M.R. Féliz

Bimolecular photoinduced electron transfer in the Marcus inverted region involving the [Re(CO)3(4-phenylpyridine)3]+ metal-to-ligand charge transfer excited state, amines and their corresponding radical products

Abstract Bimolecular electron transfer quenching of the metal-to-ligand charge transfer (MLCT) excited state of Re(CO) 3 (4-phenylpyridine) 3 (F 3 CSO 3 ) by amines was studied in acetonitrile at room temperature. The quenching rate constants ( κ q ) vary from 1.8 × 10 7 to 2.5 × 10 10 M −1 s −1 (diffusion limit). Moreover, the radical recombination reaction (back electron transfer) was also studied by flash photolysis. The back electron transfer rate constants range from 2.8 × 10 5 to 7.0 × 10 9 M −1 s −1 , lying below the diffusion-controlled limit. The rate constants show an inverse dependence on the driving force in the inverted region. All the data points can be fitted using a semiclassical model.

Effect of temperature on hydrogen peroxide photolysis in aqueous solutions

Abstract The photolysis of hydrogen peroxide in dilute aqueous solution (1 × 10−4 M) at various temperatures (15–85°C) and pH (ph 2.5–7) was studied by flash photolysis. The rate of oxygen production under continuous photolysis conditions was measured at room temperature. The rate constants and activation parameters are reported. Evidence for the formation of complexes between hydrogen peroxide and intermediate radicals is presented. The liquid phase data are discussed and compared with those available for the gas phase.

Photophysical properties of Re(CO)3L+3 (L=monoazine) complexes: Electronic delocalization effect in metal-to-ligand charge transfer excited states

The photophysical properties of the series of the [Re(CO)3L3]CF3SO3 (La4-phenylpyridine, 3-phenylpyridine, 4‐benzylpyridine, 4-(4 0 nitrobenzyl)-pyridine and 4,4 0 -bipyridine) have been studied. Emission spectral changes with temperature and biexponential decay of the luminescence both in dichloromethane fluid solution and frozen (77 K) media show two excited states, intraligand (IL) and metal-to-ligand charge transfer (MLCT) in character, emitting with intrinsic rates. Rates of MLCT emission badly correlate with the excited state-ground state energy gap. Departures from the energy gap law are rationalized in terms of excited state distortion and electronic delocalization. # 1998 Elsevier Science S.A. All rights reserved.

Photochemical and photophysical processes in fac-Re(CO)3−(4-phenylpyridine)3(CF3SO3). Steady state and flash photolysis study

Abstract The photophysics and photochemistry of fac-Re(CO)3(4-phenylpyridine)3+ were investigated by monochromatic steady state and flash photolysis. Two parallel photoprocesses, photogeneration of the emissive metal-to-ligand charge transfer (MLCT) state and photoredox dissociation in [Re(CO)3(4-phenylpyridine)22+, (4-phenylpyridine−)], were observed. In the former process, the emission originating from the lower MLCT excited state is electron transfer quenched by triethanolamine (TEOA) producing an Re(CO)3(4-phenylpyridine)3 radical and energy transfer quenched by Cu(TIM)2+ (TIM ≡ 2,3,9,10-tetramethyl-[14]-1,4,8,11-tetraene-N4). The primary species Re(CO)3(4-phenylpyridine)22+ generated in the latter photoprocess undergoes several thermal changes which finally lead to dimeric products. A third photoprocess, namely the photogeneration of the [Re(CO)3(4-phenylpyridine)22+ (4-phenylpyridine−)] species, is involved in the reduction of the copper macrocycle to Cu(TIM)+.

Solvent quenching of the 5D0 → 7F2 emission of Eu(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)3

Abstract Quenching of luminescence of Eu(fod) 3 (fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5 octanedionate) has been studied in different solvents (i.e., acetonitrile, ethanol, methanol, n -butanol, 2-propanol, bencylic alcohol, chloroform, dichloromethane, 1,1-dichloroethane, 1,2 dichloroethane, carbon tetrachloride, benzene, toluene, m -and p -xilene, acetone, ciclohexanone, methylethylketone, methylpropylketone (iso), fluorobenzene, chlorobenzene, bromobenzene, ethyl ether, dioxane and tetrahydrofurane, and deuterated solvents acetonitrile (d 3 ), ethanol (d 1 ), methanol (d 4 ), benzene (d 6 ), acetone (d 6 ) and chloroform (d)). Absorption and emission spectra of the complex in the different solvents, their luminescence lifetimes at room temperature and at the boiling temperature of liquid air, so as in solid phase were measured. Evidences of different mechanisms for quenching of the luminescence is presented and discussed.

Photochemical and catalytic properties of dimeric species of molybdenum(V)

Abstract The chemistry and photochemistry of molybdenum coordination compounds in its several oxidation states is of great importance from basic as well as applied points of view. Molybdenum is a relevant element for the synthesis of many homogeneous and heterogeneous catalysts. This element is also essential in several enzymatic systems. One of the characteristics of the molybdenum chemistry is related to the easy conversion between its oxidation states and to the changes of coordination number, observed particularly between Mo III , Mo V and Mo VI . Many aspects of the chemistry of Mo V have been reviewed. However, the thermal and photochemical reactivity of its complexes was not completely well understood until few years ago. Our studies have shown that aqueous solutions of Mo 2 O 4 (H 2 O) 6−x L x n+ complexes (where L is a monodentate ligand, such as H 2 O, Cl − , NCS − , Br − or bidentate such as cysteine), present dual reactivity; ligand–metal charge transfer (LMCT) and photolabilization of the ligand. The excitation induces photolabilization resulting in the formation of a single bridged μ-oxo-dimer that undergoes dissociative disproportionation into monomeric Mo IV and Mo VI species. Mo IV reacts with coordinated water leading to H 2 and Mo VI . Photoreduction of the complexes to Mo IV Mo V dimers which can be characterized as an LMCT process has also been reported. The main objective of the present review is to present an update of data obtained from studies performed on these systems and the contribution of our laboratory to the understanding of the photochemical and catalytic properties of the di μ-oxo-dimers of molybdenum(V).

First steps in the photochemistry of folate in alkaline medium

Abstract A laser flash photolysis study is reported on alkaline solutions of folic acid in absence of dioxygen. Spectra of the intermediates and their decays are analysed. A biexponential fit of the experimental decay can be observed in the resolution time window of the technique. Intermediates spectra obtained in laser flash photolysis were compared with those generated in pulse radiolysis experiments performed on these solutions under oxidative and reductive conditions. Conventional flash photolysis were also applied in this studies in absence of O2. In this case two processes are detected, one in the millisecond time window and a very slow process which can be followed by normal spectrophotometry after flashing solutions. The experimental behaviour of this system is compared with those reported on pterines derivatives, where absorption changes induced by flash photolysis have been attributed to triplet excited state. The evidence presented here shows that folate suffers an early photochemical transformation. Radicals generation as a consequence of the absorption of radiation is postulated in order to give a plausible explanation of the experimental results.

Equilibrium and kinetic studies of cobalt(II) complexation by folic acid

SummaryThe equilibrium and kinetics of the complexation of CoII by folic acid have been studied at 25 °C, the ionic strength being regulated with KNO3 in the 5.6–7.3 pH range. Kinetic studies were performed using the stopped-flow method. Under our experimental conditions one process was observed; a reaction which took place within a few seconds. The results are consistent with the formation of a 1∶1 complex between the reactants and a mechanism is proposed to account for the observed behaviour. Equilibrium constants for the CoII and folic acid system, as well as activation parameters, are reported.

Flash photolysis of aqueous solutions of bis(μ-oxo) bis(oxo-molybdenum(V))

Abstract The flash photolysis of bis(μ-oxo) bis(oxo-molybdenum(V)) aquo-ion Mo2O4(H2O)2+6 was studied in aqueous solutions of perchloric and hydrochloric acid in the pH range 0.5 – 2.0. Irradiation with light below 350 nm induces the formation of an intermediate which decays through a first-order reaction. The rate constant obtained depends linearly on the inverse of the hydrogen ion concentration. The end products are monomeric molybdenum(VI) and hydrogen. The overall mechanism is proposed from the consideration of the experimental observations and the evidence found in the literature. Other possible mechanisms are also discussed.

Carbon dioxide activation by ClRe(CO)3(4-phenylpyridine)2: steady state and flash photolysis study

Abstract Flash and steady state photolysis of fac-CIRe(CO) 3 (4-phpy) 2 (ph, phenyl; py, pyridine) have been studied in N 2 and CO 2 deaerated acetonitrile solutions containing triethanolamine (TEOA). Irradiations with photonic energies between 300 and 380 kJ mol −1 mostly populate the MLCT excited state which is electron transfer quenched by TEOA. The Re-containing radical ClRe(CO) 3 (4-phpy) 2 .− generated in this process reacts with CO 2 to product CO. Spectroscopic and kinetic information about this reaction is reported.