Charge Transfer Excited Triplet State Research Articles

Tuning the Ground- and Excited-State Redox Potentials of Octahedral Hexanuclear Rhenium(III) Complexes by the Combination of Terminal Halide and N-Heteroaromatic Ligands.

The present study reports that the ground- and excited-state Re6(23e)/Re6(24e) redox potentials of an octahedral hexanuclear rhenium(III) complex can be controlled by systematically changing the number and type of the N-heteroaromatic ligand (L) and the number of chloride ions at the six terminal positions. Photoirradiation of [Re6(μ3-S)8Cl6]4– with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re6(μ3-S)8Cl5(L)]3– (L = 4-dimethylaminopyridine (dmap), 3,5-lutidine (lut), 4-methylpyridine (mpy), pyridine (py), 4,4′-bipyridine (bpy), 4-cyanopyridine (cpy), and pyrazine (pz)). The bis- and tris-lut-substituted complexes, trans- and cis-[Re6(μ3-S)8Cl4(lut)2]2– and mer-[Re6(μ3-S)8Cl3(lut)3]−, were synthesized by the reaction of [Re6(μ3-S)8Cl6]3– with an excess amount of lut in refluxed N,N-dimethylformamide. The mono-L-substituted complexes showed one-electron redox processes assignable to E1/2[Re6(23e)/Re6(24e)] = 0.49–0.58 V versus Ag/AgCl. The ground-state oxidation potentials were linearly correlated with the pKa of the N-heteroaromatic ligand [pKa(L)], the 1H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re6(μ3-S)8X6–n(L)n]n−4 complexes (n = 0, X = Cl, Br, I, or NCS; n = 1–3, X = Cl) showed a linear correlation with the sum of the Lever electrochemical parameters at the six terminal ligands (ΣEL). The cyclic voltammograms of the mono-L-substituted complexes (L = bpy, cpy, and pz) showed one-electron redox waves assignable to E1/2(L0/L–) = −1.28 to −1.48 V versus Ag/AgCl. Two types of photoluminescences were observed for the complexes, originating from the cluster core-centered excited triplet state (3CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state (3MLCT) for L = bpy, cpy, and pz. The complexes with the 3CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with 3MLCT shifted to the longer wavelength in the order L = 4-phenylpyridine (ppy), bpy, pz, and cpy, which agreed with the difference between E1/2[Re6(23e)/Re6(24e)] and E1/2(L0/L–). The calculated oxidation potential of the excited hexanuclear rhenium complex with the 3CC character was linearly correlated with pKa(L), σ, and ΣEL. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.

Photocatalytic oxidation of CO with various oxidants by Mo oxide species highly dispersed on SiO 2 at 293 K

The photocatalytic oxidation of CO into CO 2 with oxidants such as NO, N 2O and O 2 proceeded efficiently on a Mo/SiO 2 with high Mo dispersion under UV light irradiation. It was found that the reaction rate greatly depended on the kind and concentration of the oxidant. Photoluminescence investigations reveal the close relationship between the reaction rate and the relative concentration of the photo-excited Mo 6+-oxide species, i.e. charge transfer–excited–triplet state (Mo 5+–O −) *, under steady-state reaction conditions. Moreover, the photocatalytic oxidation of CO with O 2 in excess H 2 was carried out to test suitability for applications to supplying pure H 2. This reaction was seen to proceed efficiently on Mo/SiO 2 with a high CO conversion of ∼100% and CO selectivity of 99% after 180 min under UV light irradiation, showing higher photocatalytic performance than TiO 2 (P-25) photocatalyst. UV–vis, XAFS, photoluminescence and FT-IR investigations revealed that the high reactivity of the charge transfer–excited–triplet state (Mo 5+–O −) *, with CO as well as the high reactivity of the photoreduced Mo-oxide species (Mo 4+-species) with O 2 to produce the original Mo-oxide species (Mo 6+ O 2−), played a crucial role in the reactions.

Selective photooxidation of methane into methanol by nitric oxide over V-MCM-41 mesoporous molecular sieves

The partial photooxidation of methane by nitric oxide selectively led to methanol formation over the V-MCM-41 catalysts under UV irradiation (λ > 270 nm) at room temperature, while only a complete oxidation of methane could occur in the presence of oxygen. The yield of methanol corresponds with the yield of the photoluminescence of the isolated tetrahedrally coordinated V-oxides species, indicating that the charge transfer excited triplet state of these species are active sites in this photooxidation reaction.

Characterization of the VS-1 catalyst using various spectroscopic techniques and its unique photocatalytic reactivity for the decomposition of NO in the absence and presence of C 3H 8

A Vanadium silicalite (VS-1) catalyst was prepared by hydrothermal synthesis. In situ characterizations were carried out and the photocatalytic reactivity of VS-1 were investigated using dynamic photoluminescence, XAFS (XANES, EXAFS), UV–Vis and FT-IR techniques, along with an analysis of the reaction products. It was found that VS-1 involves a highly dispersed tetrahedrally coordinated V-oxide species having a VO bond within the zeolite framework. VS-1 exhibits photocatalytic reactivity for the decomposition of NO under UV irradiation, leading to the formation of N 2, O 2, N 2O and NO 2. The photocatalytic reaction of NO was dramatically enhanced in the presence of propane, leading to the formation of N 2, propylene, and oxygen-containing compounds such as CH 3COCH 3 and CO 2, etc. with a good stoichiometry. The photocatalytic reactivity of VS-1 was found to be much higher than that of V/SiO 2. The efficiency of the dynamic quenching of the photoluminescence of the V-oxide species in the excited state of VS-1 by small molecules such as NO or propane was found to be larger than that of V/SiO 2. These results indicate that the high reactivity of the charge transfer excited triplet state of the V–O moieties, (V 4+–O −) *, of the VS-1 catalyst plays a significant role in the higher photocatalytic performance as compared with the V/SiO 2 catalyst.

Investigations on the photoluminescence properties of Mo-MCM-41 and the photocatalytic decomposition of NO in the presence of CO

The photocatalytic decomposition of NO into N2 and CO2 on Mo-MCM-41 was found to proceed efficiently in the presence of CO. In situ photoluminescence measurements demonstrated that this reaction proceeds in a redox cycle between alternating Mo6+ and Mo4+ ions. The yields of N2 formation in the photocatalytic reaction correspond with the yields of the photoluminescence of the tetrahedrally coordinated Mo oxide species, indicating that the charge transfer excited triplet state of the tetrahedral Mo oxide species plays a significant role in this reaction, leading to the formation of N2 and CO2 with a good stoichiometry.

Characterization of the Local Structure of the Vanadium Silicalite (VS-2) Catalyst and Its Photocatalytic Reactivity for the Decomposition of NO into N2 and O2

The vanadium silicalite (VS-2) catalyst was hydrothermally synthesized, and a molecular level characterization was carried out by means of in situ photoluminescence, XAFS, ESR, FT-IR, UV−vis, solid-state wide-line 51V NMR, and XRD in the presence or absence of reactant molecules. These spectroscopic investigations of the VS-2 catalyst clearly show that the vanadium oxide moieties are incorporated within the zeolite framework as tetrahedrally coordinated vanadium oxide species having a terminal vanadyl group (VO) with a bond distance of 1.64 Å and three VO bonds with a bond distance of 1.73 Å. VS-2 catalyst also exhibits a charge-transfer absorption band at around 270−340 nm and an intense phosphorescence spectrum as a radiative decay from the charge-transfer excited triplet state at around 450−550 nm with a vibrational fine structure at 77 K. UV irradiation of the catalyst in the presence of NO was found to lead to the efficient photocatalytic decomposition of NO into N2 and O2 at 295 K. Dynamic studies of the phosphorescence in the presence of NO clearly indicate that the charge-transfer excited triplet state of the terahedrally coordinated vanadium oxide species plays a vital role in the photocatalytic decomposition of NO into N2 and O2. The results obtained by these spectroscopic measurements and photocatalytic reactions provide important information on the local structure of the vanadium oxide moieties incorporated within the zeolite framework and on the photocatalytic reactivity of the charge-transfer excited triplet state of the vanadium oxide species at the molecular level.

Photoinduced methanol formation from methane and no at 275 k on highly dispersed vanadium oxide supported on vycor glass and its reaction intermediate species

UV irradiation at 275 K of the highly dispersed V/Vycor oxide catalyst in the presence of NO leads to the formation of N2 and O2. The decomposition reaction of NO proceeds photocatafytically. In the presence of CH4, UV-irradiation of the catalyst at 275 K leads to the formation of C2H6 and C2H4, but this reaction is found to accompany the reduction of the catalyst as well as the formation of CH3 radicals. A dynamic photoluminescence study of the catalyst in the absence and presence of the reactants indicates that the charge transfer excited triplet state of the surface vanadyl-species (V=O) plays a significant role in these photoinduced reactions of NO and CH4. On the other hand, UV-irradiation of the catalyst at 275 K in the presence of a mixture of NO and CH4 leads to the formation of CH3OH in addition to the above products. The higher the ratio of NO/CH4 in the mixture is, the larger the yield of CH3OH becomes and the smaller the yields of C2H6 and C2H4 become, the reaction proceeding catalytically. Thus, the present results not only imply that the highly dispersed supported vanadium oxide catalysts can be utilized as a potential photocatalyst for de-NOx-ing and/or methane activation at normal temperature but also suggest that the photo-formed oxygen species from NO molecules plays a significant role in the photoinduced CH3OH formation from CH4 and NO on V/Vycor oxide catalysts at 275 K.

Activation and properties of Mo=O bonds in Mo/SiO2 catalysts

The existence of the charge transfer excited triplet state [Mo5+-O-] produced by UV-irradiation of Mo/SiO2 catalysts, and its reactivity are evidenced by experiments of photoluminescence, photoinduced metathesis, and photoreduction of CO. Mo5+ ions can be produced separately by thermal activation and O- ions by further adsorption of N2O on those Mo5+ ions. The latter of which are adsorbed on Mo6+ ions are found to be more reactive than O2- of [Mo6+ =O2-] bond. They are able either to add a molecule such as CO or C2H4, or to abstract hydrogen from H2, CH4 or trans-dicyanoethylene, or a CN group form tetracyanoethylene (TCNE). The Mo5+ ions are able to coordinate gas phase ligands when their coordination sphere possesses vacant sites. This is the case for tetracoordinated Mo5+ 4c ions arising from reduction of tetrahedral Mo6+ ions (Eq. (7)). These Mo5+ 4c ions are similar to those produced by UV-irradiaiion (Eq. (2)). In addition, if the adsorbed molecule has a sufficiently large electron affinity, such as TCNE or O2, an electron transfer can occur (Eq. (9) and (17)). The [Mo5+-O-] bond obtained by thermal activation is more difficult to evidence than that obtained with UV-activation because it is not detectable by EPR. However, the EPR results obtained at low temperature show that the O- ions adsorbed on Mo/SiO2 catalysts as well as the [Mo5+-O-] excited triplet state obtained by UV-irradiation of 1Mo6+=O2] interact with methanol (Eq. (16)). They are consistent with the mechanism of methanol oxidation occurring at high temperature (Eq. (4)).

Dynamic photoluminescence studies of vanadium oxide anchored on SiO2 and the effects of added O2 and H2O

Dynamcc photoluminescence studies of V-ozide anchored onto SiO2 are investigated in the absence and presence of O2 or H2O. It is found that the added O2 molecules interact with the charge transfer excited triplet state of tetrahedrally coordinaeed V-oxide species. This results in an efficient quenching of the phosphorescence due to an enhanced radiationless deactivation. On the other hand, the added H2O molecules interact with V-oxide in the ground state, resulting in the coordination change of V-species from tetrahedral to octahedral. These results obtained by dynamic photoluminescence spectroscopy are in good agreement with those obtained by ESR measurements of the photoreduced V4+ ions. From the Stern-Volmer plots for the quenching of the phosphorescence of V-oxide, the absolute quenching rate constants (reaction rate constants) of O2 for the charge transfer excited triplet state of V-oxide anchored onto SiO2 is first determined to be 3.2 × 108 g-cat./mol s at 77 K and 9.3 × 1012 g-cat./mol s at 298 K.

Preparation of highly dispersed anchored vanadium oxides by photochemical vapor deposition method and their photocatalytic activity for isomerization of trans-2-butene

Highly dispersed anchored vanadium oxide catalysts have been first prepared by using facile reaction of the photochemically activated VOCl{sub 3} molecules with surface OH groups on Vycor glass. Anchored vanadium oxides prepared by photochemical method (photo-CVD) exhibit much higher yields in the phosphorescence with its long lifetimes (450 {mu}s). These anchored vanadium oxides exhibit a higher activity for the photocatalytic isomerization reactions of 2-butene, as compared with those obtained for the oxides prepared by conventional impregnation method. A good linear relationship between the yields of photocatalytic isomerization and the yields of phosphorescence from the charge-transfer excited triplet state of the surface vanadyl species in tetrahedral coordination is obtained, indicating that not only the surface vanadyl species play a significant role in the photocatalytic reaction on vanadium oxide catalysts, but also a photochemical vapor deposition method leads to the higher dispersion of vanadium than impregnation method.

Photoinduced metathesis reaction of C3H6 on supported MoO3 catalyst

Ultraviolet irradiation of MoO3 supported on porous Vycor glass (PVG) in the presence of C3H6 has been found to induce the formation of approximately equimolar amounts of C2H4 and 2-C4H8 as well as a small amount of CH3CHO, suggesting that the metathesis reaction of C3H6 occurs. The dependence of the yields upon the excitation wavelength is in good agreement with the excitation band of the phosphorescence of MoO3/PVG, which arises from the charge-transfer transition (Mo6+O2–) [graphic omitted] (Mo5+—O–).* The quenching of the phosphorescence of MoO3/PVG and the decrease in the yield of C3H6 metathesis on adding O2 and CO suggest that the photoinduced metathesis reaction is closely associated with the charge-transfer excited triplet state of MoO3/PVG. From these results, together with those of e.s.r. experiments reported previously, the mechanism of the photoinduced C3H6 metathesis, especially the primary process of metal–carbene formation, is discussed.