Ottó Horváth

Photochemistry of copper(I) complexes

Copper(I) complexes with simple inorganic ligands having no acceptor orbitais of low energy (e.g. Cl−, Br−, I−, and NH3) are characterized by CTTS photochemistry. UV excitation of halocuprates(I) in aqueous solutions leads to the formation of a hydrated electron, which undergoes competitive recombination and scavenging reactions. The mechanism of the whole photoinduced redox process in these systems is rather medium-dependent and involves the formation and decay of hydride intermediates. Cyano (as pseudohalo) as well as mixed-ligand cyano-halo complexes of Cu(I) show similar photoredox features, demonstrating the dominance of CTTS reactivity. On the basis of their very remarkable luminescence properties, however, CTTS (or metal-centered 3d94s1) excited states of these coordination compounds can decay via formation of emissive intermediates, which also eject an electron. This route is interpreted by an exciplex mechanism, which apparently plays an important role in the photoinduced charge-transfer properties of the homo- and heteroleptic halocuprates(I). Cluster complexes of the type Cu4X4L4 (X− = halide, L = organic amine) show remarkably rich photophysics. Multiple emissions have been observed, but excited-state assignments are somewhat ambiguous. The photochemistry of these materials has yet to be extensively studied, although some bimolecular redox reactions have been reported. Copper(I) complexes with polypyridine ligands are featured by MLCT reactivity. Formation of an exdplex (but non-emissive) via quenching of these excited states is also an essential reaction of these compounds. Differing from the CTTS character, however, they undergo photoinduced electron transfer only by means of direct redox quenching. Two-photon photochemistry is also shown by these complexes. MLCT excited states of other cationic copper(I) complexes have been involved in both intra- and inter-molecular reactions via oxidative quenching. Photocatalytic applications of Cu(I) coordination com- pounds are also demonstrated.

Photophysics and photochemistry of mercury complexes

Abstract The absorption and emission spectra as well as the photochemistry of mercury and various mercury compounds in solution are reviewed. Special attention is paid to atomic mercury, the clusters Hg22∗, Hg32+, Hg34+, and a variety of Hg(II) complexes of the type HgX2, HgX3− and HgX42− with Xhalide or organometallic fragment.

Degradation of surfactants by hydroxyl radicals photogenerated from hydroxoiron(III) complexes

The Fe(III)-photoinduced oxidation of anionic lauryl sulfate (LS-) and cationic cetyltrimethylammonium (CTA+) surfactants has been investigated in aqueous solution. Competition experiments using 2-propanol showed that the initial rate of disappearance is proportional to the concentration of the photogenerated HO* radicals scavenged by the surfactants (the degradation of lauryl sulfate involves attack by HO* only) and no direct photoinduced charge-transfer reaction occurs between the Fe(III) species and the surfactant ions. Ageing of the Fe(III) solution did not significantly influence the efficiency of photodegradation in air-saturated systems. Conversion of Fe(III) to Fe(II) in aerated solution reached a steady-state level of ca. 50% after 2 h irradiation. In nitrogen-saturated systems, the rate of surfactant oxidation decreased due to the total reduction of Fe(III). Addition of H2O2 doubled the quantum yield of the disappearance of both detergents as a result of the photo-Fenton reaction. The photoinduced oxidation of both surfactants was most efficient in acidic solutions of pH 2-3, without H2O2, and for the photo-Fenton system; the quantum yields are phi(NaLS) = 0.011, phi(CTAB) = 0.012 without H2O2, and phi(NaLS) = 0.024, phi(CTAB) = 0.027 in the photo-Fenton system with irradiation at 366 nm. For the disappearance of 4 x 10(-4) M detergent, due to the first oxidation step, 4 h of irradiation (at pH 2.6) is sufficient, whereas 100% mineralization of the total organic carbon content requires prolonged photolysis for at least 10 h. The formation of carbon dioxide dramatically accelerated after a 2 h induction period (1 h in the photo-Fenton system), indicating the cleavage of the long hydrocarbon chains to shorter intermediates in the first stage of the mechanism. The following step is total mineralization of these smaller compounds, which were identified as mostly hydroxy acids via GC-MS.

Photocatalytic degradation of benzenesulfonate on colloidal titanium dioxide

Titanium dioxide-mediated photocatalyzed degradation of benzenesulfonate (BS) was investigated by monitoring chemical oxygen demand (COD), total organic carbon (TOC) content, sulfate concentration, pH as well as the absorption and emission spectral changes in both argon-saturated and aerated systems. Liquid chromatography-mass spectrometry analysis was utilized for the detection of intermediates formed during the irradiation in the UVA range (λ(max) = 350 nm). The results obtained by these analytical techniques indicate that the initial step of degradation is hydroxylation of the starting surfactant, resulting in the production of hydroxy- and dihydroxybenzenesulfonates. These reactions were accompanied by desulfonation, which increases [H(+)] in both argon-saturated and aerated systems. In accordance with our previous theoretical calculations, the formation of ortho- and meta-hydroxylated derivatives is favored in the first step. The main product of the further oxygenation of these derivatives was 2,5-dihydroxy-benzesulfonate. No decay of the hydroxy species occurred during the 8-h irradiation in the absence of dissolved oxygen. In the aerated system much more efficient desulfonation and hydroxylation, moreover, a significant decrease of TOC took place at the initial stage. Further hydroxylation led to cleavage of the aromatic system, due to the formation of polyhydroxy derivatives, followed by ring fission, resulting in the production of aldehydes and carboxylic acids. Total mineralization was achieved by the end of the 8-h photocatalysis. It has been proved that in this photocatalytic procedure the presence of dissolved oxygen is necessary for the cleavage of the aromatic ring because hydroxyl radicals photochemically formed in the deaerated system too alone are not able to break the C-C bonds.

Equilibrium, Photophysical, Photochemical, and Quantum Chemical Examination of Anionic Mercury(II) Mono- and Bisporphyrins†

Mercury(II) ion and 5,10,15,20-tetrakis(parasulfonato-phenyl)porphyrin anion can form 1:1, 2:2, and 3:2 (metal ion/porphyrin) out-of-plane (OOP) complexes, from which Hg2P2(8-) has not been identified until now. Identification of this species significantly promoted the confirmation of the composition and the precise elucidation of the equilibrium of Hg3P2(6-). Since the formation of each complex is too fast, their kinetic behavior was studied from the side of dissociation. The rate-determining step in dissociations, as well as in the formation of the 2:2 complex, that is, the dimerization of 1:1 complex, proved to be virtually first-order under these conditions, while the consecutive formations of HgP(4-) and Hg3P2(6-) are second-order reactions. The equilibria can be spectrophotometrically investigated because the Soret- as well as the Q-absorption bands of the free-base ligand are more and more red-shifted in the series of 1:1, 2:2, and 3:2 complexes, and the split of Q-bands disappears as the singlet-1 excited states become degenerate; in the case of bisporphyrins, the bands broaden, especially in the longer-wavelength region of the spectra. The quantum yield and the lifetime of S1-fluorescence from the macrocycle is decreased by the insertion of a mercury(II) ion due to distortion, and in bisporphyrins the luminescence totally ceases because their more complicated structure promotes other ways of energy dissipation. The lifetime of the triplet excited-state is also reduced by metalation. The transient absorption measured upon excitation of Hg3P2(6-) probably originates from Hg2P2(8-) formed by efficient photodissocation during the laser pulse. This photoinduced dissociation is characteristic to out-of-plane complexes, but in metallo-monoporphyrins it needs the energetically higher Soret-excitation; in bisporphyrins, it can take place during irradiation at the longer Q-wavelengths. Investigation of the intramolecular photoredox reactions has proved that for the increased efficiency of the indirect photoinduced LMCT, not the redox potential, but the position of the metal center is responsible. The two orders of magnitude higher photoredux quantum yield for the 3:2 complex, compared to that of the 2:2 species, can be explained by the repulsive effect of the inner mercury(II) ion pushing the other two farther out of the ligand cavity. In bisporphyrins the second excited states are photochemically more reactive than the first ones, while most of the photochemical processes of HgP(4-) originate from the first excited state. According to our quantum chemical calculations, the mercury(II) ion causes the expansion of the porphyrin-cavity; therefore its out-of-plane position is smaller than the value expected based on its ionic radius. In the hitherto unknown 2:2 dimer two 1:1 saucer-shaped monomers are kept together by secondary forces, mostly by pi-pi interaction, but their relative arrangement was not unequivocally determined by the two DFT functionals used. The arrangements with a symmetry axis or plane perpendicular to both rings are not favored; instead, the two monomers are shifted along the porphyrin planes, either in a Hg-P-Hg-P or a Hg-P-P-Hg order. Our time-dependent density functional theory (TD-DFT) calculations indicate that the electronic spectra are not very sensitive to the structure of the dimer, even though the environment of the porphyrin rings is quite different if one of the metal ions is between or outside of both macrocycles. The calculated spectral shifts agree only partially with the experimental data. The TD-DFT calculations suggest that the chromophores are not fully independent in the bisporphyrins and that the observed spectral shift cannot be uniquely assigned to the geometrical distortion of the porphyrin macrocyle.

Spectra, equilibrium and photoredox chemistry of tri- and tetraiodoplumbate(II) complexes in acetonitrile

Abstract The stepvise formation constant of the kinetically labile PbI42 complex was determined in acetonitrile (K4 = 27 - 4 dm3 mol−1). The lowest-energy bands in the absorption spectra of the tri- and tetraiodoplumbate(II) complexes are attributed to ligand-to-metal charge transfer transitions. Photolysis of these complexes in solution results in the formation of Pb(I) and I or I2 as primary products. The quantum yield of this reaction is 0.10 ± 0.005 for both complexes at 355-nm excitation. The efficiency of the overall reaction (with the formation of I3− as end product) is higher for the PbI3− than for the PbI42 complexes (Φ3 = 0.026 ± 0.002 and Φ4 = 0.010 ±0.004) in air-saturated system. λirr=366 nm, Cpb(11)= 1 × 100−4 mol dm 3) due to the different repulsion towards the negatively charged reactants. The overall quantum yield is essentially determined by the competitive dark reactions of the primary products, mostly by those of I2 . One key step in the mechanism is the reaction between the I2− intermediate and the ground-state lead(II) complexes, with rate constants of 3.4 ± 1.1 × 107 dm3 mol−1s−1 and 7.0±2.0 × 107 dm3 mol−1 s−1 at 37 and 94 partial molar percentages of PbI3− . respectively.

Air-stable, heme-like water-soluble iron(II) porphyrin: in situ preparation and characterization

Preparation of the water-soluble, kinetically labile, high-spin iron(II) tetrakis(4-sulfonatophenyl)porphyrin, Fe(II)TPPS4−, has been realized in neutral or weakly acidic solutions containing acetate buffer. The buffer played a double role in these systems: it was used for both adjusting pH and, via formation of an acetato complex, trapping trace amounts of iron(III) ions, which would convert the iron(II) porphyrins to the corresponding iron(III) species. Fe(II)TPPS4− proved to be stable in these solutions even after saturation with air or oxygen. In the absence of acetate ions, however, iron(II) ions play a catalytic role in the formation of iron(III) porphyrins. While the kinetically inert iron(III) porphyrin, Fe(III)TPPS3−, is a regular one with no emission and photoredox properties, the corresponding iron(II) porphyrin displays photoinduced features which are typical of sitting-atop complexes (redshifted Soret absorption and blueshifted emission and Q absorption bands, photoinduced porphyrin ligand-to-metal charge transfer, LMCT, reaction). In the photolysis of Fe(II)TPPS4− the LMCT process is followed by detachment of the reduced metal center and an irreversible ring-opening of the porphyrin ligand, resulting in the degradation of the complex. Possible oxygen-binding ability of Fe(II)TPPS4− (as a heme model) has been studied as well. Density functional theory calculations revealed that in solutions with high acetate concentration there is very little chance for iron(II) porpyrin to bind and release O2, deviating from heme in a hydrophobic microenvironment in hemoglobin. In the presence of an iron(III)-trapping additive that is much less strongly coordinated to the iron(II) center than the acetate ion, Fe(II)TPPS4− may function as a heme model.

Photocatalytic degradation of 1,5-naphthalenedisulfonate on colloidal titanium dioxide.

Photocatalytic degradation of 1,5-naphthalenedisulfonate (NDS) was investigated by monitoring the absorption and emission spectral changes, chemical oxygen demand, total organic carbon (TOC) content as well as pH and sulfate concentration. Intermediates formed during the irradiation were also detected by liquid chromatographic-mass spectrometric analysis. The results obtained by the applied analytical techniques clearly indicate that the initial step of degradation is oxygenation (hydroxylation) of the starting surfactant resulting in the formation of an 8-hydroxy derivative, although desulfonation and some mineralization, that is, decrease of TOC indicating carbon dioxide generation, also take place at this stage. Further oxygenation and desulfonation lead to the destruction of the diaromatic naphthalene system, then to ring fission, producing diols, aldehydes, and carboxylic acids on the side-chains. A tentative scheme involving possible pathways of degradation is proposed, taking the intermediates detected by mass spectrometry into consideration. On the basis of the results of quantum chemical calculations, the most possible points of attack by HO radical were identified, supplementing the MS results, and elucidating the initial oxidation step in the degradation of NDS and the benzenesulfonate (BS) intermediate. Thus, in the case of NDS para position is favored for hydroxylation, while for BS, formation of the ortho-hydroxy derivative is preferred.

Photoinduced electron transfer and luminescence in aqueous bromocuprate(I) complexes

Abstract Luminescence, laser flash photolysis and continuous photolysis studies of equilibrated solutions of CuBr 2 − and CuBr 3 2− were carried out in the UV region. Excitation of the absorption band at 279 nm in CuBr 3 2− results in emission centered at 475 nm, with a lifetime of 710 ns in neutral solution, and quenched by hydronium ions with a rate constant of 6.2 × 10 8 M −1 s −1 . Neutral solutions of the complexes produce hydrated electrons when they absorb 15 ns pulses of laser light at 266 nm. The electrons are scavenged by the copper(I) species itself with a second-order rate constant of 7.5 × 10 9 M −1 s −1 , and by hydronium ions with a second-order rate constant of 1.3 × 10 10 M −1 s −1 at 0.5 M ionic strength. Individual quantum yields of electron production, determined at 1 M ionic strength, are 0.67 for CuBr 2 − and 0.34 for CuBr 3 2− . Continuous photolysis of acidic solutions of the complexes reveals a dependnece on hydronium ion concentration which is different from that for the scavenging of electrons, a dependence on Br − concentration and an action spectrum consistent with the 279 nm absorption band as the photoactive state. These plus other observations and arguments support a mechanism for dihydrogen evolution, involvin the formation of a steady state hydride intermediate which reacts with H + to form dihydrogen.

Equilibrium, photophysical, photochemical and quantum chemical examination of anionic mercury(I) porphyrins

Hg2 2+ ion and 5,10,15,20-tetrakis(parasulphonato-phenyl)porphyrin anion can form 2:1 (2 clusters:1 porphyrin) and 2:2 complexes, while the formation of the 1:1 species is not observable: it is only an intermediate, similarly to the cases of other large metal ions of small charge-density. The differences between mercury(I) and mercury(II) porphyrins in the composition of monoporphyrins (2:1 vs. 1:1), in the stability and the Soret absorption based on the arrangement of 2:2 complexes (asymmetric vs. probably symmetric sandwich-structure), in the kinetic behavior (molecularities and the special dimerization of Hg II P 4- ), in the product of the photoinduced dissociations of 2:2 bisporphyrins (free-base ligand vs. 1:1 complex) can prove that no mercury(II) porphyrins can form due to the possible disproportion of dimercury(I) ions. However, the similarities in the absorption, photophysical and photochemical features (also to other out-of-plane metalloporphyrins) suggest that the out-of-plane position of metal center and the distorted structure of complexes may be responsible for these common properties, the so-called sitting-atop characteristics. Moreover, the calculated structural data of the theoretically studied 1:1 mercury(I) porphyrin are very similar to those of Hg II P as a consequence of the charge separation in the cluster based on the strength of metal-nitrogen bonds. In the case of the 2:2 species, neither the increased distance (because of the Hg-Hg bond), nor the absence of 45 o rotation of the two ligands can significantly modify the π-π interaction because its both measured and calculated absorption spectra are similar to those of Hg II 2P2.