Transient Absorption Spectroscopy Experiments Research Articles

Reversible Optical Switching of Polyoxovanadates and Their Communication via Photoexcited States.

The 2-bit Lindqvist-type polyoxometalate (POM) [V6O13((OCH2)3CCH2N3)2]2- with a diamagnetic {V6O19} core and azide termini shows six fully oxidized VV centers in solution as well as the solid state, according to 51V NMR spectroscopy. Under UV irradiation, it exhibits reversible switching between its ground S0 state and the energetically higher lying states in acetonitrile and water solutions. TD-DFT calculations demonstrate that this process is mainly initialized by excitation from the S0 to S9 state. Pulse radiolysis transient absorption spectroscopy experiments with a solvated electron point out photochemically induced charge disproportionation of VV into VIV and electron communication between the POM molecules via their excited states. The existence of this unique POM-to-POM electron communication is also indicated by X-ray photoelectron spectroscopy (XPS) studies on gold-metalized silicon wafers (Au//SiO2//Si) under ambient conditions. The amount of reduced vanadium centers in the "confined" environment increases substantially after beam irradiation with soft X-rays compared to non-irradiated samples. The excited state of one POM anion seems to give rise to subsequent electron transfer from another POM anion. However, this reaction is prohibited as soon as the relaxed T1 state of the POM is reached.

Photonic Properties of Thin Films Composed of Gallium Nitride Quantum Dots Synthesized by Nonequilibrium Plasma Aerotaxy.

Gallium nitride quantum dots (GaN QDs) are a promising material for optoelectronics, but the synthesis of freestanding GaN QDs remains a challenge. To date, the size-dependent photonic properties of freestanding GaN QDs have not been reported. Here, we examine the photonic properties exhibited by thin films composed of GaN QDs synthesized by nonequilibrium plasma aerotaxy. Each film exhibited two photoluminescence peaks after exposure to ambient air. The first peak was in the ultraviolet spectral region, and the second peak was in the visible region. Both peak positions depended on the QD size. Our findings, supported by transient absorption spectroscopy experiments, suggest that conduction band to valence band recombination was the cause of the ultraviolet photoluminescence and that recombination between the conduction band and an acceptor level was the cause of visible photoluminescence. Furthermore, we show that coating the surface of fresh QDs with Al2O3 suppressed the visible region photoluminescence, corroborating the conclusion that the photoactive defect was caused by oxidation in air.

Designing Ionic Ir(III) Cyclometalated Complexes as Photocatalysts for Light Assisted ATRP of MMA. A Combined Experimental and Mechanistic Study.

A new family of ionic Ir(III) cyclometalated complexes with general formula [Ir(CN)2(NN)][Br], was designed and prepared to be assessed as photocalysts for the visible light assisted ATRP polymerization of MMA. To this purpose, our design strategy involved both: i) the decoration of the cyclometalating (CN) and the ancillary (NN) ligands with various electron withdrawing and/or electron donor substituents and, ii) the use of Br- as the counter anion for these cationic Ir(III) species. After an extensive screening in which the [Ir(CN)2(NN)][Br]-type compounds were compared to the model neutral complex fac-[Ir(ppy)3], the "fully" amino-substituted ion pairs abbreviated as [10][Br] and [11][Br], exhibited the best photocatalytic performances under irradiation with CFL lamps. It is worth noting that the outcomes of transient absorption spectroscopy (TAS) experiments combined with theoretical DFT calculations, enlightened the role played by the Ir(III) complexes in the mechanism of the photoATRP process, and suggested the rationalization of the different performances that were highlighted by our Ir(III) catalyst in the visible light assisted photopolymerization of MMA.

Optical Variation and Molecular Transformation of Brown Carbon During Oxidation by NO3• in the Aqueous Phase.

The NO3•-driven nighttime aging of brown carbon (BrC) is known to greatly impact its atmospheric radiative forcing. However, the impact of oxidation by NO3• on the optical properties of BrC in atmospheric waters as well as the associated reaction mechanism remain unclear. In this work, we found that the optical variation of BrC proxies under environmentally relevant NO3• exposure depends strongly on their sources, with enhanced light absorptivity for biomass-burning BrC but bleaching for urban aerosols and humic substances. High-resolution mass spectrometry using FT-ICR MS shows that oxidation by NO3• leads to the formation of light-absorbing species (e.g., nitrated organics) for biomass-burning BrC while destroying electron donors (e.g., phenols) within charge transfer complexes in urban aerosols and humic substances, as evidenced by transient absorption spectroscopy and NaBH4 reduction experiments as well. Moreover, we found that the measured rate constants between NO3• with real BrCs (k = (1.8 ± 0.6) × 107 MC-1s-1, expressed as moles of carbon) are much higher than those of individual model organic carbon (OC), suggesting the reaction with OCs may be a previously ill-quantified important sink of NO3• in atmospheric waters. This work provides insights into the kinetics and molecular transformation of BrC during the oxidation by NO3•, facilitating further evaluation of BrC's climatic effects and atmospheric NO3• levels.

Protective action of graphene oxide against singlet oxygen generation by cosmetic dye methylene blue

Nanohybrid methylene blue/graphene oxide (MB/GO) was prepared via non-covalent functionalization with the focus on spectroscopic characterization of MB molecules adsorbed on GO. Fluorescence was not detected for the MB/GO hybrid which indicates that other competitive fast singlet excited state deactivation process must take place. It was demonstrated by measuring singlet oxygen (1O2) phosphorescence at 1270 nm that MB functionalized with GO does not generate 1O2. Nanosecond transient absorption spectroscopy experiments pointed to a lack of population of triplet state by MB/GO hybrid material. Femtosecond pump–probe experiments revealed much faster dynamics of the singlet excited state of MB/GO in comparison to free MB, which was attributed to the photoinduced electron transfer from MB to GO. Suppressing of the singlet oxygen quantum efficiency by MB upon functionalization to GO might be useful for designing graphene-based dye additives to color cosmetics with a reduced risk of the 1O2 generation on the skin.

Photoinduced Dynamics of 13,13'-Diphenylpropyl-β-carotene.

Carotenoids are ubiquitous pigment systems in nature which are relevant to a range of processes, such as photosynthesis, but the detailed influence of substitutions at the polyene backbone on their photophysics is still underexplored. Here, we present a detailed experimental and theoretical investigation of the carotenoid 13,13'-diphenylpropyl-β-carotene using ultrafast transient absorption spectroscopy and steady-state absorption experiments in n-hexane and n-hexadecane, complemented by DFT/TDDFT calculations. In spite of their bulkiness and their potential capability to "fold back" onto the polyene system, which could result in π-stacking effects, the phenylpropyl residues have only a minor impact on the photophysical properties compared with the parent compound β-carotene. Ultrafast spectroscopy finds lifetimes of 200-300 fs for the S2 state and 8.3-9.5 ps for the S1 state. Intramolecular vibrational redistribution with time constants in the range 0.6-1.4 ps is observed in terms of a spectral narrowing of the S1 spectrum over time. We also find clear indications of the presence of vibrationally hot molecules in the ground electronic state (S0*). The DFT/TDDFT calculations confirm that the propyl spacer electronically decouples the phenyl and polyene π-systems and that the substituents in the 13 and 13' positions point away from the polyene system.

Identification of Alkoxy Radicals as Hydrogen Atom Transfer Agents in Ce-Catalyzed C-H Functionalization.

The intermediacy of alkoxy radicals in cerium-catalyzed C-H functionalization via H-atom abstraction has been unambiguously confirmed. Catalytically relevant Ce(IV)-alkoxide complexes have been synthesized and characterized by X-ray diffraction. Operando electron paramagnetic resonance and transient absorption spectroscopy experiments on isolated pentachloro Ce(IV) alkoxides identified alkoxy radicals as the sole heteroatom-centered radical species generated via ligand-to-metal charge transfer (LMCT) excitation. Alkoxy-radical-mediated hydrogen atom transfer (HAT) has been verified via kinetic analysis, density functional theory (DFT) calculations, and reactions under strictly chloride-free conditions. These experimental findings unambiguously establish the critical role of alkoxy radicals in Ce-LMCT catalysis and definitively preclude the involvement of chlorine radical. This study has also reinforced the necessity of a high relative ratio of alcohol vs Ce for the selective alkoxy-radical-mediated HAT, as seemingly trivial changes in the relative ratio of alcohol vs Ce can lead to drastically different mechanistic pathways. Importantly, the previously proposed chlorine radical-alcohol complex, postulated to explain alkoxy-radical-enabled selectivities in this system, has been examined under scrutiny and ruled out by regioselectivity studies, transient absorption experiments, and high-level calculations. Moreover, the peculiar selectivity of alkoxy radical generation in the LMCT homolysis of Ce(IV) heteroleptic complexes has been analyzed and back-electron transfer (BET) may have regulated the efficiency and selectivity for the formation of ligand-centered radicals.

Removal of As(III) via adsorption and photocatalytic oxidation with magnetic Fe-Cu nanocomposites.

Magnetic Fe-Cu nanocomposites with high adsorption capacity and photocatalytic properties were prepared via the precursor method using soluble substances isolated from urban biowaste (BBS) as carbon sources and different temperatures of the pyrolysis treatment (400, 600, and 800°C). BBS is used as complexing agent for the Fe3+ and Cu2+ ions in the precursors. The as-prepared magnetic materials were tested in As(III) removal processes from water. Dark experiments performed with the materials obtained at 400 and 600°C showed excellent adsorption capacities achieving a significant uptake of 911 and 840mgg-1 for As(III), respectively. Experiments conducted under steady-state irradiation showed a reduction of 50-71% in As(III) levels evidencing the meaningful photocatalytic capacity of Fe-Cu nanocomposites. The best photocatalytic performance was obtained for the nanocomposite synthesized at the highest pyrolysis temperature, in line with the reported trend of HO· radicals production. Transient absorption spectroscopy experiments revealed the occurrence of an alternative oxidation pathway involving the valence band holes and yielded relevant kinetic information related to the early stages of the As(III) photooxidation. The higher absorption of the electron-hole pairs observed for the samples treated at lower temperature means that controlling the pyrolysis temperature during the synthesis of the Fe-Cu nanocomposites allows tuning the photocatalyst activity for oxidation of substrates via valence band holes, or via HO· radicals.

Shape influence on the ultrafast plasmonic properties of gold nanoparticles.

The aim of shape-controlled colloidal synthesis of gold (Au) is to produce Au nanoparticles (NPs) with fine control of shapes, sizes, and dispersities. We show how transient absorption spectroscopy (TAS) can be used to rapidly and accurately quantify the vast ensemble of shapes of Au NPs in solution within minutes, including the synthesized nanorods, decahedra, and nanospheres. Colloidal solutions containing Au NPs were measured in TAS and their localized surface plasmon resonance (LSPR) modes were classified according to the shape, wavelength and number of peaks. Then their excited-state relaxation dynamics were used to ascertain their electron-phonon (e-ph) coupling time constant and frequency of optomechanical modes. TAS can quickly show that an Au nanosphere sample contains a tiny fraction of Au nanorods, whereas steady-state absorbance is totally blind to the presence of nanorods. Additionally, the TAS experiments indicate that the characteristic e-ph coupling time constants in Au nanorods depend on the NPs dimensions at high excitation intensity (> 6 µJ/cm2) which can help identify if there are any elongated Au NPs in Au spheres samples. Finally, optomechanical oscillations formed by NPs breathing modes were observed, providing information related to the average size and monodispersity of Au nanospheres and nanorods.

Tracking excited state decay mechanisms of pyrimidine nucleosides in real time

DNA owes its remarkable photostability to its building blocks—the nucleosides—that efficiently dissipate the energy acquired upon ultraviolet light absorption. The mechanism occurring on a sub-picosecond time scale has been a matter of intense debate. Here we combine sub-30-fs transient absorption spectroscopy experiments with broad spectral coverage and state-of-the-art mixed quantum-classical dynamics with spectral signal simulations to resolve the early steps of the deactivation mechanisms of uridine (Urd) and 5-methyluridine (5mUrd) in aqueous solution. We track the wave packet motion from the Franck-Condon region to the conical intersections (CIs) with the ground state and observe spectral signatures of excited-state vibrational modes. 5mUrd exhibits an order of magnitude longer lifetime with respect to Urd due to the solvent reorganization needed to facilitate bulky methyl group motions leading to the CI. This activates potentially lesion-inducing dynamics such as ring opening. Involvement of the 1nπ* state is found to be negligible.

Ribose Alters the Photochemical Properties of the Nucleobase in Thionated Nucleosides.

Substitution of exocyclic oxygen with sulfur was shown to substantially influence the properties of RNA/DNA bases, which are crucial for prebiotic chemistry and photodynamic therapies. Upon UV irradiation, thionucleobases were shown to efficiently populate triplet excited states and can be involved in characteristic photochemistry or generation of singlet oxygen. Here, we show that the photochemistry of a thionucleobase can be considerably modified in a nucleoside, that is, by the presence of ribose. Our transient absorption spectroscopy experiments demonstrate that thiocytosine exhibits 5 times longer excited-state lifetime and different excited-state absorption features than thiocytidine. On the basis of accurate quantum chemical simulations, we assign these differences to the dominant population of a shorter-lived triplet nπ* state in the nucleoside and longer-lived triplet ππ* states in the nucleobase. This explains the distinctive photoanomerziation of thiocytidine and indicates that the nucleoside will be a less efficient phototherapeutic agent with regard to singlet oxygen generation.

Insights into unexpected photoisomerization from photooxidation of tribromoacetic acid in aqueous environment using ultrafast spectroscopy

Haloacetic acids are carcinogenic disinfection by-products (DPBs) and their photo-decomposition pathways, especially for those containing bromine and iodine, are not fully understood. In this study, femtosecond transient absorption (fs-TA) spectroscopy experiments were introduced for the first time to investigate the photochemistry of tribromoacetic acid. The fs-TA experiments showed that a photoisomerization intermediate species HOOCCBr2−Br (iso-TBAA) was formed within several picoseconds after the excitation of TBAA. The absorption wavelength of the iso-TBAA was supported by time-dependent density calculations. With the Second-order Møller-Plesset perturbation theory, the structures and thermodynamics of the OH-insertion reactions of iso-TBAA were elucidated when water molecules were involved in the reaction complex. The calculations also revealed that the isomer species were able to react with water with its reaction dynamics dramatically catalyzed by the hydrogen bonding network. The proposed water catalyzed OH-insertion/HBr elimination mechanism predicted three major photoproducts, namely, HBr, CO and CO2, which was consistent with the photolysis experiments with firstly reported CO formation rate and mass conversion yield as 0.096 min-1 and 0.75 ± 0.1 respectively. The spectroscopic technique, numerical tool and disclosed mechanisms provided insights on photodecomposition and subsequent reactions of polyhalo-DPBs contain heavy atom(s) (e.g., Br, I) with water, aliphatic alcohols or other nucleophiles.

Observation of quantum-confined exciton states in monolayer WS2 quantum dots by ultrafast spectroscopy.

Monolayer transition metal dichalcogenide quantum dots (TMDC QDs) could exhibit unique photophysical properties, because of both lateral quantum confinement effect and edge effect. However, there is little fundamental study on the quantum-confined exciton dynamics in monolayer TMDC QDs, to date. Here, by selective excitations of monolayer WS2 QDs in broadband transient absorption (TA) spectroscopy experiments, the excitation-wavelength-dependent ground state bleaching signals corresponding to the quantum-confined exciton states are directly observed. Compared to the time-resolved photophysical properties of WS2 nanosheets, the selected monolayer WS2 QDs only show one ground state bleaching peak with larger initial values for the linear polarization anisotropy of band-edge excitons, probably due to the expired spin-orbit coupling. This suggests a complete change of the band structure for monolayer WS2 QDs. In the femtosecond time-resolved circular polarization anisotropy experiments, a valley depolarization time of ∼100 fs is observed for WS2 nanosheets at room temperature, which is not observed for monolayer WS2 QDs. Our findings suggest a strong state-mixing of band-edge valley excitons responsible for the large linear polarization in monolayer WS2 QDs, which could be helpful for understanding the exciton relaxation mechanisms in colloidal monolayer TMDC QDs.

Efficient table-top beamline for attosecond molecular and solid-state dynamics at the carbon K-edge

SynopsisA harmonic continuum extending up to 375 eV is generated from 750 μJ, 9 fs mid-IR pulses centered at 1300 nm. The development of this source and its use for attosecond transient absorption spectroscopy experiments at the carbon K-edge are discussed.

Picosecond to millisecond tracking of a photocatalytic decarboxylation reaction provides direct mechanistic insights

The photochemical decarboxylation of carboxylic acids is a versatile route to free radical intermediates for chemical synthesis. However, the sequential nature of this multi-step reaction renders the mechanism challenging to probe. Here, we employ a 100 kHz mid-infrared probe in a transient absorption spectroscopy experiment to track the decarboxylation of cyclohexanecarboxylic acid in acetonitrile-d3 over picosecond to millisecond timescales using a photooxidant pair (phenanthrene and 1,4-dicyanobenzene). Selective excitation of phenanthrene at 256 nm enables a diffusion-limited photoinduced electron transfer to 1,4-dicyanobenzene. A measured time offset in the rise of the CO2 byproduct reports on the lifetime (520 ± 120 ns) of a reactive carboxyl radical in solution, and spectroscopic observation of the carboxyl radical confirm its formation as a reaction intermediate. Precise clocking of the lifetimes of radicals generated in situ by an activated C-C bond fission will pave the way for improving the photocatalytic selectivity and turnover.

Simulation of ultrafast excited-state dynamics and elastic x-ray scattering by quantum wavepacket dynamics.

Simulation of the ultrafast excited-state dynamics and elastic X-ray scattering of the [Fe(bmip)2]2+ [bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-4-pyridine] complex is presented and analyzed. We employ quantum wavepacket dynamics simulations on a 5-dimensional potential energy surface (PES) calculated by time-dependent density functional theory with 26 coupled diabatic states. The simulations are initiated by explicit inclusion of a time-dependent electromagnetic field. In the case of resonant excitation into singlet metal-to-ligand charge transfer (1MLCT) states, kinetic (exponential) population dynamics are observed with small nuclear motion. In agreement with transient optical absorption spectroscopy experiments, we observe a subpicosecond 1MLCT → 3MLCT intersystem crossing and a subsequent decay into triplet metal-centered (3MC) states on a picosecond time scale. The simulated time-resolved difference scattering signal is dominated by the 3MC component, for which the structural distortions are significant. On the other hand, excitation into 1MC states leads to ballistic (nonexponential) population dynamics with strong nuclear motion. The reason for these ballistic dynamics is that in this case, the excitation occurs into a nonequilibrium region, i.e., far from the minimum of the 1MC PES. This results in wavepacket dynamics along the principal breathing mode, which is clearly visible in both the population dynamics and difference scattering. Finally, the importance of decomposing the difference scattering into components by electronic states is highlighted, information which is not accessible from elastic X-ray scattering experiments.

Carbene Spin Multiplicity in Solution Probed Using Time-Resolved EPR Spectroscopy

Time-resolved (CW) EPR spectroscopy at X-band has been used to detect free radicals created by H-atom abstraction reactions of photochemically generated carbenes in n-hexadecane solutions at room temperature. Three different carbenes were studied, formed via loss of nitrogen after photo-excitation of a substituted biphenyl diazo precursor. The TREPR signals from two different alkyl radicals and the benzylic-type radical from the carbene are detected for two of the three diazo substrates. The radicals were unambiguously identified by spectral simulation using literature parameters (g-factors and hyperfine coupling constants). Previous transient optical absorption spectroscopy experiments have concluded that these two carbenes have triplet ground states, which is confirmed by the low field emissive, high field absorptive phase of the TREPR signals reported here. The spectra are strongly spin polarized by the Radical Pair Mechanism of CIDEP, and their overall intensities track with the measured quantum yields for carbene production. A third substrate, known to react via a concerted Wolff reaction or excited state rearrangement via a singlet state, showed no TREPR signal, as expected. Reactions run in acetonitrile, where the spin multiplicity is reversed to give a singlet ground state, did not lead to detectable TREPR signals for all three substrates. The observations are consistent with previously published laser flash photolysis results.

Phonon-Assisted Auger Process Enables Ultrafast Charge Transfer in CdSe Quantum Dot/Organic Molecule

Charge transfer between photoexcited quantum dots and molecular acceptors is one of the key limiting processes in most applications of colloidal nanostructures, most prominently in photovoltaics. An atomistic detailed description of this process would open new ways to optimize existing and create new structures with targeted properties. We achieve a one-to-one comparison between ab-initio non-adiabatic molecular dynamics calculations and transient absorption spectroscopy experiments, which allows us to draw a comprehensive atomistic picture of the charge transfer process, following the time evolution of the charge carrier across the electronic landscape and identifying the thereby induced vibrations. For two quantum dot sizes we find two qualitatively different processes. For the larger structure we find a relatively slow (\tau = 516 fs) transfer process that we explain by the existence of a large energy detuning and weak vibronic coupling. For the smaller structure the process is ultrafast (\tau = 20 fs) due to an efficient, phonon-assisted Auger process triggered by a strong electron-hole coupling.

Evaluating Photooxidation of Phospholipid Membranes by a Novel Switchable Photosenstizer

Photodynamic Therapy (PDT) is a non-invasive treatment modality that utilizes photosensitizers for the excitation of singlet oxygen, a highly reactive oxidative species, for the destruction of undesired tissues. It is currently used for the treatment of certain types of cancers, acne, and age-related macular degeneration but existing photosensitizers have very limited molecular specificity and rely on local irradiation to avoid off-target oxidation at the macroscale. We investigate here a novel class of conjugated polyelectrolytes (OPEs) as switchable photosensitizers based on their unique fluorescence turn-on capacity when bound to a molecular scaffold, including detergent assemblies and amyloid protein aggregates implicated in the pathogenesis of neurodegenerative diseases. Controllable photo-activation has the potential to reduce the risk of off-target oxidation and prolonged photosensitivity while increasing the specificity of targeted delivery of singlet oxygen at the microscale that limit current photosensitizers. To evaluate switchable photooxidation by OPEs, we used large unilamellar vesicles composed of varying combinations of 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG), and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) lipids. Fluorescence, dynamic light scattering, cryo-TEM, and synchrotron surface X-ray diffraction and reflectivity measurements were made to characterize OPE-membrane interactions and membrane morphology. Ultrafast transient absorption spectroscopy and chemical trapping experiments were additionally carried out to directly probe OPE triplet state and singlet oxygen generation, respectively. Our results show that while OPE exhibited little to no interaction with physiologically relevant POPC and mixed POPC:POPG vesicles, it exhibited profound binding, fluorescence turn-on, and photooxidation of cationic POPC:DOTAP membranes. Our findings provide further evidence that OPEs are switchable photosensitizers and suggests that they may be useful theragnostic agents with minimal off-target oxidation.

The Effect of Residual Palladium Catalyst Contamination on the Photocatalytic Hydrogen Evolution Activity of Conjugated Polymers

AbstractThe effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the hydrogen evolution reaction (HER) rate. The HER rate increases linearly with increasing Pd concentration from <1 ppm to approximately 100 ppm, at which point the rate begins to saturate. Transient absorption spectroscopy experiments support these conclusions, and suggest that residual Pd mediates electron transfer from the F8BT nanoparticles to protons in the aqueous medium.