Search

The surface of commercial TiO2 compounds (P25 and ST01) has been modified with Ag nanoparticles induced by radiolysis. On P25, the Ag nanoclusters are very small (1–2 nm) and homogeneous in size while on ST01, two populations of nanoparticles are obtained, small nanoparticles (1–2 nm) and larger ones (mean diameter 7 to 12 nm depending on the silver loading). The photocatalytic properties of Ag-modified TiO2 have been studied for phenol photodegradation in aqueous suspensions under UV and visible light. Their electronic properties have been studied by time resolved microwave conductivity (TRMC) to follow the charge-carrier dynamics. TRMC measurements show that the TiO2 modification (P25 and ST01) with Ag nanoparticles plays a role in charge-carrier separations increasing the activity under UV light. Indeed, Ag nanoparticles act as electron scavengers, decreasing the charge carrier recombination. TRMC measurements show also that more electrons are produced in the conduction band of P25 under UV illuminatio...

Our original approach consists of observing the dynamics of homogeneous crystal growth throughout the volume of highly supersaturated solutions. The crystal growth is induced at room temperature by a powerful single nanosecond laser pulse and is observed using a synchronized time-resolved optical detection. The supersaturated solution readily transforms after the pulse into a homogeneous gel-like slurry consisting of nanometric crystals. From the analysis of the signals via a kinetic model, we derive the elementary accretion rate constant, the nuclei concentration, and its dependence upon the initial supersaturation concentration. This approach could permit a quantitative study of the homogeneous phase transition dynamics for various other systems.

The surface of four commercial TiO2 compounds (Cristal Global PC-series) has been modified by Pd nanoparticles induced by gamma radiolysis. Their photocatalytic properties have been followed by phenol and rhodamine B photocatalytic degradation in aqueous suspensions under UV and visible light. Their electronic properties have been studied by time resolved microwave conductivity (TRMC) method to follow the charge-carrier dynamics. The experiments evidence a complex behavior of the surface-Pd. Its influence depends on the pollutant and on the irradiation. The modification may be strongly favorable to the photocatalytic activity. The results have been interpreted in terms of modification of charge-carrier dynamics with TRMC measurements. The surface-Pd plays a role in charge-carrier separations, increasing the photocatalytic activity under UV-light, but shows no effect on the absorption properties, preventing the creation of an activity under visible light.

The photophysical and electrochemical properties of the tetracationic zinc 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis( N -pyridiniumyl) porphyrin chloride (ZnOEP(Py) 4 4+ ,4Cl − ) were studied in aqueous solutions. The steady state and time-resolved absorption and emission measurements indicate that the porphyrin skeleton adopts a severely nonplanar conformation which minimizes steric crowding between the 12 peripheral substituents. The absorption spectrum of [ZnOEP(Py) 4 4+ ,4Cl − ] in water exhibits significant red shifts of the visible Q and Soret bands as well as considerable broadening and decrease in intensity of the latter compared to the spectrum recorded for the planar [ZnTMPyP 4+ ,4Cl − ] porphyrin. The S 2 → S 1 internal conversion is faster than the experimental resolution ( 1 excited state has a lifetime of 170 ps. The electrochemical properties of [ZnOEP(Py) 4 4+ ,4Cl − ] were investigated in water at pH 6.5 and 3.0 by cyclic and differential pulse voltammetry as well as spectroelectrochemistry. Reductions take place initially at the pyridinium sites with four successive one-electron steps at pH 6.5 or a one-electron step followed by a three-electron process at pH 3.0. Both oxidation and reduction processes undergone by the porphyrin are irreversible.

New complexes based on a coordination interaction between a pyridyl-porphyrin (namely 5,10,15-tritolyl-20-(4-pyridyl)porphyrin, 5,10,15-tritolyl-20-(3-pyridyl)porphyrin or 5,10,15-triphenyl-20-(4-pyridyl)porphyrin) and a Keggin-type polyoxometalate (α-[MSiW 11 O 39 ] 6− , M = Co 2+ or Ni 2+ ) are formed in solution. The formation of these complexes is clearly evidenced by steady-state and time-resolved luminescence measurements. A strong quenching of the porphyrin fluorescence, accompanied by an important shortening of the fluorescence lifetime, is observed upon addition of the POM and formation of the complexes. Using a variant of the Job’s method from the luminescence spectra, the association constants of the complexes have been estimated to be around 10 6 L mol −1 . Paramagnetic 1 H NMR experiments confirm the formation of the complexes. Indeed, in addition to broadenings of the signals, the coordination binding of the POM to the porphyrin induces large high-frequency shifts for the protons of the pyridyl group coordinated to the paramagnetic metal, and low-frequency shifts for all the other resonances.

Absorption spectrum of derived Dawson sandwich polyoxometalates (POM) [M(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](n-) with n = 16 for M = Zn(II), Ni(II), and n = 12 for M = Fe(III) have been extended in the visible range forming electrostatic complexes with a chromophore, the zinc tetracationic porphyrin [ZnTMePyP](4+). Formation of such complexes was followed by steady-state absorption and luminescence spectroscopies. The electrostatic complexation gives in all cases a strong, neutral, and nonluminescent complex. A charge transfer between the two units was shown by transient absorption spectroscopy. Upon a visible excitation of the porphyrin subunit, an electron transfer from the porphyrin to the POM occurs and imparts it a catalytic activity. This has been demonstrated studying a model reaction such as the reduction of silver cations leading to nanoparticles. In all cases, the reduction of the silver cations takes place. We showed that the catalytic activity depends of the nature of the metal of the tetraoxometallic central cluster of the Dawson sandwich POM.

New complexes based on electrostatic interactions between the tetracobalt Dawson-derived sandwich polyanion [Co 4 (H 2 O)(OH)(P 2 W 15 O 56 ) 2 ] 17− and the tetracationic zinc porphyrins, ZnTMPyP 4+ or ZnOETPyP 4+ , are formed in aqueous solutions. In these complexes, porphyrins can be excited by visible light and then play the role of photosensitizers able to give electrons to polyoxometalate. Consequently, these complexes exhibit photocatalytic behaviour in the visible domain. Indeed, the photocatalytic reduction of silver ions in the presence of these complexes and propan-2-ol is observed in aerated and deaerated aqueous solutions. The formed silver nanoparticles are stable in air conditions without illumination.

Photocatalytic reduction of Ag 1 2 SO 4 from aqueous solutions is observed in the presence of Dawson-derived sandwich type polyoxometalates (POMs) [M 4 (P 2 W 15 O 56 ) 2 ] 16- , M=Co 2+ , Ni 2+ and Zn 2+ as photocatalyst and an organic substrate (propan-2-ol) as sacrificial electron donor. The direct photochemical excitation of the Dawson-derived sandwich type polyoxometalates in the presence of propan-2-ol leads to its reduction. That first reduction step induces electron transfer to Ag + ions to give Ag° metal atoms which then form by aggregation colloidal metal nanoparticles stabilized by POM. In the case of [Co 4 (P 2 W 15 O 56 )] 16- , TEM experiments reveal that the Ag n particles obtained with a slight excess of Ag+ are almost spherical with size in the range 20 - 50 nm. However, in a large excess of Ag + , the obtained colloids are more oblate and assembled together to give larger aggregates.

The present work reports the first pulse-probe absorbance measurements of the hydrated electron at picosecond range and at elevated temperature. The decay of the hydrated electron, from room temperature up to 350 °C, is observed at 790 nm using a laser triggered picosecond electron accelerator. With increasing temperature, the decay of the hydrated electron becomes less and less pronounced. At 350 °C, almost a flat signal from 100 ps to 2.75 ns is observed, suggesting an efficient escape of hydrated electrons from spur reactions.

C 2 F 4 was excited by using a 150 fs pulse in its longest-wavelength band to the Rydberg (3 s) state and then probed by photoionization techniques at 810 nm. The molecule relaxes in two consecutive steps (time constants 29 and 118 fs), probably via the ππ* state, which is lowered in energy by stretching and twisting the C=C bond. A coherent oscillation (350 fs) was found, which we assign to an overtone of the twist vibration (47.6 cm -1 ) in this state. We also conclude that dissociation to singlet and some triplet CF 2 only takes place in the hot ground state of C 2 F 4 from where also the C 2 F 4 triplet state is populated. The potentials and their conical intersections are discussed with respect to relaxation and dissociation, including also some new considerations of thermal processes.

The dynamics of ionic monomer and multimers of the ethylenic molecule TDMAE (tetrakis(dimethylamino)ethylene) have been studied under femtosecond stepwise excitation using an energy surface of the neutral molecule (or cluster) as a relay. This specific excitation takes advantage of the wave packet evolution in the neutral intermediate state to (i) allow access to different regions of the ionic potential surface and (ii) identify some aspects of the dynamics in the ionic state of either TDMAE+ or TDMAEn+. This original experimental approach we shall name double pump excitation. We have identified the existence of a double well in TDMAEn+ corresponding to a different localization of the positive charge, the most stable corresponding to a complete delocalization over the nitrogen atoms. A reaction in the dimer and higher multimer ions is observed that involves the formation of compounds of general mass 200m116n+. This reaction is only observed in clusters and not in thermal ion molecule collisions between TDMAE+ and TDMAE. A tentative mechanism of formation for this product is proposed that involves the creation of a distonic ion via H-atom migrations. The double pump method could be extended to many other systems to explore competing pathways in ionic reactions, differently accessed via transient states of a neutral intermediate.

International audience; We present a new strategy to form thickness-adjusted and ultra-smooth films of very large and unwrinkled graphene oxide (GO) flakes through the transfer of both hemispherical and vertical water-films stabilized by surfactants. With its versatility in terms of substrate type (including flexible organic substrates) and in terms of flakes density (from isolated flakes to continuous and multilayer films), this wafer-scale assembly technique is adapted to a broad range of experiments involving GO and rGO (reduced graphene oxide). We illustrate its use through the evaluation of transparent rGO electrodes.

The diethyl carbonate, DEC, is an ester that is used as a solvent in Li-ion batteries, but its behavior under ionizing radiation was unknown. The transient optical absorption spectra, the decay kinetics, and the influence of various scavengers have been studied by using the picosecond laser-triggered electron accelerator ELYSE. In neat DEC, the intense near-IR (NIR) absorption spectrum is assigned to the solvated electron. It is overlapped in the visible range by another transient but longer-lived and less intense band that is assigned to the oxidized radical DEC(−H). The solvated electron molar absorption coefficients and radiolytic yield evolution from 25 ps, the geminate recombination kinetics, and the rate constants of electron transfer reactions to scavengers are determined. The radiolytic mechanism, indicating a certain radioresistance of DEC, is compared with that for other solvents.

International audience; Two covalently linked porphyrin-polyoxometalate hybrids have been prepared: an Anderson-type hexamolybdate [N(C(4)H(9))(4)](3)[MnMo(6)O(18){(OCH(2))(3)CNHCO(ZnTPP)}(2)] with two pendant zinc(ii)-tetraphenylporphyrins, and a Dawson-type vanadotungstate [N(C(4)H(9))(4)](5)H[P(2)V(3)W(15)O(59){(OCH(2))(3)CNHCO(ZnTPP)}] with one porphyrin. Electrochemical studies show independent redox processes for the organic and inorganic parts at usual potentials. Photophysical studies reveal an electron transfer from the excited porphyrin to the Dawson polyoxometalate, but not to the Anderson polyoxometalate. Time resolved absorption spectroscopy allows the identification of the electron transfer pathways and the determination of the time constants.

Picosecond pulse radiolysis of neat tetrahydrofuran (THF) shows a fast decay of the solvated electron within 2.5ns. The decay of the solvated electron observed at 790nm is because of spur reaction. A numerical simulation using time dependent Smoluchowski equation containing a sink term with a distance dependent reaction rate is used to fit the pulse-probe data and shows that the geminate reaction can proceed at long distance in this low polar solvent.

A systematic study of the ultrafast decay of metalloporphyrins containing various transition metals with partially filled 3d shells and zinc (3d filled) is reported here after excitation in the second excited state of the system (Soret band). Both time-of-flight mass spectrometry and velocity map imaging have been used for detection. A general biexponential decay with a short time constant tau1 approximately 100 fs is observed for the transition metal porphyrins, followed by a tau2 approximately 1 ps time decay. This evolution is interpreted as a porphyrin-to-metal charge transfer, tau1, followed by a back transfer, tau2, which leads to an excited state (d,d*) localized on the metal. These conclusions stem from the different behaviors of zinc and the transition metal porphyrins. A porphyrin-to-metal charge transfer model is chosen to describe the relaxation mechanism, based upon the fact that transition metalloporphyrins can accept electrons on the metal site, in contrast to zinc porphyrins.

We report the first study of picosecond pulse radiolysis of neat tetrahydrofuran (THF) by pulse–probe method. It shows a fast decay of absorbance at 790 nm within 2.5 ns. This decay is assigned to solvated electron. We observe that, at 30 ps the G value of solvated electron in THF is much lower than that of hydrated electron. Our data on solvated electron survival probability are compared with the literature data on the yield measurements with scavenging method.

Infrared spectra of unstable species such as CO-free ruthenium tetraphenylporphyrin RuTPP and RuCOTPP (species with vacant coordination sites) isolated in solid argon at 8 K have been recorded. Selective deposition conditions allow the isolation of either RuTPP and RuCOTPP or RuCOTPP and Ru(CO)2TPP. This depends on the preparation conditions of the sample. A specific Ru-CO bending mode has been characterized at 590.1 cm(-1) for Ru(CO)2TPP. The behavior of each vibrational mode of RuTPP, RuCOTPP, and Ru(CO)2TPP has been analyzed. Modes such as gamma8 at 721.3 cm(-1) (out-of-plane stretching mode gamma(Cbeta-H)sym) and nu41 at 1342.8 cm(-1) (nuCalpha-N coupled with deltaCalpha-Cm) reflect the charge transfer in the porphyrin. Indeed, the addition of one or two CO ligands to RuTPP reduces the charge transfer between the metal center and the porphyrin, which appears as an increase in the frequency of the nu41 mode and in a decrease in that of the gamma8 mode.

A femtosecond pump (266 nm)–probe (800 nm) experiment as been performed on small clusters of tetrakis(dimethylamino)ethylene (TDMAE) with argon (TDMAE(Ar) ≪10 ). The pump excites TDMAE and initiates deformations of the molecule within the cluster. Then, the probe ionises TDMAE and disposes enough excess energy in the cluster to evaporate all argon atoms. However, ionisation during the first 150 fs after the pump gives preferentially access to a metastable configuration of the TDMAE + ion, unquenched by the argon environment. This takes up enough of the excess energy to prevent complete argon evaporation. Observing the non-total-evaporation of weakly bond clusters formed of an organic cation with argon appears as a useful tool to reveal the existence of such metastable configurations in organic cations.

Photochemistry in polyatomic molecules is often controlled by initial electronic relaxation that occurs at very short times. The formation pathway of the reacting electronic state conditions its subsequent photochemical evolution. It is found that this evolution could proceed within the femtosecond time domain, and could not be explained by the existence of a multitude of dissipative channels degenerate with the excited state. Rather, a direct mechanism involving very few coordinates could explain the very rapid evolution from the accessed excited potential energy surface to a lower one of different configuration. Conical intersections (CI) between potential energy surfaces are recognized now as playing this role: the connection between surfaces relating to different electronic configurations. This allows for the very rapid downhill descent of a wavepacket prepared by femtosecond excitation, the mechanism being simply controlled by the dynamics on the connected surfaces. These properties of potential energy surfaces, known for a long time, are now found to be ubiquitous when searched by extensive quantum chemistry calculations, in the ultrafast dynamics of electronically excited organic molecules.

A supersonic expansion of the title molecule (TDMAE) with helium and polar solvent molecules S (ammonia or acetonitrile) generates TDAME(S) ⪆10 clusters. By applying the femtosecond pump–probe technique, a wavepacket is launched on the S 1 potential energy surface of TDMAE. The electronic configuration of TDMAE changes from valence V(ππ*) to Zwitterionic C + C − when the wavepacket passes in the vicinity of a conical intersection. While argon as solvent slows down this switch by an inertial effect, the present polar molecules move the conical intersection, reduce the length of the way to access it and accelerate the corresponding V-to-Z energy transfer.

The supersonic expansion of a mixture of the title molecule (TDMAE) with argon generates a beam carrying a log-normal distribution of TDAME(Ar)n clusters, broadly centered at n ≈60 . The femtosecond pump–probe technique is used to investigate the excited state dynamics of these clusters up to a 220 ps delay between the pump and the probe. This documents the effect of the argon environment on the TDMAE dynamics. The TDMAE molecule is excited in the valence state V within the cluster by the pump laser at 266 nm. It undergoes deformation in the excited potential energy surface that brings the initial wavepacket to a conical intersection (CI), where the electronic configuration of the molecule switches to a zwitterionic configuration Z. Compared to the behaviour of free TDMAE, the effect of the argon environment is a slow down of the wavepacket movement and an increase of the time scale of the V–Z energy transfer from 300 to 400±50 fs. This slow down effect, that we call a chistera effect, differs from a standard cage effect. Here, the deforming molecule does not experience a hard sphere collision with the argon cage, rather it pushes it away. Furthermore, umbrella oscillations of the dimethylamino groups are excited when the initial wavepacket passes the CI region. Because of the argon environment, the sharp 250 fs oscillation period of the free molecules is transformed into a broad structure of 40 fs width (FWHM) centred at about 240 fs. In addition, breathing oscillations of the argon environment with respect to the TDMAE molecule are observed with a period of 410±40 fs. Finally, long delays between the pump and the probe lasers allow us to investigate the nonradiative energy transfer from the Z electronic configuration of TDMAE(Ar)n to a charge transfer state. The effect of the evaporation of argon atoms in the neutral and the ionised clusters has been taken into account, as its time scale accompanies that of the observed phenomena.

Double ionisation of ruthenium–CO octaethyl porphyrin has been observed in the gas phase by femtosecond sequential 400/800 nm multi-photon excitation. Time resolved measurements show the important effect of femtosecond delayed two colour ionisation in producing efficiently doubly ionised metalloporphyrins. This is related to ultrafast electronic relaxation after the first 400 nm pump laser. The stability of such metallophyrin doubly charged ions is exceptional owing to the ability of the ring and the metal to accommodate two positive charges. These measurements relate to the positive charge distribution in the metalloporphyrin and should allow a detailed investigation of the redox properties of these important molecules.

International audience; A femtosecond pump-probe experiment is performed on tetrakis(dimethylamino)ethylene. The evolution resulting from the π-π* excitation of the CC double bond corresponds to movement along a single adiabatic potential surface with deformation along several coordinates and passage near a conical intersection. Surprisingly, this movement excites the umbrella mode of the amino groups, resulting in a measurable oscillation regime.

International audience; A soft desorption technique has been tested in order to vaporize metalloporphyrins in a supersonic jet and study their energetics and dynamics in the gas phase at low temperatures. Several metalloporphyrins: ZnTPP, NiTPP, CoTPP and ligated RuTPP–CO have been detected by resonant 2 photon ionization. ZnTPP was observed with high sensitivity owing to its long S1 lifetime, while the transition metal porphyrins displayed less intense ion yields due to short excited state lifetimes. The ligated ruthenium complex could be observed intact and femtosecond excitation at 400 nm revealed the ultrashort excited state dissociation of the ruthenium metalloporphyrin.

International audience; Au, Cu and bimetallic Au–Cu nanoparticles were synthesized on the surface of commercial TiO2 compounds (P25) by reduction of the metal precursors with tetrakis(hydroxymethyl) phosphonium chloride (THPC) (0.5% in weight). The alloyed structure of Au–Cu NPs was confirmed by HAADF-STEM, EDS, HRTEM and XPS techniques. The photocatalytic properties of the modified TiO2 have been studied for phenol photodegradation in aqueous suspensions under UV-visible irradiation. The modification by the metal nanoparticles induces an increase in the photocatalytic activity. The highest photocatalytic activity is obtained with Au–Cu/TiO2 (Au–Cu 1 : 3). Their electronic properties have been studied by time resolved microwave conductivity (TRMC) measurements to follow the charge-carrier dynamics. TRMC measurements show that the TiO2 modification with Au, Cu and Au–Cu nanoparticles plays a role in charge-carrier separations increasing the activity under UV-light. Indeed, the metal nanoparticles act as a sink for electrons, decreasing the charge carrier recombination. The TRMC measurements also show that the bimetallic Au–Cu nanoparticles are more efficient in electron scavenging than the monometallic Au and Cu ones.