Periodate-Mediated Aerobic Oxidation of Sulfides over a Bifunctional Porphyrin-polyoxometalate Catalyst: Photosensitized Singlet Oxygen Oxidation of Iodate to Periodate.

Regeneration of terminal oxidants by molecular oxygen in metal-catalyzed oxidations of organic substrates has the advantage of avoiding the use of stoichiometric amounts of hazardous and/or expensive reagents to meet (some of) the green chemistry requirements. In the present study, photosensitized singlet oxygen oxidation of iodate to periodate has been used to regenerate the oxidant in polyoxometalate (POM)-catalyzed oxidation of sulfides to sulfoxides with periodate in water. To the best of our knowledge, it is the first report on singlet oxygen oxidation of iodate to periodate. In order to determine the contribution of photooxidation and oxidation pathways in the formation of sulfoxide, the oxidation of diphenyl sulfide with a very low reactivity toward aerobic photooxidation was studied; a sevenfold increase in the conversion of the sulfide to the diphenyl sulfoxide was observed for the reaction conducted in the presence of H2TMPyP-PW12O40/IO3-/O2/hν compared to that in the presence of H2TMPyP-PW12O40/O2/hν. Also, under the same conditions, a ca. 1.5-fold increase was observed in the case of methyl phenyl sulfide, which shows high reactivity toward both the oxidation and photooxidation reactions. A porphyrin-POM nanocomposite formed by the electrostatic immobilization of meso-tetra(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) on PW12O40 was employed for the one-pot oxidation and photooxidation reactions. Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), diffuse-reflectance UV-vis spectroscopy, thermal gravimetric analysis, and Fourier transform infrared were used to characterize the formation of the hybrid compound. An average particle size of 42 nm was estimated for H2TMPyP-PW12O40 from XRD peak broadening using the Scherrer equation. Also, FESEM images showed the formation of nearly spherical nanoparticles with a size of ca. 200 nm. The redshift of the Soret band of H2TMPyP upon immobilization on POM was attributed to strong N-H···O hydrogen-bond interactions between POM and porphyrin.

Significantly Increased Stability of Donor-Acceptor Molecular Complexes under Heterogeneous Conditions: Synthesis, Characterization, and Photosensitizing Activity of a Nanostructured Porphyrin-Lewis Acid Adduct.

While the BF3 complexes of meso-tetra(aryl)porphyrins are readily decomposed into their components under aqueous conditions, immobilization of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) on a nanosized polymer (sodium salt of Amberlyst 15, nanoAmbSO3Na) formed a water-stable BF3 complex applicable for efficient aerobic photooxidation of 1,5-dihydroxylnaphthalene and sulfides under green conditions. NanoAmbSO3@H2TMPyP(BF3)2 was characterized by diffuse reflectance UV-vis spectroscopy, dynamic light scattering, thermal gravimetric analysis, Brunauer-Emmett-Teller analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques. The catalyst was successfully used for 10 consecutive reactions with no detectable degradation of the complex and decrease in the catalyst activity. NanoAmbSO3@H2TMPyP(BF3)2 was also completely stable toward dissociation to its components under different light conditions in acetonitrile. The singlet oxygen quantum yields φΔ of H2TMPyP, nanoAmbSO3@H2TMPyP, and their molecular complexes with BF3, determined chemically by using 1,3-diphenylisobenzofuran, revealed substantially higher values in the case of the heterogenized porphyrin and molecular complex.

Effect of the Number of Anchoring and Electron-Donating Groups on the Efficiency of Free-Base- and Zn-Porphyrin-Sensitized Solar Cells.

A series of porphyrin compounds, free base (H2P) and their Zn (II) metallated analogues (ZnP), bearing one, two or three carboxylic acid groups, have been synthesized, characterized, and used as sensitizers in dye sensitized solar cells (DSSCs). The performance of these devices has been analyzed, showing higher efficiencies of those sensitized with ZnP compounds. These results have been explained, on one hand, taking into account the electronic character of the metal ion, which acts as mediator in the injection step, and, on the other, considering the number of anchoring groups, which determines both the stereoelectronic character of the dye and the way it binds to TiO2 surface.

Unusual near-white electroluminescence of light emitting diodes based on saddle-shaped porphyrins.

In contrast to the red electroluminescence emission frequently observed in porphyrins based OLED devices, the present devices exhibit a nearly white emission with greenish yellow, yellowish green and blue green hues in the case of Fe(II)(TCPPBr6) (TCPPBr6 = ß-hexabromo-meso-tetrakis-(4-phenyl carboxyl) porphyrinato), Zn(II)(TPPBr6) and Co(II)(TPPBr6), respectively.

Saddle-shaped porphyrins for dye-sensitized solar cells: new insight into the relationship between nonplanarity and photovoltaic properties.

We report on the theoretical and experimental studies of the new dye-sensitized solar cells functionalized with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin zinc(II) complexes bearing 2- and 8-bromo substituents at the ß positions. In agreement with the results of TD-DFT calculations, the absorption maxima of di- and octa-brominated Zn(II) complexes, ZnTCPPBr2 and ZnTCPPBr8, exhibited large red-shift compared to that of the non-brominated free base porphyrin (H2TCPP). Furthermore, DFT calculations showed that the higher stabilization of the LUMO levels relative to the HOMO ones makes the HOMO-LUMO gap of the brominated Zn-porphyrins models smaller compared to that of the nonbrominated counterparts, which explains the red shifts of the Soret and Q bands of the brominated compounds. Solar cells containing the new saddle-shaped Zn(II) porphyrins were subjected to analysis in a photovoltaic calibration laboratory to determine their solar to electric energy conversion. In this regard, we found that the overall conversion efficiency of ZnTCPPBr8 adsorbed on TiO2 nanocrystalline films was 5 times as large as that of ZnTCPPBr2 adsorbed on the same films. The effect of the increasing number of Br groups on the photovoltaic performance of the complexes was compared to the results of computational methods using ab initio DFT molecular dynamics simulations and quantum dynamics calculations of electronic relaxation to investigate the interfacial electron transfer (IET) in TCPPBrx/TiO2-anatase nanostructures. Better IET in ZnTCPPBr8 compared to ZnTCPPBr2, and in H2TCPP was evaluated from interfacial electron transfer (IET) simulations. The IET results indicate that electron injection in ZnTCPPBr8-TiO2 (τ = 25 fs) can be up to 5 orders of magnitude faster than ZnTCPPBr2-TiO2 (τ = 125 fs). Both experimental and theoretical results demonstrate that the increase of the number of bromo-substituents at the ß-pyrrole positions of the porphyrin macrocycle created a new class of porphyrin-based DSSC, which exhibits a remarkable increase in the photovoltaic performance compared to non-brominated porphyrins.

Hydrogen bond controlled adduct formation of meso-tetra(4-sulfonatophenyl)porphyrin with protic acids: a UV-vis spectroscopic study.

Interaction of meso-tetra(4-sulfonatophenyl)porphyrin (H2tppS4) with weak and strong protic acid have been studied by UV-vis spectroscopy in water, dichloromethane and methanol. Different shifts of the Soret and Q(0,0) bands in the three solvents, the aggregation of diprotonated species and the stability of porphyrin-acid adducts in the solution, may be explained by the inter- and intramolecular hydrogen bonds. Whilst, the addition of excess amounts of tetra-n-butylammonium chloride to H2tppS4(Cl)2 in dichloromethane has little to no effect on the UV-vis spectrum of the dication, gradual addition of tetra-n-butylammonium hydrogen sulfate to the dichloromethane solution of H2tppS4(H2SO4)2 leads to the degradation of adducts and the release of porphryin. The results of this study clearly show the crucial role played by hydrogen bonds between the porphyrin diprotonated species and the counter ion in the stability of porphyrin diacids in solution.

A UV-vis spectroscopic study of 1:2 adduct formation of some free-base meso-tetraaryl- and meso-tetraalkylporphyrins with PhSnCl3 and (CH3)2SnCl2.

Molecular complexation of meso-tetraalkylporphyrins (with primary alkyl residues) (alkylpor), para-substituted meso-tetraphenylporphyrins (para=H, CH(3), Cl, OCH(3), NO(2)) (arylpor) and meso-teramesitylporphyrin (H(2)tmp) with PhSnCl(3) (A(I)) and (CH(3))(2)SnCl(2) (A(II)) led to the red shift of the Soret bands. Comparison of the Soret bands of A(I) (or A(II))(2)arylpor with those of (A(I) or A(II))(2)alkylpor shows a red shift of ca. 10-20nm for the formers with respect to the latters. According to the very similar saddled porphyrin core reported for the dications of H(2)tpp and H(2)t(n-Pr)p with CF(3)COOH, it may be concluded that in addition to the Q(0,0) bands the Soret bands may be also affected by the pi-donation of meso-aryl substituents to the porphyrin aromatic system. An overlap between one of the e(1g) orbital of phenyl groups and the a(1u) orbital of porphyrin core is suggested to explain the observed red shifts in the case of pi-donor para-substituents. Very similar red shift of the Soret bands of (A(I) or A(II))(2)H(2)t(4-OMe)pp and (A(I) or A(II))(2)H(2)t(4-NO(2))pp with respect to that of (A(I) or A(II))(2)H(2)t(n-Pr)p, in spite of the pi-electron withdrawing effects of para-NO(2) groups, seems to be resulted in by the pi-interaction of LUMOs of (4-NO(2))phenyl substituent with e(g) orbital of porphyrin core; this interaction would lead to the stabilization of e(g) orbital and the observed red shift of the Soret band. However, due to the distribution of electron densities of the a(2u) orbital on meso-positions, as well as the central nitrogens, the Q(0,0) bands are more strongly affected by the pi-donation of meso-aryl groups to the porphyrin core.