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Excitation energy transfer in the far-red absorbing violaxanthin/vaucheriaxanthin chlorophyll a complex from the eustigmatophyte alga FP5
摘要： This work highlights spectroscopic investigations on a new representative of photosynthetic antenna complexes in the LHC family, a putative violaxanthin/vaucheriaxanthin chlorophyll a (VCP) antenna complex from a freshwater Eustigmatophyte alga FP5. A representative VCP-like complex, named as VCP-B3 was studied with both static and time-resolved spectroscopies with the aim of obtaining a deeper understanding of excitation energy migration within the pigment array of the complex. Compared to other VCP representatives, the absorption spectrum of the VCP-B3 is strongly altered in the range of the chlorophyll a Qy band, and is substantially red-shifted with the longest wavelength absorption band at 707 nm at 77 K. VCP-B3 shows a moderate xanthophyll-to-chlorophyll a efficiency of excitation energy transfer in the 50–60% range, 20–30% lower from comparable VCP complexes from other organisms. Transient absorption studies accompanied by detailed data fitting and simulations support the idea that the xanthophylls that occupy the central part of the complex, complementary to luteins in the LHCII, are violaxanthins. Target analysis suggests that the primary route of xanthophyll-to-chlorophyll a energy transfer occurs via the xanthophyll S1 state.
关键词： Chlorophyll a,Light-harvesting complex,Transient absorption,Violaxanthin,Photosynthesis,Vaucheriaxanthin
更新于2025-11-19 16:46:39
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Artifacts in Transient Absorption Measurements of Perovskite Films Induced by Transient Reflection from Morphological Microstructures
摘要： Organolead halide perovskites MAPbX3 (MA = CH3NH3+; X = Cl?, Br?, I?) have attracted broad tremendous interest in the past 10 years for applications in solar cells and light-emitting devices. In evaluating the quality of the perovskite materials, spectroscopic characterizations such as static and time-resolved absorption and photoluminescence measurements are essential to examine their photophysical properties. A recent report found that the correct measurement of static absorption spectra of MAPbX3 films is indeed difficult due to the strong light scattering caused by their poor surface coverage or complex microstructures. These morphological complexities seem to be inevitable in thin-film fabrication and should not only affect the steady-state spectroscopic measurements but also can significantly impact the time-resolved spectroscopic characterizations, whose results are crucial for understanding photoinduced carrier dynamics in the examined materials. Photoexcited states in semiconductor materials induce changes in the real and imaginary parts of the dielectric function. This leads to changes in absorption (imaginary part) and reflectivity (real part), which can be substantial for materials with significant values of refractive index such as lead halide perovskites. Transient absorption (TA) spectroscopy is a typical technique that has been broadly used to probe photoexcited state dynamics in perovskites and other semiconductor materials. In TA measurements, a pump laser pulse is used to excite the perovskite films, and the induced absorption changes (ΔA) are recorded as a function of both wavelength and time. With the transmitted light as the probe (Figure 1a), the TA signal (ΔA) is mainly decided by the ratio of the intensity of transmitted probe light with and without pump excitation (see eq S1 in the SI), assuming that the loss of transmitted probe light completely results from the sample absorption. On the basis of the same experimental setup, transient reflection (TR) measurements can also be carried out by using the reflected probe light as detection signal (Figure 1b). The TR signal (ΔR/R) can also be determined by the ratio of the intensity of reflected probe light with and without pump excitation (see eq S4 in the SI). Unlike the TA measurements that mainly probe the bulk property of samples, the TR signal mainly detects the photoinduced reflection variations due to the refractive index change at the sample surface. Therefore, the TR spectrum and kinetics can be significantly different from those of TA even in the same sample. For example, previous TA and TR measurements have found dramatically faster carrier recombination kinetics on the surface than in the bulk of MAPbX3 perovskite films or single crystals because of the presence of more surface defects. There is an abnormal case in the regular TA measurements particularly when performed on the films with large and heterogeneous microstructures (e.g., films with poor coverage, large grains, and pinholes) because the loss of transmitted probe light in their TA measurements likely results not only from the sample absorption but also from the reflection of the film surface or the boundary of microstructures in samples. In this case, the measured transient spectrum, though collected in the transmittance mode as in TA, can contain contributions from both TA and TR signals (see Figure 1c and eq S6 in the SI). This could lead to distorted TA spectra and thus inaccurate analysis of photoinduced kinetics. A solution-processed organic or inorganic halide perovskite thin film is a typical material whose morphological microstructures were found to have significant impact on device performance. Although the photoinduced carrier dynamics in perovskite films has been extensively studied using TA spectroscopy, the possible artifacts in TA results induced by TR signal originating from the photoinduced reflectivity variation of film surfaces and microstructures have been overlooked. Herein, in order to clarify the influence of TR signal in the regular TA measurements, we performed a careful transient spectroscopic analysis on a series of MAPbBr3 perovskite films with different microstructure morphology. Meanwhile, TR measurements on MAPbBr3 single crystals (SCs) were carried out for comparison. We confirmed that the TA spectra measured in MAPbBr3 perovskite films with large and heterogeneous microstructures do comprise non-negligible TR signals from the photoinduced reflection of microstructures, with the weight of contribution increased from ～20 to ～100% as the size of the microstructure increased from <200 nm to 1?2 μm. The presence of TR signal leads to an “artifact” feature in the TA spectra and faster observed kinetics owing to the faster surface carrier recombination, which will thus mislead the analysis of bulk carrier dynamics. We also provided a method to reduce the TR signal in actual TA measurements by adding solvent with its refractive index close to the samples, by which the TR distortion can be suppressed to some extent.
关键词： artifacts,transient reflection,transient absorption,carrier dynamics,perovskite films,microstructures
更新于2025-09-23 15:23:52
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Understanding Spatiotemporal Photocarrier Dynamics in Monolayer and Bulk MoTe <sub/>2</sub> for Optimized Optoelectronic Devices
摘要： Semiconducting molybdenum ditelluride has emerged as a promising transition-metal dichalcogenide with a number of novel properties. In particular, its bandgap in infrared range makes it an attractive candidate for ultrathin and high-performance infrared optoelectronic applications. Dynamical properties of photocarriers play a key role in determining performance of such devices. We report an experimental study on spatiotemporal dynamics of photocarriers in both monolayer and bulk MoTe2. Transient absorption measurement in reflection geometry revealed ultrafast thermalization and relaxation processes of photocarriers and lifetimes of about 60 and 80 ps in monolayer and bulk MoTe2, respectively. By spatially resolved transient absorption measurements on monolayer, we obtained an exciton diffusion coefficient of 20 ± 10 cm2 s?1, a mean free time of 200 fs, a mean free path of 20 nm, and a diffusion length of 350 nm. The corresponding values for the bulk sample are 40 ± 10 cm2 s?1, 400 fs, 40 nm, and 570 nm, respectively. These results provide fundamental information for understanding and optimizing performance of MoTe2-based optoelectronic devices.
关键词： two-dimensional material,exciton,transient absorption,molybdenum ditelluride,transition-metal dichalcogenide,diffusion,photocarrier dynamics
更新于2025-09-23 15:23:52
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Ultrafast spectroscopy of the primary charge transfer and ISC processes in 9-anthraldehyde
摘要： The ultrafast charge transfer (CT) and the following intersystem crossing (ISC) processes of 9-anthraldehyde were investigated in ethanol and hexane using femtosecond transient absorption spectroscopy combined with quantum chemical calculations. The CT was observed within 0.11 ps in ethanol and 0.23 ps in hexane. Due to the stronger polarity in ethanol, the CT is faster than in hexane. The following ISC is determined to be 22.3 ps and 21.4 ps in two solvents, respectively. However, the timescales of ISC are similar in both solvents since the excited energies of the S1 and triplet states are close.
关键词： Time-resolved transient absorption spectroscopy,intramolecular charge transfer,radiationless intersystem crossing
更新于2025-09-23 15:23:52
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Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Selected Solutions
摘要： Nitrenium ions are important reactive intermediates in chemistry and biology. In this work, femtosecond and nanosecond transient absorption (fs-TA and ns-TA) along with nanosecond time-resolved resonance Raman (ns-TR3) experiments were employed to examine the photochemical pathways of N-(4,4′-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF4? (salt (DN) from just absorption of a photon of light to the production of the important N,N-di(4-bromophenyl)nitrenium ion 2. In acetonitrile (MeCN), the formation of halogenated diarylnitrenium ion 2 was observed within 4 ps, showing the vibrational spectra with strong intensity. The nucleophilic adduct reaction of ion 2 with H2O was also examined in aqueous solutions. The direct detection of the unique ortho adduct intermediate 3 shows that there is an efficient and exclusive reaction pathway for 2 with H2O. The results shown in this paper give new characterization of 2, which can be used to design time-resolved spectroscopy investigations of covalent addition reactions of nitrenium ions with other molecules in future studies.
关键词： transient absorption,reactive intermediate,resonance Raman,nitrenium ion
更新于2025-09-23 15:22:29
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Photodetachment, electron cooling, and recombination, in a series of neat aliphatic room temperature ionic liquids
摘要： Transient absorption following photodetachment of a series of neat methyl-alkyl-pyrrolidinium bis(trifluoromethylsulfonyl)amides at 6.20 eV was measured with sub-picosecond time resolution in the visible and near-IR portions of the spectrum. This series spans the onset of structuring in the liquids in the form of polarity alternation. Excitation promotes the electron into a delocalized state with a very large reactive radius. Strong transient absorption is observed in the visible spectrum with a ～700 fs lifetime, and much weaker, long-lived absorption is observed in the near-IR spectrum. Absorption in the visible is shown to be consistent with the hole, and absorption in the near-IR is assigned to the free solvated electron. Yield of free electrons is estimated at ～4%, is insensitive to the size of the cation, and is determined in less than 1 ps. Solvation of free electrons depends strongly on the size of the cation and correlates well with the viscosity of the liquid. In addition to radiolytic stability of the aliphatic cations, ultrafast, efficient recombination of separated charge in NTf2? based ionic liquids following photo-excitation near the band-gap may prevent subsequent reactive damage associated with anions.
关键词： solvated electron,electron cooling,recombination,photodetachment,ionic liquids,transient absorption
更新于2025-09-23 15:22:29
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Ultrafast X-ray Transient Absorption Spectroscopy of Gas-Phase Photochemical Reactions: A New Universal Probe of Photoinduced Molecular Dynamics
摘要： Time-resolved spectroscopic investigations of light-induced chemical reactions with universal detection capitalize recently on single-photon molecular probing using laser pulses in the extreme ultraviolet or X-ray regimes. Direct and simultaneous mappings of the time-evolving populations of ground-state reactants, Franck?Condon (FC) and transition state regions, excited-state intermediates and conical intersections (CI), and photoproducts in photochemical reactions utilize probe pulses that are broadband and energy-tunable. The limits on temporal resolution are set by the transit- or dwell-time of the photoexcited molecules at specific locations on the potential energy surface, typically ranging from a few femtoseconds to several hundred picoseconds. Femtosecond high-harmonic generation (HHG) meets the stringent demands for a universal spectroscopic probe of large regions of the intramolecular phase-space in unimolecular photochemical reactions. Extreme-ultraviolet and soft X-ray pulses generated in this manner with few-femtosecond or sub-femtosecond durations have enormous bandwidths, allowing the probing of many elements simultaneously through excitation or ionization of core?electrons, creating molecular movies that shed light on entire photochemical pathways. At free electron lasers (FELs), powerful investigations are also possible, recognizing their higher flux and tunability but more limited bandwidths. Femtosecond time-resolved X-ray transient absorption spectroscopy, in particular, is a valuable universal probe of reaction pathways that maps changes via the fingerprint core-to-valence resonances. The particular power of this method over valence-ionization probes lies in its unmatched element and chemical-site specificities. The elements carbon, nitrogen, and oxygen constitute the fundamental building blocks of life; photochemical reactions involving these elements are ubiquitous, diverse, and manifold. However, table-top HHG sources in the “water-window” region (280?550 eV), which encompasses the 1s-absorption edges of carbon (284 eV), nitrogen (410 eV), and oxygen (543 eV), are far from abundant or trivial. Recent breakthroughs in the laboratory have embraced this region by using long driving-wavelength optical parametric amplifiers coupled with differentially pumped high-pressure gas source cells. This has opened avenues to study a host of photochemical reactions in organic molecules using femtosecond time-resolved transient absorption at the carbon K-edge. In this Account, we summarize recent efforts to deploy a table-top carbon K-edge source to obtain crucial chemical insights into ultrafast, ultraviolet-induced chemical reactions involving ring-opening, nonadiabatic excited-state relaxation, bond dissociation and radical formation. The X-ray probe provides a direct spectroscopic viewport into the electronic characters and configurations of the valence electronic states through spectroscopic core-level transitions into the frontier molecular orbitals of the photoexcited molecules, laying fertile ground for the real-time mapping of the evolving valence electronic structure. The profound detail and mechanistic insights emerging from the pioneering experiments at the carbon K-edge are outlined here. Comparisons of the experimental methodology with other techniques employed to study similar reactions are drawn, where applicable and relevant. We show that femtosecond time-resolved X-ray transient absorption spectroscopy blazes a new trail in the study of nonadiabatic molecular dynamics. Despite table-top implementations being largely in their infancy, future chemical applications of the technique will set the stage for widely applicable, universal probes of photoinduced molecular dynamics with unprecedented temporal resolution.
关键词： time-resolved spectroscopy,photochemical reactions,high-harmonic generation,X-ray transient absorption spectroscopy,nonadiabatic molecular dynamics
更新于2025-09-23 15:21:21
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Salt Effect in Ion-Pair Dynamics after Bimolecular Photoinduced Electron Transfer in a Room-Temperature Ionic Liquid
摘要： Bimolecular photoinduced electron transfer between perylene and two quenchers was investigated in an imidazolium room-temperature ionic liquid (RTIL) and in a dipolar solvent mixture of the same viscosity using transient absorption on the subpicosecond to submicrosecond timescales. Whereas charge separation dynamics were similar in both solvents, significant differences were observed in the temporal evolution of the ensuing radical ions: although small, the free ion yield is significantly larger in the RTIL, and recombination of the ion pair to the triplet state of perylene is more efficient in the dipolar solvent. The temporal evolution of reactant, ion and triplet state populations could be well reproduced using Unified Encounter Theory. This analysis reveals that the observed differences can be explained by the strong screening of the Coulomb potential in the ion pair by the ionic solvent. In essence, RTILs favor free ions compared to highly dipolar solvents of the same viscosity.
关键词： unified encounter theory,free ions,spin conversion,charge recombination,transient absorption,charge separation
更新于2025-09-23 15:21:21
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Bi-Exciton Dissociation Dynamics in Nano-Hybrid Au-CuInS <sub/>2</sub> Nanocrystals
摘要： Multiexciton harvesting from semiconductor quantum dot has been a new approach for improving the solar cell efficiency in Quantum Dot Sensitized Solar Cells (QDSC). Till date, relation between multiexciton dissociation in metal?semiconductor nanohybrid system and boosting the power conversion efficiency (PCE) of QDSC were never discussed. Herein we report a detailed spectroscopic investigation of biexciton dissociation dynamics in copper indium sulfide (CuInS2, also referred as CIS) and Au-CIS nanohybrid, utilizing both time-resolved PL and ultrafast transient absorption (TA) techniques. Ultrafast transient absorption suggests the formation of bi-exciton in CIS NCs which efficiently dissociates in Au-CIS nanohybrids. Maximum multiexciton dissociation (MED) efficiency is determined to be ~ 80% at higher laser fluency, however it was observed to be 100% at lower laser fluency. Prior to exciton dissociation electrons are captured by Au NP in the nanohybrid from the conduction band of CIS NCs which is energetically higher than Fermi level of Au. Here we demonstrate the proof-of-concept in multi-electron dissociation which may provide a new approach for improving the efficiency in QDSSCs, where we found power conversion efficiency (PCE) of Au-CIS nanohybrids up to 2.49% as compared to ~1.06% ~for pure CIS NCs in similar condition. This finding can be an efficient approach towards the design and development of efficient solar cell and optoelectronic devices using the principles of multiexciton generation and extracting multiexcitons in metal-semiconductor nanohybrid system.
关键词： copper indium sulfide,biexciton dissociation dynamics,Quantum Dot Sensitized Solar Cells,power conversion efficiency,ultrafast transient absorption,Multiexciton harvesting,Au-CIS nanohybrid
更新于2025-09-23 15:21:21
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Fluorinating ??a??Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells
摘要： The synthesis and characterization of new semiconducting materials is essential for developing high-efficiency organic solar cells. Here, the synthesis, physiochemical properties, thin film morphology, and photovoltaic response of ITN-F4 and ITzN-F4, the first indacenodithienothiophene nonfullerene acceptors that combine π-extension and fluorination, are reported. The neat acceptors and bulk-heterojunction blend films with fluorinated donor polymer poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]-dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PBDB-TF, also known as PM6) are investigated using a battery of techniques, including single crystal X-ray diffraction, fs transient absorption spectroscopy (fsTA), photovoltaic response, space-charge-limited current transport, impedance spectroscopy, grazing incidence wide angle X-ray scattering, and density functional theory level computation. ITN-F4 and ITzN-F4 are found to provide power conversion efficiencies greater and internal reorganization energies less than their non-π-extended and non-fluorinated counterparts when paired with PBDB-TF. Additionally, ITN-F4 and ITzN-F4 exhibit favorable bulk-heterojunction relevant single crystal packing architectures. fsTA reveals that both ITN-F4 and ITzN-F4 undergo ultrafast hole transfer (<300 fs) in films with PBDB-TF, despite excimer state formation in both the neat and blend films. Taken together and in comparison to related structures, these results demonstrate that combined fluorination and π-extension synergistically promote crystallographic π-face-to-face packing, increase crystallinity, reduce internal reorganization energies, increase interplanar π–π electronic coupling, and increase power conversion efficiency.
关键词： impedance spectroscopy,organic solar cells,crystal structures,femtosecond transient absorption spectroscopy,computational chemistry
更新于2025-09-23 15:21:01