowledge.