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Quasi-2D layers of transition metal dichalcogenides are promising candidates for creating saturable absorbers for pulsed lasers. However, the peculiarities of intense electromagnetic radiation’s influence on such structures have not been thoroughly studied. This paper explores the dynamics of photoexcited carriers in WSe2 flakes through experimental studies. These studies found that WSe2 flakes significantly change their optical properties under the influence of a high-power optical pump, allowed estimating the thermalization time of these structures (about 2 ps), and found that full relaxation takes more than 10 ps. The concentration of carriers in the semiconductor surface layer was estimated to be about 1028 m–3. It was found that standard description models of the optical response based on exciton resonances and absorption by free carriers could not adequately describe the experiments’ results. Thus, for an accurate description of the optical response, it was necessary to consider the effects associated with Coulomb screening that are caused by the high concentration of photo-excited carriers of the optical pumping densities used in this experiment.

Trp146 of the p53 DNA-binding domain (DBD) was investigated by static and time-resolved fluorescence combined with molecular dynamics (MD) simulations at different temperatures (25, 30, 37, and 45 °C). Static emission spectra exhibit an intensity maximum at 30 °C without any substantial peak shift, while the time-resolved fluorescence displays a peculiar stretched exponential decay, indicative of a structural disorder, at all of the investigated temperatures. The stretched exponential parameter was found to increase at 37 °C. An analysis of the MD simulation trajectories evidenced the occurrence of jumps in the temporal evolution of the distances between Trp146 and residues Arg110, Asp228, Cys229, and Gln144, which are mainly responsible for Trp146 fluorescence quenching. The times that these quenchers spend close to or far from Trp146 can provide an explanation for the static fluorescence behavior. Further essential dynamics analysis of the MD trajectories indicates a significant restriction of protein global motions above 37 °C. These results are consistent with a decrease in the structural heterogeneity of DBD as the temperature increases. The results are also discussed in view of understanding how temperature can modulate the p53 capability to binding partners, including DNA.

To study the influence of thermal annealing on the charge generation and transport in PM6-based non-fullerene solar cells. Morphology, optical and electrochemical properties of active layers as well as electrical properties of polymer solar cells were studied. Furthermore, the photoelectric conversion processes of annealed and unannealed devices were also examined by means of time resolved spectroscopy. The results showed that thermal annealing had a weak influence on the dynamics of exciton states. Besides, annealed device is found to suppress bimolecular recombination owing to its higher charge carrier mobility in ordered donor and acceptor aggregation phases, which led to higher photocurrent and power conversion efficiency than unannealed photovoltaic device.

We introduce a new approach to transient spectroscopy, fluorescence‐detected pump–probe (F‐PP) spectroscopy, that overcomes several limitations of traditional PP. F‐PP suppresses excited‐state absorption, provides background‐free detection, removes artifacts resulting from pump–pulse scattering, from non‐resonant solvent response, or from coherent pulse overlap, and allows unique extraction of excited‐state dynamics under certain conditions. Despite incoherent detection, time resolution of F‐PP is given by the duration of the laser pulses, independent of the fluorescence lifetime. We describe the working principle of F‐PP and provide its theoretical description. Then we illustrate specific features of F‐PP by direct comparison with PP, theoretically and experimentally. For this purpose, we investigate, with both techniques, a molecular squaraine heterodimer, core–shell CdSe/ZnS quantum dots, and fluorescent protein mCherry. F‐PP is broadly applicable to chemical systems in various environments and in different spectral regimes., Ultrafast pump–probe spectroscopy detected by fluorescence is introduced. Combining sensitive fluorescence detection with ultrafast pulse‐limited time resolution, this approach provides a complementary technique to standard transient absorption.

Multichromophoric triads 1 and 2 based on aza-BODIPY as the central chromophore and bay-substituted (tetrachloro- and tetraphenoxy-)perylenediimides (PDI) as peripheral chromophores have been designed and synthesized that are panchromatic absorbers and near infrared (NIR) emitters. Both triads 1 and 2 exhibited ∼99% Forster resonance energy transfer (FRET) from the peripheral PDIs to central aza-BODIPY. The excitation energy transfer from PDI to aza-BODIPY was studied via steady state emission, fluorescence quantum yield, time resolved fluorescence emission and theoretical calculations. These studies revealed quantitative singlet excitation energy transfer efficiencies for 1 and 2. Electrochemical studies revealed the strong electron deficient character of these triads and thus electron mobilities of these triads were measured using space charge limited current (SCLC) method. Triads 1 and 2 exhibited appreciable electron mobilities of 2.44 ± 1.70 × 10−3 cm2 V−1 s−1 and 4.00 ± 1.50 × 10−3 cm2 V−1 s−1 respectively, an order of magnitude higher mobility than aza-BODIPY based small molecules reported in the literature. Leveraging upon the dual emission behaviour of these triads, ratiometric FRET sensing as well as ratiometric temperature sensing behaviour were investigated via steady state absorption and fluorescence measurements. Triads 1 and 2 showed remarkable ratiometric FRET-off sensing where the addition of metals such as Co2+ and Fe3+ led to near-quantitative FRET off for both the triads. Triads 1 and 2 also serve as efficient ratiometric temperature sensors with positive temperature coefficients and small temperature sensitivities of ∼0.29% °C−1 and ∼0.14% °C−1 respectively that suggest the possibility of precise physiological temperature measurements using these triads.

With an aim to understand the difference in the behaviour of imidazolium and pyrrolidinium-based dicationic ionic liquids (DILs) in terms of the intermolecular interactions, microscopic-structure and dynamics, two DILs, the imidazolium-based 1,9-bis(3-methylimidazolium-1-yl)nonane bis(trifluoromethanesulfonyl)imide and the pyrrolidinium-based 1,9-bis(1-methylpyrrolidinium-1-yl)nonane bis(trifluoromethanesulfonyl)imide, have been synthesized and subsequently investigated by exploiting combined steady sate and time resolved fluorescence, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic techniques. Data obtained for DILs have also been compared with their corresponding mono-cationic counterpart (MILs) to evaluate and understand the distinctive characteristics of the DILs in contrast with the corresponding MILs. Steady state emission and EPR data have revealed that the pyrrolidinium-based DIL is slightly less polar than the imidazolium-based DIL. Temperature-dependent fluorescence anisotropy decay of two probes, perylene and MPTS (8-methoxypyrene-1,3,6-trisulfonate), has been measured in DILs as well as in MILs. Solute-solvent coupling constants obtained from the experimentally measured rotational correlation times with the aid of Stokes-Einstein-Debye hydrodynamic theory have indicated appreciable differences in the dynamics of both the solutes on going from MILs to DILs. More interestingly, the outcome of the NMR study has suggested that the alkyl spacer chain in the imidazolium-based DIL exists in the folded form, but the pyrrolidinium-based DIL remains in the straight chain conformation. Inherently, the outcomes of all of these studies have depicted that the microscopic structural organisations in imidazolium and pyrrolidinium-based DILs are different from each other as well as from their respective mono-cationic counterparts.

The dynamics of photo-driven charge transfer-induced spin transition (CTIST) in two Fe/Co Prussian Blue Analogues (PBAs) is revealed by femtosecond IR and UV/vis pump-probe spectroscopy. Depending on temperature the known tetranuclear square-type complex [Co2Fe2(CN)6(tp*)2(4,4’-dtbbpy)4](PF6)2 (1) exists in two electronic states. In acetonitrile solution at LT) phase is prevalent consisting of low-spin Fe(II) and low-spin Co(III), [FeIILSCoIIILS]2. Temperature rise causes thermally induced CTIST towards the high temperature (HT) phase consisting of low-spin Fe(III) and high-spin Co(II), [FeIIILSCoIIHS]2, being prevalent at >300 K. Photo-excitation into the intervalence charge transfer (IVCT) band of the LT phase at 800 nm induces electron transfer in one Fe-Co edge of PBA 1 and produces a [FeIIILSCoIILS] intermediate which by spin crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [FeIIILSCoIIHS]. In contrast, IVCT excitation of the HT phase at 400 nm generates a [FeIILSCoIIIHS] species with a lifetime of 3.6 ps. Subsequent back-electron transfer populates the vibrationally hot ground state, which thermalizes within 8 ps. The newly synthesized dinuclear PBA, [CoFe(CN)3(tp*)(pz*4Lut)]ClO4 (2), provides a benchmark of the HT phase of 1, i.e. [FeIIILSCoIIHS], as verified by variable temperature magnetic susceptibility measurements and 57Fe Mößbauer spectroscopy. The photo-induced charge transfer dynamics of PBA 2 indeed is almost identical to that of the HT phase of phase of PBA 1 with a lifetime of the excited [FeIILSCoIIIHS] species of 3.8 ps.

Singlet fission in organic semiconducting materials has attracted great attention for the potential application in photovoltaic devices. Research interests have been concentrated on identifying working mechanisms of coherent SF processes in crystalline solids as ultrafast SF is hailed for efficient multiexciton generation. However, as long lifetime of multiexcitonic triplet pair in amorphous solids facilitates the decorrelation process for triplet exciton extractions, a precise examination of incoherent SF processes is demanded in delicate model systems to represent heterogeneous structures. Heterogeneous coupling and energetics for SF were developed in our oligoacene dendrimers, which mimic complicated SF dynamics in amorphous solids. SF dynamics in dendritic structures was thoroughly investigated by time-resolved spectroscopic techniques and quantum chemical calculations in respect of the relative orientation/distance between chromophores and though-bond/-space interactions.

Capability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types. However, application of such molecular-level switching has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump-probe spectroscopy in the visible and mid-infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization-switching material realized using the metastable state. Moreover, the pump-probe data in the visible region reveal the pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm-1 , which we attribute to a lattice vibrational mode. The pronounced non-Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.

In this paper, we fabricated NaYF4: Eu3+ nanocrystals via a facile hydrothermal approach. By a technique of in situ optical spectroscopy, the luminescence kinetics of the NaYF4: Eu3+ nanocrystals were real-time monitored by a confocal optical microscopy system. It is discovered that the most intense emission peak of Eu3+ ions is at 615 nm (5D0→7F2) that belongs to the electric dipole (ED) transition and its intensity is sensitive to the crystal environment. The 5D0→7F2 emission band could appear obvious stark-splitting phenomenon during the heat process of the nanocrystals at 1000 °C. The luminescence mechanism of NaYF4: Eu3+ nanocrystals is analyzed by Judd–Ofelt theory. The special spectroscopic properties of NaYF4: Eu3+ nanocrystals at high temperature could have an impact on the optical properties of trivalent lanthanide ion doped fluoride materials and associated applications. Furthermore, this finding may serve as a promising and powerful technique to better investigate the PL mechanism.

A simple antipyrine based fluorescent probe, 4-[(2-hydroxy-3-methoxy-benzylidene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (OVAP), has been successfully synthesized using a one-step condensation method. It exhibits dual sensing properties toward Al3+ and Zn2+ in the presence of other relevant metal ions and also displays novel aggregation induced emission enhancement (AIEE) characteristics in its aggregated/solid state. Aggregated OVAP microstructures with interesting morphologies have been synthesized using SDS as a morphology directing agent. Morphologies of the particles are characterized using optical microscopy. Photophysical properties of the as-synthesized OVAP hydrosol are studied using UV–Vis absorption, steady state and time resolved fluorescence spectroscopy. The ‘turn on’ luminescence property of OVAP is used for the selective detection of trace amounts of Al3+ and Zn2+ and a significant turn on fluorescence enhancement over ∼100-fold is triggered via chelation-enhanced fluorescence (CHEF) through complex formation. The 1 : 1 stoichiometry of each sensor metal ion complex is observed from Job's plot based on UV-Vis absorption titration. The LODs for Al3+ and Zn2+ are found to be 1.05 nM and 2.35 nM, respectively. Notably, the sensor, OVAP, is further demonstrated using a molecular INHIBIT logic gate.

This paper introduces some applications of time-of-flight or time-resolved spectroscopy to cellulosic materials like wood. Time-of-flight or time-resolved spectroscopy uses short pulses of light illumination where a portion of the light is scattered but most of the light propagates through the sample. By using such technique, we could know that the light scattering contribution to near infrared spectra in wood is significant because it is a complex and highly scattering medium due to its cellulosic structure.

Time-evolutions of excited state hydrogen transfer (ESHT) in phenol (PhOH)-(NH3)2 clusters have been measured by three-color picosecond (ps) ultraviolet (UV)-near infrared (NIR)-UV pump-probe ion dip spectroscopy. The formation of a reaction product, ˙NH4NH3, is detected by its NIR absorption due to a 3p-3s Rydberg transition. The ESHT reactions from all of the vibronic levels show biexponential time-evolutions, even from the S1 origin. Based on the biexponential time-evolution, it is suggested that there is a second reaction path via the triplet πσ* state, which gives the slow component. The fast time-evolution of the ESHT reaction from the S1 origin is measured to be 268 ps, which is 10-times slower than that in PhOH-(NH3)3, and a higher barrier between the ππ* and reactive πσ* states is suggested. The size dependence of the ESHT reaction rates is discussed based on a potential distortion due to the proton transferred state in the ππ* potential surface.

This work addresses the synthetic optimization of carbon dots (CDs) and their application in sensing picric acid from latent fingerprints by exploiting a smartphone-based RGB tool. The optimization of the synthesis of CDs is investigated towards achieving shorter reaction time, better product yield and fluorescence quantum efficiency. Precursors such as citric acid and thiourea were chosen for the synthesis of CDs. Among the various synthetic methodologies, it is found that the pyrolysis method offers ∼50% product yield within 15 min. The morphology and optical properties of the prepared CDs are characterized using the typical microscopic and spectroscopic techniques, respectively. The synthesized CDs exhibit quasi-spherical shape with an average particle size of 1.7 nm. The excitation dependent emissive properties of CDs are investigated by time resolved fluorescence spectroscopy. Furthermore, the excellent fluorescence properties (φ = 11%) of CDs are explored as a fluorescent fingerprint powder for the identification of latent fingerprints on various substrates. In addition, the presence of picric acid in latent fingerprints was detected. Furthermore, this study is extended to perform real time detection of fingerprints and harmful contaminants in fingerprints by utilizing a smartphone-based RGB color analysis tool. Based on these investigations, the prepared CDs could be a prospective fluorescent material in the field of forensics.

Defects in the lattice have both positive and negative implications for the optical properties of light emitting materials and a great effort is required to understand the defect structure surrounding the activator ion in phosphor materials. The fluoride based perovskite class of compounds such as KMgF3 is always associated with cationic and anionic vacancies, which may give rise to many interesting optical properties. They may also play an important role in determining the phosphor characteristics, when an activator such as Eu3+ ions is doped in KMgF3. This is because defects may alter the local structure of Eu3+ ions, whose characteristic transitions are very sensitive to the local environment. In the first attempt of its kind, we have tried to present a complete picture of the defect structures both in undoped and Li+, Eu3+ ion doped KMgF3 compounds and their influence on the phosphor characteristics. By analysing the changes in the defect structure due to changes in the dopant ions using positron annihilation lifetime spectroscopy supported by EPR and theoretical calculations, it was observed that different defect clusters are being evolved upon changing the dopant ions from Eu3+ to Li+ as co-dopant ions. These changes have a direct impact on the emission characteristics, especially the colour can be tuned to a more deep red region. Using time resolved spectroscopy two different Eu3+ components adjacent to different defect clusters are isolated. The clusters due to K-vacancies were found to change their configuration upon co-doping Li+ ions; VC1 for Eu3+ doped and VC2 for Eu3+ and Li+ co-doped compounds. From the relative percentage of these cluster vacancies, it was possible to make a conclusion about the distribution of Eu3+ and Li+ ions among the different unit cells of KMgF3, which have different defect structures. The study will be helpful in tailoring the defect structure while designing an efficient phosphor material.

In this study, ultrafast dynamics of photoexcited carriers in an intrinsic CdTe film are investigated deeply and systematically as a function of carrier density and excess energy using femtosecond-resolved pump-probe transient differential transmission spectroscopy. A dynamic model is developed based on an interband transition absorption model in direct bandgap semiconductors by introducing the dynamics of thermalization, cooling and recombination of photoexcited carriers into the interband transition absorption model. Then, the model is used to best fit the measured ultrafast dynamics to retrieve the initial temperature of thermalized electrons and time constants of the thermalization, cooling and recombination dynamic processes as a function of carrier density and excess energy. The increase with carrier density and the decrease with excess energy of thermalization time constant reveal that thermalization process of photoexcited carriers is dominated by carrier-phonon scatterings. We find for the first time that cooling of thermalized carriers depend strongly on carrier density and excess energy, and cooling time constant varies in a wide range from 0.2 to 5 ps. It is also found that carrier recombination in the intrinsic CdTe film is dominated by the radiative recombination of electron-hole pairs, rather than Auger recombination as reported in doped CdTe films. These new results are very important to understanding of microscopic mechanism of relaxation processes of photoexcited carriers and applications of CdTe-based optoelectronic devices.

We proposed a complex method based on a combination of shadow photography and time-resolved Raman spectroscopy to observe the non-stationary laser-induced supercritical state in molecular media. Shadow photography is applied for retrieving pressure values, while Raman spectroscopy with molecular dynamics for temperature estimation. Time resolution of 0.25 ns is achieved by varying the delay between the pump (creating an extreme energy delivery) and the probe laser pulses by the self-made digital delay electronic circuit . The proposed method was employed in liquid carbon dioxide and water. Under nanosecond laser pulse impact, the estimated temperatures and pressures (∼700 K and ∼0.5 GPa) achieved in media are higher than the critical parameters of the samples.

Inspired by the active center of the natural [FeFe] hydrogenases, we designed a compact and precious metal-free photosensitizer-catalyst dyad (PS-CAT) for photocatalytic hydrogen evolution under visible light irradiation. PS-CAT represents a prototype dyad comprising pi-conjugated oligothiophenes as light absorbers. PS-CAT and its interaction with the sacrificial donor 1,3-dimethyl-2-phenylbenzimidazoline were studied by steady-state and time-resolved spectroscopy coupled with electrochemical techniques and visible light-driven photocatalytic investigations. Operando EPR spectroscopy revealed the formation of an active [Fe(I)Fe(0)] species – in accordance with theoretical calculations – presumably driving photocatalysis effectively (TON ≈ 210).

Dual-comb spectroscopy (DCS) is a powerful spectroscopic technique, which is developing for the detection of transient species in reaction kinetics on a short time scale. Conventionally, the simultaneous determination of multiple species is limited to the requirement of broadband spectral measurement at the cost of the measurement speed and spectral resolution owing to the inherent trade-off among these characteristics in DCS. In this study, a high-speed multi-molecular sensing is demonstrated and achieved through using a programmable spectrum-encoded DCS technique, where multiple narrow encoding spectral bands are reserved selectively and other comb lines are filtered out. As a dual-comb spectrometer with a repetition rate of 108 MHz is encoded spectrally over a spectral coverage range of 1520 to 1580 nm, the measurement speed is increased 6.15 times and single-shot absorption spectra of multiple molecules (C2H2, HCN, CO, CO2) at a time scale of 208 µs are obtained. Compared to conventional single-shot dual-comb spectra, encoded dual-comb spectra have improved short-term signal-to-noise ratios (SNRs) by factors of 3.65 with four encoding bands and 5.68 with two encoding bands. Furthermore, a fiber-Bragg-grating-based encoded DCS is demonstrated, which reaches 17.1 times higher average SNR than that of the unencoded DCS. This spectrum-encoded technique can largely improve the DCS measurement speed, and thus is promising for use in studies on multi-species reaction kinetics.

International audience; Spin Crossover (SCO) is a promising switching phenomenon when implemented in electronic devices as molecules, thin films or nanoparticles. Among the properties modulated along this phenomenon, optically induced mechanical changes are of tremendous importance as they can work as fast light-induced mechanical switches or allow to investigate and control microstructural strains and fatigability. The development of characterization techniques probing 2 nanoscopic behaviour with high spatio-temporal resolution allows to trigger and visualize such mechanical changes of individual nanoscopic objects. Here we use Ultrafast Transmission Electron Microscopy (UTEM) to precisely probe the length changes of individual switchable nanoparticles induced thermally by nanosecond laser pulses. This allows us to reveal the mechanisms of spin switching, leading to the macroscopic expansion of SCO materials. This study was conducted on individual pure SCO nanoparticles and SCO nanoparticles encapsulating gold nanorods that serve for plasmonic heating under laser pulses. Length changes are compared with time-resolved optical measurements performed on an assembly of these particles.

In the last 10 years, there has been a boom in the organic photovoltaic (OPV) community, with new devices achieving power conversion efficiencies (PCEs) of ~18%. This significant increase in device performance is due to a switch from polymer:fullerenes, such as P3HT:PCBM, to polymer:non-fullerenes, like PM6:Y6. These films are often created with the use of solvent additives which, when correctly chosen, increase the device performance. However, because many of these polymer:non-fullerene systems have only recently been developed the exact effect that these solvent additives have on the morphology, performance and electronic properties is not well understood. Here we use a combination of photoluminescence and time-resolved spectroscopy along with grazing-incidence wide-angle x-ray scattering to fully understand what is occurring at both an electronic and morphological level in a series of PM6:Y6 films with varying amounts of acetone used as a solvent additive. From these data we find that acetone changes the degree of mixing and crystal grain size, which leads to changes in polaron yield and recombination, which is reflected in device performance.

Time-resolved near-infrared absorption spectroscopy of single non-repeatable transient events is performed at high spectral resolution with a dual-comb interferometer using a continuous-wave laser followed by a single electro-optic amplitude modulator. By sharing high-speed electrical/optical components, our spectrometer greatly simplifies the implementation of dual-comb spectroscopy and it offers a high mutual coherence time, measured up to 50-s, without any active stabilization system and/or data processing. The time resolution, which can be reconfigured a posteriori, is as short 100 microseconds in our experimental demonstration. For a span of 36 GHz, the mean signal-to-noise ratio of 80, at 100-MHz spectral resolution and 100-microsecond measurement time, enables the precise determination of the parameters of rovibrational lines, including intensity or concentration., Comment: 8 pages, 4 figures

2D perovskites, consisting of alternating layers of metal halide sheets and cations, tend to be more environmentally robust compared to their bulk 3D counterpart and have broad technological appeal because of their tunable mechanical, optical, and electrical properties. While these materials have promise for flexible optoelectronic applications, it is necessary to determine the impact of strain on the perovskite optical and electronic properties. Here, we discuss our work in understanding how strain modifies the carrier dynamics of 2D perovskites using time resolved spectroscopy. We compare the photoluminescence lifetime of two different 2D perovskite materials, synthesized using either phenethylammonium or butylammonium cations. Both perovskite materials exhibit about a 50% decrease in the lifetime for tensile strains

The results of time-resolved fluorescence measurements of flavin mononucleotide (FMN) in rigid polyvinyl alcohol films (PVA) demonstrate that fluorescence intensity decays are strongly accelerated in the presence of fluorescent dimers and nonradiative energy transfer processes ⟨τ⟩fl = 2.66 ns (at λexc = 445 nm) and ⟨τ⟩fl&nbsp, &nbsp, = 2.02 (at λexc = 487 nm) at λobs = 600 nm and T = 253 K from H and J state of dimers, respectively. We show that inhomogeneous orientational broadening of energy levels (IOBEL) affects the shape of the fluorescence decay and leads to the dependence of the average monomer fluorescence lifetime on excitation wavelength. IOBEL affected the nonradiative energy transfer and indicated that different flavin positioning in the protein pocket could (1) change the spectroscopic properties of flavins due to the existence of “blue” and “red” fluorescence centers, and (2) diminish the effectiveness of energy transfer between FMN molecules.