Transient Emission Spectra Research Articles

Pt-S Bond-Enabled Temperature-Dependent Phosphorescence in S-Heteroaryl Tetradentate Pt(S^C^N^O) Complexes.

This study presents the rare examples of S-heteroaryl tetradentate Pt(S^C^N^O) luminescent complexes (PtSZ and PtSZtBu) containing a Pt-S bond. The presence of the Pt-S bond allows the novel Pt(S^C^N^O) complexes to exhibit temperature-dependent phosphorescent emission behavior. The PtSZtBu exhibits dual-emission phenomena and biexponential transient decay spectra above 250 K, indicating the presence of two minimal excited states in the potential energy surface (PES) of the T1 state. Through complementary experimental and computational studies, we have identified changes in orbital composition between Pt(dxy)-S(px) and Pt(dyz)-S(pz) in excited states with increasing temperature. This results in two energy minima, enabling the excited states to decay selectively and radiatively at different temperatures. Consequently, this leads to remarkable steady-state and transient emission spectra changes. Our work not only provides valuable insights for the development of novel Pt-S bond-based tetradentate Pt(II) complexes but also enhances our understanding of the distinctive properties governed by the Pt-S bond.

Amplified Spontaneous Emission from Electron-Hole Quantum Droplets in Colloidal CdSe Nanoplatelets.

Two-dimensional cadmium selenide nanoplatelets (NPLs) exhibit large absorption cross sections and homogeneously broadened band-edge transitions that offer utility in wide-ranging optoelectronic applications. Here, we examine the temperature-dependence of amplified spontaneous emission (ASE) in 4- and 5-monolayer thick NPLs and show that the threshold for close-packed (neat) films decreases with decreasing temperature by a factor of 2-10 relative to ambient temperature owing to extrinsic (trapping) and intrinsic (phonon-derived line width) factors. Interestingly, for pump intensities that exceed the ASE threshold, we find development of intense emission to lower energy in particular provided that the film temperature is ≤200 K. For NPLs diluted in an inert polymer, both biexcitonic ASE and low-energy emission are suppressed, suggesting that described neat-film observables rely upon high chromophore density and rapid, collective processes. Transient emission spectra reveal ultrafast red-shifting with the time of the lower energy emission. Taken together, these findings indicate a previously unreported process of amplified stimulated emission from polyexciton states that is consistent with quantum droplets and constitutes a form of exciton condensate. For studied samples, quantum droplets form provided that roughly 17 meV or less of thermal energy is available, which we hypothesize relates to polyexciton binding energy. Polyexciton ASE can produce pump-fluence-tunable red-shifted ASE even 120 meV lower in energy than biexciton ASE. Our findings convey the importance of biexciton and polyexciton populations in nanoplatelets and show that quantum droplets can exhibit light amplification at significantly lower photon energies than biexcitonic ASE.

Phosphine-Oxide-Balanced Intra- and Interchain Through-Space Charge Transfer in Thermally Activated Delayed Fluorescence Polymers: Beyond 30% External Quantum Efficiency.

Through-space charge transfer (TSCT) is crucial for developing highly efficient thermally activated delayed fluorescence polymers. The balance of intra- and interchain TSCT can markedly improve performance, but it is still a big challenge. In this work, an effective strategy for "intra- and interchain TSCT balance" is demonstrated by way of a series of non-conjugated copolymers containing a 9,9-dimethylacridine donor and triazine-phosphine oxide (PO)-based acceptors. Steady-state and transient emission spectra indicate that compared to the corresponding blends, the copolymers can indeed achieve balanced intra- and interchain TSCT by accurately optimizing the inductive and steric effects of the acceptors. The DPOT acceptor with the strongest electron-withdrawing ability and the second bigger steric hindrance endows its copolymers with state-of-the-art photoluminescence and electroluminescence quantum efficiencies beyond 95% and 32%, respectively. This demonstrates that, compared to other congeners, the synergistic inductive and steric effects effectively enhance TSCT in DPOT-based copolymers for radiation, and suppress singlet and triplet quenching. The record-high efficiencies of its devices make this kind of copolymers hold the potential for low-cost, large-scale, and high-efficiency applications.

Applications of time‐resolved step‐scan Fourier‐transform infrared spectroscopy in revealing the light‐initiated reactions in condensed phases

AbstractStep‐scan Fourier‐transform infrared spectroscopy (ssFTIR) simultaneously provides the spectroscopic and kinetic information of a given reaction. ssFTIR has been extensively employed to acquire the transient absorption and emission spectra in gas phase for identifying unstable species, for example, various Criegee intermediates, and elucidating the dynamics and kinetics of the reaction, such as the molecular elimination dynamics of haloalkenes and the bimolecular reactions involving chlorine atoms and singlet oxygen atoms. In addition to gaseous studies, ssFTIR has been also utilized to record the time‐resolved difference spectra of the photochemical reactions in condensed phases, such as the photolysis of metal–ligand complexes, photocycles of the retinal proteins, coordination capability of solvents to unstable transient species, chemical reactions of atmosphere‐related molecules in aqua, and the exciplex dynamics of organic light emitting materials. Moreover, my group has pioneered the recording of the transient thermal infrared emission of gold nanostructures upon photoexcitation. The experimental setups and the working principles for probing the time‐resolved infrared absorption and emission in condensed phases will be revealed and a number of studies on chemical, biological, and materials systems will be described. These reported results demonstrate that ssFTIR is a versatile tool for exploring the properties of novel materials and photoreactions in condensed phases.

In Situ Encapsulation of Graphene Quantum Dots in Highly Stable Porphyrin Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction.

Photocatalytic CO2 reduction to valuable hydrocarbon solar fuel is of great significance but still challenging. Strong CO2 enrichment ability and easily adjustable structures make metal-organic frameworks (MOFs) potential photocatalysts for CO2 conversion. Even though pure MOFs have the potential for photoreduction of CO2, the efficiency is still quite low due to rapid photogenerated electron-hole recombination and other drawbacks. In this work, graphene quantum dots (GQDs) were in situ encapsulated into highly stable MOFs via a solvothermal method for this challenging task. The GQDs@PCN-222 with encapsulated GQDs showed similar Powder X-ray Diffraction (PXRD) patterns to PCN-222, indicating the retained structure. The porous structure was also retained with a Brunauer-Emmett-Teller (BET) surface area of 2066 m2/g. After incorporation of GQDs, the shape of GQDs@PCN-222 particles remained, as revealed by the scanning electron microscope (SEM). As most of the GQDs were covered by thick PCN-222, it was hard to observe those GQDs using a transmission electron microscope (TEM) and a high-resolution transmission electron microscope (HRTEM) directly, the treatment of digested GQDs@PCN-222 particles by immersion in a 1 mM aqueous KOH solution can make the incorporated GQDs visible in TEM and HRTEM. The linker, deep purple porphyrins, make MOFs a highly visible light harvester up to 800 nm. The introduction of GQDs inside PCN-222 can effectively promote the spatial separation of the photogenerated electron-hole pairs during the photocatalytic process, which was proved by the transient photocurrent plot and photoluminescence emission spectra. Compared with pure PCN-222, the obtained GQDs@PCN-222 displayed dramatically enhanced CO production derived from CO2 photoreduction with 147.8 μmol/g/h in a 10 h period under visible light irradiation with triethanolamine (TEOA) as a sacrificial agent. This study demonstrated that the combination of GQDs and high light absorption MOFs provides a new platform for photocatalytic CO2 reduction.

Excited state trap in erbium doped borate glass

Erbium doped borate glass is widely used in luminescent materials, the luminescence dynamics of erbium doped borate glass is of great significance for optimizing and improving the luminous efficiency. The 2% molar ratio erbium doped borate glass was synthesized by the traditional melt quenching method, and annealed at 260 °C below the borate glass transition temperature. The thermal performance parameters of borate glass undoped and doped with Er3+ were measured by differential scanning calorimetry with 10 °C/min. The transient emission spectrum and decay kinetics curves were measured for the luminescence mechanism of erbium doped borate. Er3+ ions have different lifetime when emitted at 556 nm with different excitation wavelengths, the excited state trap may exist in erbium doped borate glass.

The regulatory effect of triphenylphosphine oxide on perovskites for morphological and radiative improvement

The influence of a triphenylphosphine oxide (TPPO) additive on the morphology and emission of FAPbBr3 perovskites is investigated by transient and temperature-dependent emission spectra, which indicates the defect passivation through P=O coordination.

Transient Spectral Fluctuations of Single Molecules Revealed using an Optical Antenna

Fluorescence spectra and lifetimes of single molecules reflect their specific local environments, which is not available from the ensemble-averaged measurements. Usually, a low signal-to-noise ratio of single-molecule spectroscopy impedes access to transient emission spectra of single molecules at room temperature, especially for the molecules with a low quantum yield. Here, we realize detection of transient single-molecule spectra with an effective integration time of ∼2.14 milliseconds at room temperature of methylene blue (MB) or crystal violet (CV) molecules. In the experiments, the transient emission spectra detection of single molecules is achieved through the enhanced signal using a plasmonic antenna, the suppressed background noise due to a low quantum yield, and the short dwelling time of molecular adsorption on the antenna surface. The transient spectra show apparent fluctuation in the emission intensity and shifts of the peak maximum. Interestingly, the emission spectrum presents a narrow band which can be fitted well with a single Lorentzian lineshape. The specific molecule configuration should dominate the transient spectral lineshape mainly, which is supported by the numerical ab initio simulations. These spectral fluctuations should attribute to the transitions between metastable minima in the molecular potential-energy surface. These findings provide us more understanding of the transient behaviors of single molecules at the solid-aqueous interfaces.

Opto-thermal Radiometry for Detection of Surface Contamination

The aim of this work is to conduct a proof-of-concept study on detection of surface contamination by biological and chemical agents using opto-thermal transient emission radiometry (OTTER). OTTER is an infrared remote sensing technology that has been developed in our research group for skin measurements. It is based on the measurement of laser-induced transient thermal emission spectra in the infrared fingerprint region, and has advantages of non-contact, non-invasive, spectroscopic in nature, and can work on arbitrary surfaces of any materials. In this paper, two types of experiments were conducted, different ingredients on the surface of a sample material and different ingredients on the skin surface. The results show that by measuring the OTTER signal at different detection wavelengths, e.g., 6.05 μm, 6.48 μm, 7.79 μm, 8.5 μm, 9.5 μm, 11.5 μm, 13.1 μm, it is possible to differentiate from skin and skin with different ingredients on the surface. The next step is to develop mathematical models and analytical algorithms to quantify the ingredient concentration on the surface.

Aldehyde group functionalized iridium(III) complexes for the selective sensing of homocysteine

Two aldehyde group functionalized iridium(III) complexes were synthesized and characterized. The electronic absorption, emission and transient emission spectra were studied. The crystal structure of one of the complexes has been determined. Their interaction with homocysteine (Hcy) was investigated by UV–vis absorption, photoluminescence (PL), 1H NMR and mass spectroscopies. The photoluminesce of the complexes was enhanced significantly after addition of Hcy due to the formation of thiazide product, while there are no obvious changes in the emission spectra after addition of cysteine (Cys) and glutathione (GSH). These probes can be used for discriminating Hcy from Cys and GSH efficiently.

Coupled Excited-State Dynamics in N-Substituted 2-Methoxy-9-Acridones.

Fluorophores of the acridone family have been widely employed in many applications, such as DNA sequencing, the detection of biomolecules, and the monitoring of enzymatic systems, as well as being the bases of intracellular sensors and even antitumoral agents. They have been widely used in fluorescence imaging due to their excellent photophysical properties, in terms of quantum yield and stability. However, frequently, the fluorescence emission data from acridones are not easily interpretable due to complex excited-state dynamics. The formation of π-stacking aggregates and excimers and excited-state proton transfer (ESPT) reactions usually result in emission features that are dependent on the experimental conditions. Therefore, an in-depth understanding of the dynamics involved in the excited-state transients of these dyes is mandatory for their appropriate application. Herein, we synthesized and fully characterized different 2-methoxy-9-acridone dyes. Their transient fluorescence emission spectra exhibited a complex dynamic behavior that can be linked to several excited-state reactions. We performed a thorough study of the excited-state dynamics of these dyes by means of time-resolved fluorimetry supported by computational calculations. All this allowed us to establish a multistate kinetic scheme, involving an ESPT reaction coupled to an excimer formation process. We have unraveled the rich dynamics behind this complex behavior, which provides a better understanding of the excited states of these dyes.

Stilbene-benzophenone dyads for free radical initiating polymerization of methyl methacrylate under visible light irradiation

The AB and AB2 trans-stilbene-benzophenone dyads were synthesized through multi-step pathway. The target dyads showed remarkable absorption in visible light region. Visible light photoinitiating polymerization of methyl methacrylate by the new dyads was studied by photo-differential scanning calorimetry. The results suggested that the AB2 dyad showed more efficient photoinitiating polymerization of methyl methacrylate than the AB one under visible light irradiation. Furthermore, the new dyads yielded the greater visible light photoinitiaing polymerization effect than the intermolecular photoinitiating systems and the commercial photoinitiator titanocene respectively. The molecular weights of visible light photopolymers produced by the new dyads were determined by gel permeation chromatography. The visible light photoinitiating mechanism of the new dyads was analyzed by the static and transient absorption and emission spectra, electron spin resonance spectra and cyclic voltammograms. The thermal stabilities of the new dyads were further studied by differential scanning calorimetry and thermograving.

Lasing dynamics study by femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy**Project supported by the National Natural Science Foundation of China (Grant Nos. 20925313 and 21503066), the Innovation Program of Chinese Academy of Sciences (Grant No. KJCX2-YW-W25), the Postdoctoral Project of Hebei University, China, and the Project of Science and Technology Bureau of Baoding City, China (Grant No. 15ZG029).

Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy (FNOPAS) is a versatile technique with advantages of high sensitivity, broad detection bandwidth, and intrinsic spectrum correction function. These advantages should benefit the study of coherent emission, such as measurement of lasing dynamics. In this letter, the FNOPAS was used to trace the lasing process in Rhodamine 6G (R6G) solution and organic semiconductor nano-wires. High-quality transient emission spectra and lasing dynamic traces were acquired, which demonstrates the applicability of FNOPAS in the study of lasing dynamics. Our work extends the application scope of the FNOPAS technique.

Time and species dependent ambient air’s effects on carbon clusters generated during femtosecond laser ablation of highly oriented pyrolytic graphite

Near infrared femtosecond laser is employed to ablate highly oriented pyrolytic graphite (HOPG) in ambient air and in vacuum respectively. The recorded transient emission spectra of the ablated plume with a time resolution of 2 ns indicate that the effects of air on the plume are dependent on both time and species. This finding provides important insights into the generation and decay mechanisms of different carbon radicals or clusters. At 1 or 2 ns after the laser pulse strikes the target, air weakens the Swan bands of C2 compared with the case in vacuum, an effect that may be caused by the quenching collisions between air molecules and C2 radicals. This result shows that C2 may be mainly generated by direct spallation from the target surface at the early stage of ablation. Emission spectra at large time delays present that the existence time of the Swan bands in air is longer than the lifetime of the upper electronic state of the Swan system, suggesting that the air-involved three-body recombination and collisional excitation that enhance the generation of emitting C2 overcome quenching collisions at large time delays. A spectral band at ~416 nm assigned to the transition from 1Σ u + to X 1Σ g + of C15 is more intense in air than in vacuum, indicating that C15 clusters are generated at least partially by the combination of smaller clusters in air. It is also found that air-assisted heat transfer makes the temperature of carbon clusters decrease more quickly in air than in vacuum, leading to a much shorter lifetime of the continuum in air.

Optical Properties of Carbazole-Based Photorefractive Polymers

The optical properties of several carbazole-based photorefractive polymers which we prepared were determined. Through analysis of the UV-vis spectrum, the ratio of the azo nitrobenzene was adjusted to control the content of azo chromophore functional components. The solid transient fluorescence emission spectra of the photorefractive polymers were tested with a 325 nm emission wavelength. The results showed that the J-aggregates and several large closed-loop structures formed in the photorefractive polymers resulted in a long lifetime of their transient fluorescence. Spin-orbit coupling among the molecules increased the intersystem crossing rate, and induced delayed fluorescence.

Transient Absorption of N719 and its Electron Transfer Kinetics on ZnO Nanoparticles Surface

The transient absorption and emission spectra of ruthenium complex sensitizer N719 under excitation in different solvents were studied. Isopropanol was found to stabilize the singlet excited state of N719. An emission band centered at 460 nm resulting from the singlet excited state of N719 was observed at 1 ns time delay, which is much longer than the reported observation time of the singlet excited state of N719. The triplet excited state of N719 undergoes photophysical decay in acetonitile and isopropanol with lifetimes of around 40 ns, while it may encounter photochemical reactions in water resulting in long living intermediate. The sensitizer was introduced to the surface of three types of ZnO nanoparticles with different morphology, which were used as the electron acceptors upon photoexcitation. The electron transfer dynamics between sensitized N719 and ZnO interface both in the presence and absence of electrolyte were studied by time-correlated single photon counting technique, nanosecond transient absorption and emission spectroscopies. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics at the N719-ZnO interface. In the absence of electrolyte, the electron transfer process at the N719-ZnO interface is dependent on the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, ZnO defects show no impacts on the electron transfer process at the N719-ZnO interface and effective electron injection happens similarly from the excited N719 to ZnO in spite of ZnO particle sizes.

Enhanced broadband ultrafast detection of ultraviolet emission using optical Kerr gating.

We demonstrate a high-sensitivity ultrafast emission spectrometer based on the optical Kerr effect that time resolves emission simultaneously in the ultraviolet and visible ranges. We show that using benzene as the Kerr medium leads to the optimal balance between time-resolution and sensitivity of the optical shutter with low losses due to ultraviolet absorption. Using this medium together with high contrast broadband polarizers and charge-coupled device detection, we achieve efficient detection of emission transients (bandwidth >1.5 eV) in a time bin of ~500 fs. To highlight the distinctive insights that can be gained by resolving complex subpicosecond dynamics in a single experiment, we present UV-visible transient emission spectra of technologically relevant wide bandgap zinc oxide. With an enhanced broadband detection, subpicosecond effects such as thermalization, bandgap renormalization, and carrier trapping can be easily assessed, with ramifications for optoelectronics and energy-related technologies.

瞬态发射光谱法研究正十二烷燃烧反应特性

Using a heated shock tube and an intensified spectroscopic detector CCD,transient emission spectra of n-dodecane in the combustion reaction were measured in the range of 200~900 nm.Experiments were conducted at pressure of 4.0atm,temperatures of(1 200~1 300)K,equivalence ratios of 0.5,1.0,and 2.0.Results show that the main emission bands are attributed to OH,CH and C2 radicals produced during the combustion process.In the different equivalence ratios,the emission intensities of OH(306.4 nm)/CH(431.4 nm)/C2(516.4 nm)are significantly different in the combustion process,which indicates that fuel-lean situation is good for generation of OH radicals,and fuel-rich situation is good for C2.Different diluent gases lead to different combustion temperatures when the fuel burns,which causes the different patterns of the emission spectra of the fuel.The variation of OH,CH and C2 radicals produced in the combustion of n-dodecane are shown clearly by the time-resolved spectrum obtained.This research is helpful for understanding the properties and validating the mechanism of n-dodecane combustion reaction.

Photochemistry of fac‐[Re(bpy)(CO)3Cl

The photochemistry of fac-[Re(bpy)(CO)3Cl] (1 a; bpy=2,2′-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1 b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO)2Cl(MeCN)] (2 c; bpy=2,2′-bipyridine). In MeCN, two isomers, 2 c and its (OC-6-34) form (2 a), were produced. Only 2 c thermally isomerized to produce the (OC-6-44) form 2 b. A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a. Irradiation of 1 a using the higher-energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (3HES(1)). In THF, 3HES(1) was competitively converted to both the triplet ligand field (3LF) and metal-to-ligand charge transfer (3mLCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)2Cl(thf)]+ (1 c) within 10 ps by means of a dissociative mechanism. Re-coordination of CO to 1 c gives both 1 a and 1 b. In MeCN, irradiation of 1 a by using high-energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c. A seven-coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.

Ultrafast Twisting Dynamics in the Excited State of Auramine

Relaxation dynamics of the excited state of bis-[4-(dimethylamino)-phenyl] methaniminium chloride (Auramine) has been investigated using subpicosecond time-resolved absorption spectroscopic technique in both aprotic and alcoholic solvents. The locally excited (LE) state, formed following photoexcitation of Auramine using 400 nm light, undergoes intramolecular charge transfer (ICT) process, which is accompanied by the twisting of the dimethylanilino groups. Time evolution of the transient absorption-stimulated emission spectra as well as the wavelength dependence of the temporal dynamics investigated in each kind of solvents suggest that the relaxation process proceeds via the formation of at least two transient states (TS I and TS II), which are geometrical conformers and consecutively formed following the decay of the LE state. Twisting of the dimethylaniline groups are nearly barrierless processes, the rates of which show linear correlation both with the macroscopic or shear viscosities as well as the solvation times of the solvents. Time-dependent and fractional viscosity dependence of the relaxation rates of the LE and the TS I states in aprotic solvents suggest the multidimensionality of the reaction coordinate as well as reveal the viscoelastic property of the solvents. However, in normal alcohols, in addition to these two factors, activation energy of the solvent viscosity may be another important factor for the slower twisting dynamics of Auramine in alcohols. To explain the viscosity dependence of the decay time of the TS II state, which undergoes an efficient internal conversion process to the ground state, the possibility of occurrence of different mechanisms, such as, energy gap law, involvement of intramolecular high frequency modes, as well as the phenyl group twisting motion on a potential energy surface having a photochemical funnel, have been discussed. TDDFT method has been applied to obtain the optimized electronic structures of the transient states but it has been possible to obtain only that for the TS II state.