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International audience; The [Re(bpy)(CO)3Cl] catalyst pioneered by Lehn for the two-electron reduction of CO2 has constantly revealed unique facets in the mechanistic understanding of the selective transformation of CO2. A novel triazole-linked ruthenium photosensitizer and a rhenium catalyst dyad was synthesized and investigated for photo-induced charge accumulation using time-resolved absorption spectroscopy. The triazole bridging ligand promoted weak electronic communication between the two units, resulting in an anodic shift of the reduction potentials of the Re moiety. Upon excitation of the photosensitizer, the first reduction of the catalyst occurred with a fast apparent rate of >5×107 s−1. Using a double-excitation nanosecond pump-pump-probe setup to track the second electron accumulation on the catalytic unit was not conclusive as no observable absorption changes occurred upon the second excitation, suggesting a pathway for an efficient intramolecular reverse electron transfer preventing the two-electron accumulation at the catalyst under our experimental conditions. Nevertheless, under continuous irradiation and with the use of sacrificial electron donors, photocatalytic CO2 reduction assays showed good turnover numbers, hinting at the non-innocent role of byproducts in solution.

In this work, online spontaneous Raman spectroscopy was applied to study the soot inception and growth zones of a low pressure premixed ethylene/oxygen flame. Firstly, we measured online Raman spectrum of aerosol soot extracted from the flame. The spectrum was compared to ex situ Raman measurements of the same soot after being deposited on a window. In the aerosol soot particles, the presence of an abundant sp hybridized carbon chain component is inferred accompanying a polycyclic aromatic component. Both cumulenic and polyynic chains are affected by the deposition under vacuum and by the pressure as it is raised to atmospheric, revealing that the sp phase is no longer visible in the ex situ characterization of the sampled soot. Secondly, we monitored the Raman spectra of aerosol soot as a function of the height above the burner. Comparing spectral differences of by-products along the blue to orange zone of the flame permitted the soot formation and evolution to be probed. The online spectra, by avoiding structural modifications of the real soot structures by deposition, highlight the role of unsaturated carbon chains in the chemical condensation scenario.

Raman spectra of soot particles were monitored in the gaseous flow extracted from the burning regions of two low-pressure premixed ethylene flames, for the first time. The flame conditions were chosen to explore the diversity of soot nanostructure in slightly sooting flames. Evaluation of the Raman spectral parameters revealed that the soot particles exhibited a strongly disordered structure and a large proportion of sp hybridization of the carbon. The appearance of sp carbon chains composing up to 30% of the total carbon content as well as an olefinic component may indicate their important role in soot nucleation and growth in low pressure ethylene flames. Hence, Raman spectroscopy of soot particles in the aerosol phase revealed that accretion and cyclization of the aliphatic carbon including sp carbon chains could thus be of importance for the initial soot growth and require more investigation.

International audience; The design of robust and inexpensive molecular photocatalysts for the conversion of abundant stable molecules like H2O and CO2 into an energetic carrier is one of the major fundamental questions for scientists nowadays. The outstanding challenge is to couple single photoinduced charge separation events with the sequential accumulation of redox equivalents at the catalytic unit for performing multielectronic catalytic reactions. Herein, double excitation by nanosecond pump-pump-probe experiments was used to interrogate the photoinduced charge transfer and charge accumulation on a molecular dyad composed of a porphyrin chromophore and a ruthenium-based catalyst in the presence of a reversible electron acceptor. An accumulative charge transfer state is unattainable because of rapid reverse electron transfer to the photosensitizer upon the second excitation and the low driving force of the forward photodriven electron transfer reaction. Such a method allows the fundamental understanding of the relaxation mechanism after two sequential photon absorptions, deciphering the undesired electron transfer reactions that limit the charge accumulation efficiency. This study is a step toward the improvement of synthetic strategies of molecular photocatalysts for light-induced charge accumulation and more generally, for solar energy conversion.

Methylviologen (MV2+) is perhaps the most used component as a reversible electron acceptor in photophysical studies. While MV2+ is most commonly implicated as a reversible one-electron mediator, its electrochemical properties clearly evidence two successive one-electron reduction processes. In this report, we have investigated on the light driven two-charge accumulation on MV2+ using a multicomponent system composed of the prototypical molecular photosensitizer [Ru(bpy)3]2+ and MV2+ in the presence of ascorbate as reversible electron donor. The sequential addition of two electrons on the methylviologen was tracked upon sequential excitation of the [Ru(bpy)3]2+ at optimized concentration of the electron acceptor. The charge accumulated state carries an energy of 0.9 eV above the ground state and has a lifetime of ca. 50 μs. We have reached a fairly good global yield of approximately 9% for the two-charge accumulation. This result clearly demonstrates the potential of this simple approach for applications i...

Biomimetic models that contain elements of photosynthesis are fundamental in the development of synthetic systems that can use sunlight to produce fuel. The critical task consists of running several rounds of light-induced charge separation, which is required to accumulate enough redox equivalents at the catalytic sites for the target chemistry to occur. Long-lived first charge-separated state and distinct electronic signatures for the sequential charge accumulated species are essential features to be able to track these events on a spectroscopic ground. Herein, we use a double-excitation nanosecond pump-pump-probe experiment to interrogate two successive rounds of photo-induced electron transfer on a molecular dyad containing a naphthalene diimide (NDI) linked to a [Ru(bpy)3 ](2+) (bpy=bipyridine) chromophore by using a reversible electron donor. We report an unprecedented long-lived two-electron charge accumulation (t=200 μs).

The photochromic reaction dynamics of 6 ′ -cyano-spiroindolinenaphthoxazine (6 ′ CN-SNO) in solution is followed by femtosecond transient absorption spectroscopy. In addition to the formation of the merocyanine coloured form, we demonstrate the presence of a new metastable species with intense absorption bands in the 400–600 and 650–975 nm spectral regions that was not observed for the non-substituted parent molecule. The formation of this species parallels the photochromic reaction. The existence of this concurrent photophysical process can explain the 50% reduction in the photocoloration quantum yield of this molecule relative to that of the parent molecule.

Femtosecond and nanosecond transient absorption and picosecond time-correlated single photon counting techniques have been used to study the mechanism and dynamics of the efficient quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT) in the S2 state, by acetonitrile. The results suggest the involvement of two aborted processes in the quenching mechanism: exciplex formation and hydrogen abstraction. The occurrence of the latter process is supported by the observation of a clear isotope effect on going from acetonitrile to deuterated acetonitrile. 2003 Elsevier B.V. All rights reserved.

New time-resolved resonance Raman spectra of the radical cation of N,N,N′,N′-tetramethylbenzidine (TMB) in solution have been probed at 752 nm, in resonance with a strong electronic transition, for several isotopic derivatives. These spectra and those obtained previously at 488 nm, in resonance with a second electronic transition, have been interpreted by means of the density functional theory (B3LYP) calculations. An unambiguous assignment of the observed vibrations has been deduced. For comparison, similar calculations have been done for the parent ground state molecule. In addition, configuration interaction singles (CIS) calculations have been used to assign the two resonant electronic excited states of the radical cation and identify the molecular orbitals involved in the corresponding transitions. On this basis, the Raman intensities of the radical cation in resonance with these two transitions have been evaluated in the Franck–Condon approximation by projecting the excited state potential energy surface gradients, determined from CASSCF calculations, on the ground state normal modes. The analysis of both the vibrational frequencies and resonance Raman intensities suggests that the TMB radical cation adopts a coplanar structure (D2h point group).

We present the femtosecond transient absorption spectra of trans-4,4‘-diphenylstibene (DPS) in dioxane, methylene chloride, and acetonitrile. The transient absorption spectra feature two bands that are assigned to different electronic transitions. We interpret the spectral changes in the transient absorption spectra in terms of vibrational cooling and conformational dynamics. Vibrational cooling is evidenced by changes in the integrated peak intensity of the transient absorption spectra. For the two transient absorption bands observed in S1 DPS, the vibrational cooling time constants correlate well with the thermal diffusivity of the solvent. The vibrational cooling rates for the two absorption bands are statistically different in all solvents. We attribute this difference to the relative energy exchange rates between the weighted average of the vibrational modes in the Franck−Condon region that contribute to the S1−Sn transition and the weighted average of the vibrational modes that contribute to the S1−...

International audience; Circumventing the limit imposed by diffraction is a major issue in the instrumental development to realize finer resolutions in biological samples. With STED microscopy, we exploit the molecular transitions of the fluorescent marker to image well below the Rayleigh criterion. Also in combination with STED, we propose to use an alternative technique for optically sectioning fluorescent emitters close to the water-glass interface by selectively filtering the supercritical emission at the pupil plane. We discuss the instrumental development of such a system and its combination with other imaging techniques.

The photoinduced reaction of electron transfer (ET) from N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and N,N,N′,N′-tetramethylbenzidine (TMB) to the dichloromethane solvent has been studied by picosecond transient absorption and time-resolved resonance Raman spectroscopy. In neat CH2Cl2, “static” quenching of the lowest excited singlet state (S1) of these amines is demonstrated. For TMPD, the reaction shows two kinetic contributions (τ1

Vibrational data on the first excited singlet state (S 1 ) of N,N,N',N'-tetramethylbenzidine (TMB) and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) have been obtained by using picosecond time-resolved Raman spectroscopyfor different isotopic derivatives and for the N,N,N',N'-tetraethyl analogues, TEB and TEPD. Reliable vibrational assignments are proposed. On this basis, the S 1 state structure and electronic configuration of these diamines are discussed and compared with the results previously reported for the ground state, the lowest excited triplet state, and the radical cation species.

Femtosecond transient absorption has been used to study the mechanism and dynamics of the quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT), in the S2 state, by saturated hydrocarbons (n-hexane, dodecane, hexadecane, cyclohexane, cyclohexane-d12). The results demonstrate the involvement of the hydrocarbon C–H bonds in the quenching process. Two quenching mechanisms are proposed: efficient H-atom abstraction followed by ultrafast back hydrogen transfer, or ‘aborted hydrogen abstraction'. In the latter case, the progress along the reaction path to a conical intersection is assumed to deactivate the S2 state through transition to the S1 state energy surface.

The photoreduction of 4,4'-bipyridine (44BPY) by 1,4-diazabicyclo[2.2.2]octane (DABCO) has been studied in water by picosecond transient absorption spectroscopy and time-resolved resonance Raman spectroscopy. The kinetics of formation of the anion radical 44BPY- by electron transfer have been analyzed and compared with those obtained previously for the same reaction in acetonitrile. The 44BPY- anion is linked via H-bonding to the surrounding water molecules immediately after its formation and undergoes proton transfer along the H-bond to reach an acid-base equilibrium of pKa 15. The nature of the ion pair produced initially in the electron-transfer reaction is discussed.

We have obtained picosecond (ps) photoluminescence (PL) decays of phenyl disubstituted polyacetylenes (PDPA) with an attached n-butyl group (PDPA-n-Bu) in solution and as thin films. In solution, we observe only a single exponential decay at all emission wavelengths over the range from 2.15 to 2.50 eV. The PL lifetime in solution is 190 ps. The PL decays in thin films cannot be fit well using a single exponential, and are significantly longer in thin films than in solution. In addition, the PL decay times vary with emission wavelength in thin films with longer PL decay times observed at lower emission energies. We have also obtained the femtosecond (fs) photoinduced absorption (PA) spectra of PDPA-n-Bu in solution over the range from 1.70 to 2.95 eV. These data exhibit fast ground state bleaching at high energy, along with the growth of a broad PA feature at low energies that decays with a time constant consistent with the PL decay time in solution. At long times (>200 ps) we observe the growth of a long-lived PA feature peaked at 2.3 eV that persists for hundreds of nanoseconds. We also observe a broad feature in the infrared PA at 0.23 eV whose dynamics are the same as the band at 2.3 eV. We assign these two long-lived bands to polaron transitions.

The photoreduction of 4,4'-bipyridine (44BPY) by 1,4-diazabicyclo[2.2.2]octane (DABCO) and triethylamine (TEA) has been examined in acetonitrile solution by picosecond transient absorption and time-resolved resonance Raman spectroscopy. Both the 44BPY S1 and T1 states are quenched by electron transfer via the formation of radical ion pairs. The S1-state reaction is essentially unproductive as the singlet ion pair undergoes ultrafast intrapair back electron transfer. The T1-state reaction leads to the anion radical upon reduction by DABCO and to a mixture of anion radical and N-hydro radical upon reduction by TEA. In the latter case, a contact ion pair of 70 ps lifetime is first produced and decays mainly by intrapair proton transfer (kpt = 1.2 × 1010 s-1) and to a minor extent by dissociation into free ions (kdis = 2.3 × 109 s-1). The free and ion-paired 44BPY anion species have similar absorption spectra but are distinguishable from Raman spectroscopy by slightly different vibrational data. From the analysis of the Raman frequencies, it is suggested that the proton transfer is preceded by a rate-limiting intrapair reorientation step.