Femtosecond Time-resolved Transient Absorption Research Articles

Ultrafast Dynamics of a Solvatochromic Dye, Phenol Blue: Tautomerization and Coherent Wavepacket Oscillations

Femtosecond time-resolved transient absorption spectroscopy was performed for a nonfluorescent solvatochromic dye, phenol blue, N-(4-dimethylaminophenyl)-1,4-benzoquinoneimine, which exhibits ultrafast nonradiative decay due to its flexible molecular structure. By exciting the molecule in ethanol (EtOH) solution with two excitation wavelengths located at shorter- and longer-wavelength sides of the visible absorption band, we observed ultrafast nonradiative decay from the excited state, followed by spectral evolution in the ground state. The nonradiative decay in the subpicosecond range creates a vibrationally hot ground state with the lifetime in the picosecond range. Subsequently, a tautomer that absorbs at shorter wavelengths is produced from the hot state, which causes a red shift of the ground-state bleach (GSB). The tautomerization presumably involves twisting of the benzoquinoneimine moiety induced by the breaking of the hydrogen bond (H-bond) between the solute and the solvent molecules. The recombination of the H-bond occurs with a time constant of ∼30 ps, and the system returns to its original state. We also observed low-frequency coherent wavepacket oscillations that modulate the GSB with dephasing times similar to the excited-state lifetime.

Plasmonic gold nanorods decorated Ti3C2 MXene quantum dots-interspersed nanosheet for full-spectrum photoelectrochemical water splitting

Efficient utilization of solar energy for economical photoelectrochemical (PEC) water splitting requires highly active and stable photoelectrodes with the abilities of harvesting broadband light from ultraviolet to infrared. Herein, we report on the synthesis and characterization of plasmonic gold nanorods (NRs) in situ decorated onto the Ti3C2 MXene quantum dots-interspersed Ti3C2 nanosheet (TDTS) as well as their implementation as photoanodes for full-spectrum solar-light-activated PEC water splitting. The TDTS is prepared by a hydrothermal method, followed the plasmonic Au NRs chemically anchored on the surface of TDTS via a seed growth method. Under the illumination of simulated solar light (100 mW cm−2), the optimized Au NRs/TDTS hybrid composite displays a stable and remarkable gas production rate that is about 3.4 times higher than that of pristine TDTS, and broadened spectral response range of 300–1100 nm. Such high performance arises from the generating strong plasmon-induced hot electrons of Au NRs that inject into TDTS with efficient charge separation and transfer at the interface. X-ray photoelectron spectroscopy and femtosecond time-resolved transient absorption measurements provide direct evidences that the plasmon-induced interfacial hot electrons could efficient transfer from Au to TDTS with the quantum efficiency of 16 ± 5%. This strategy provides an avenue for designing plasmon-mediated composite systems to effectively harvest full-spectrum solar light conversion into chemical energy.

Mechanistic Insights into Rapid Generation of Nitroxyl from a Photocaged N-Hydroxysulfonamide Incorporating the (6-Hydroxynaphthalen-2-yl)methyl Chromophore.

HNO is a highly reactive molecule that shows promise in treating heart failure. Molecules that rapidly release HNO with precise spatial and temporal control are needed to investigate the biology of this signaling molecule. (Hydroxynaphthalen-2-yl)methyl-photocaged N-hydroxysulfonamides are a new class of photoactive HNO generators. Recently, it was shown that a (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM)-photocaged derivative of N-hydroxysulfonamide incorporating the trifluoromethanesulfonamidoxy group (1) quantitatively generates HNO. Mechanistic studies have now been carried out on this system and reveal that the ground state protonation state plays a key role in whether concerted heterolytic C-O/N-S bond cleavage to release HNO occurs versus undesired O-N bond cleavage. N-Deprotonation of 1 can be achieved by adding an aqueous buffer or a carboxylate salt to an aprotic solvent. Evidence is presented for C-O/N-S bond heterolysis occurring directly from the singlet excited state of the N-deprotonated parent molecule on the picosecond time scale, using femtosecond time-resolved transient absorption spectroscopy, to give a carbocation and 1NO-. This is consistent with the observation of significant fluorescence quenching when HNO is generated. The carbocation intermediate reacts rapidly with nucleophiles including water, MeOH, or even (H)NO in the absence of a molecule that reacts rapidly with (H)NO to give an oxime.

In situ construction of elemental phosphorus nanorod-modified TiO2 photocatalysts for efficient visible-light-driven H2 generation

In this work, we constructed a red phosphorus (RP)/TiO2 composite system via a chemical vapor deposition strategy. The evolution of the fibrous phased RP nanorods on the surface of anatase TiO2 spheres was rationally adjusted by controlling the composition of precursors. The optimal RP/TiO2 composite exhibited a significantly enhanced visible-light-driven photocatalytic H2 generation rate of 681 μmol h−1 g−1. The tightly interior surface bonding between RP and TiO2 greatly promoted the efficient charge transfer and redistribution by suppressing the charge recombination and self-trapping processes within TiO2, resulting in prolonged lifetimes of the active charges during the photocatalytic process as investigated in detailed using femtosecond time-resolved transient absorption (fs-TA) spectroscopy.

Excitation Energy-Dependent Branching Dynamics Determines Photostability of Iron(II)-Mesoionic Carbene Complexes.

Photoactive metal complexes containing earth-abundant transition metals recently gained interest as photosensitizers in light-driven chemistry. In contrast to the traditionally employed ruthenium or iridium complexes, iron complexes developed to be promising candidates despite the fact that using iron complexes as photosensitizers poses an inherent challenge associated with the low-lying metal-centered states, which are responsible for ultrafast deactivation of the charge-transfer states. Nonetheless, recent developments of strongly σ-donating carbene ligands yielded highly promising systems, in which destabilized metal-centered states resulted in prolonged lifetimes of charge-transfer excited states. In this context, we introduce a series of novel homoleptic Fe-triazolylidene mesoionic carbene complexes. The excited-state properties of the complexes were investigated by time-resolved femtosecond transient absorption spectroscopy and quantum chemical calculations. Pump wavelength-dependent transient absorption reveals the presence of distinct excited-state relaxation pathways. We relate the excitation-wavelength-dependent branching of the excited-state dynamics into various reaction channels to solvent-dependent photodissociation following the population of dissociative metal centered states upon excitation at 400 nm.

Nature of Linear Spectral Properties and Fast Electronic Relaxations in Green Fluorescent Pyrrolo[3,4-c]Pyridine Derivative.

The electronic nature of 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT) was comprehensively investigated in liquid media at room temperature using steady-state and time-resolved femtosecond transient absorption spectroscopic techniques. The analysis of the linear photophysical and photochemical parameters of HPPT, including steady-state absorption, fluorescence and excitation anisotropy spectra, along with the lifetimes of fluorescence emission and photodecomposition quantum yields, revealed the nature of its large Stokes shift, specific changes in the permanent dipole moments under electronic excitation, weak dipole transitions with partially anisotropic character, and high photostability. Transient absorption spectra of HPPT were obtained with femtosecond resolution and no characteristic solvate relaxation processes in protic (methanol) solvent were revealed. Efficient light amplification (gain) was observed in the fluorescence spectral range of HPPT, but no super-luminescence and lasing phenomena were detected. The electronic structure of HPPT was also analyzed with quantum-chemical calculations using a DFT/B3LYP method and good agreement with experimental data was shown. The development and investigation of new pyrrolo[3,4-c]pyridine derivatives are important due to their promising fluorescent properties and potential for use in physiological applications.

Enhanced optical nonlinearity of Mxene Ti3C2Tx nanosheets decorated with silver nanoparticles

Two-dimensional Ti3C2Tx nanosheets have drawn much attention due to their unique nonlinear optical properties. To enhance the optical nonlinearity of the material, we synthesized Ti3C2Tx composites decorated with silver nanoparticles (Ti3C2Tx/Ag) through self-assembling of Ag nanoparticles on the surface of Ti3C2Tx. The nonlinear optical properties the composite were studied via nanosecond laser Z-scan method, and enhanced saturable absorption (SA) and reversed saturable absorption (RSA) effects were demonstrated. Using the femtosecond time-resolved transient absorption measurements, the carrier dynamics in the nonlinear response, as well as the enhancement mechanism of the composites was clarified.

Ab Initio Surface-Hopping Simulation of Femtosecond Transient-Absorption Pump-Probe Signals of Nonadiabatic Excited-State Dynamics Using the Doorway-Window Representation.

An ab initio theoretical framework for the simulation of femtosecond time-resolved transient absorption (TA) pump-probe (PP) spectra with quasi-classical trajectories is presented. The simulations are based on the classical approximation to the doorway-window (DW) representation of third-order four-wave-mixing signals. The DW formula accounts for the finite duration and spectral shape of the pump and probe pulses. In the classical DW formalism, classical trajectories are stochastically sampled from a positive definite doorway distribution, and the signals are evaluated by averaging over a positive definite window distribution. Nonadiabatic excited-state dynamics is described by a stochastic surface-hopping algorithm. The method has been implemented for the pyrazine molecule with the second-order algebraic-diagrammatic construction (ADC(2)) ab initio electronic-structure method. The methodology is illustrated by ab initio simulations of the ground-state bleach, stimulated emission, and excited-state absorption contributions to the TA PP spectrum of gas-phase pyrazine.

Hot excitons cooling and multiexcitons Auger recombination in PbS quantum dots

In the past few years, lead chalcogenide quantum dots (QDs) have attracted attention as a new system with a strong quantum confinement effect. In this paper, the hot-excitons cooling and Auger recombination of multiexcitons in PbS QDs are investigated by the femtosecond time-resolved transient absorption spectroscopy. The results show that the excitons dynamics in PbS QDs are closely related to the pump-photon energy and pump-pulse energy. Multiexcitons generate when the excess energy of the absorbed photons is larger than the bandgap energy in PbS QDs. The hot-excitons cooling lifetime increases but the Auger recombination lifetime decreases as the pump-photon energy and the pump-pulse energy increase. Besides, there is a competitive relation between multiple-excitons generation and hot-excitons cooling. The dynamics results of the formation and relaxation of multiexcitons in PbS QDs would shed light on the further understanding of the interaction between excitons and photons in the optoelectronic application based on PbS QDs.

A combined experimental and theoretical investigation of the excited-state dynamics of 2,4,6-trinitrotoluene (TNT) in DMSO solvent.

In the present contribution we carried out a TDDFT and femtosecond transient absorption study of the excited state dynamics of TNT in DMSO solvent. Vertical excitation and excited state relaxation were calculated at the SMD/M06-2X/TZVP level of theory. The electron absorption spectrum for the DMSO solvated TNT was calculated and compared with the experimental results. The results of the electronic excitation energies and the spin-orbital constants imply an intersystem crossing for the S1-T2 transition. The femtosecond time-resolved transient absorption measurements of the TNT in DMSO show the presence of two absorption signals around 650 nm and 540 nm, which are assigned to the population in the lowest singlet and triplet excited states, S1 and T1, respectively. The fast decay of the S1 state population is assigned to an efficient S1-T2 intersystem crossing, which soon internally converts to the T1 state. The slow decay of the T1 population is attributed to the nonradiative transition to the S0 state. The combined theoretical and experimental results present a mechanistic view of the photophysical dynamics of TNT in DMSO solution.

不同核心尺寸及壳层厚度CdTe/CdS量子点非线性光学及超快动力学特性

CdTe core-shell semiconductor quantum dots are being widely explored due to their special nonlinear optics and ultrafast dynamics characteristics that include solar cells, optoelectronic devices, biological labeling, and optics fiber sensing fields. In this work, the six kinds of CdTe/CdS core-shell quantum dots were researched in the different core sizes and shell thicknesses for the nonlinear optics and ultrafast dynamics characteristics. The nonlinear absorption and refraction coefficients of the samples were measured by using Z-scan technology under the action of 400 nm wavelength and 130 fs laser pulse width. The experimental results show that the shell thickness of CdTe/CdS core-shell quantum dots affects the nonlinear absorption and refraction characteristics, in which the nonlinear absorption and refraction coefficients increase with the shell thickness. And the core size mainly affects the nonlinear absorption characteristics, while the nonlinear absorption coefficient decreases with the increase of the core size. At the same time, femtosecond time-resolved transient absorption spectroscopy technology was used to measure the ultrafast dynamics characteristics of the samples under the conditions of 400 nm wavelength, 130 fs pulse width, 1 kHz frequency, and 400 nJ single-pulse energy. The transient absorption spectra and ultrafast dynamics curves were obtained. The results suggest that the rising time of bleaching signal increases with the shell thickness. The decay time of the fast process increases with the shell thickness and core size. The decay time of the slow process increases with the shell thickness. The research reveals the influence of the core size and shell thickness of CdTe core-shell quantum dots on the nonlinear optics and ultrafast dynamics, providing a theoretical basis for the preparation of core-shell quantum dots and the research of the photophysical properties.

Nonradiative Decay and Vibrational Dephasing of Malachite Green in Trehalose Glass Monitored by Femtosecond Time-Resolved Transient Absorption Spectroscopy

Nonradiative Decay and Vibrational Dephasing of Malachite Green in Trehalose Glass Monitored by Femtosecond Time-Resolved Transient Absorption Spectroscopy

Ultrafast carrier relaxation in SnSe (x=1, 2) thin films observed using femtosecond time-resolved transient absorption spectroscopy

Abstract In this study, SnSex (x = 1, 2) films are prepared using pulsed laser deposition (PLD) and characterized using X-ray diffraction spectroscopy (XRD) and X-ray photoelectron spectroscopy (XPS). The crystal structure and valence state of SnSex films can be observed using XRD patterns and XPS spectra. The carrier dynamics are investigated using femtosecond time-resolved TA spectroscopy. The carrier relaxation process is strongly influenced by thin film stoichiometry. The TA spectra observed in the SnSe and SnSe2 films associated with photoinduced absorption, free carrier absorption, and state filling effects. The carrier ultrafast dynamics of two films are well fitted using a tri-exponential decay model with fast (several picoseconds), medium (tens of picoseconds), and slow (several nanoseconds) components. These components are associated with carrier scattering, Auger recombination, and diffusion, respectively. The component with medium lifetime (Auger recombination) in the SnSe2 film is faster than that of SnSe film because defect state trapping. The slow relaxation component in SnSe2 film, on the other hand, is much longer than that of SnSe film. This is also due to trapping by defect states in SnSe2, which affects the diffusion of the photo-induced carriers. We investigated the carrier relaxation processes in SnSex (x = 1, 2) films and offered an insight in the ultrafast dynamic mechanism in the two tin selenides.

Vibrational decoherence induced by ultrafast intramolecular charge separation of an asymmetric bianthryl derivative.

Coherent wavepacket oscillation accompanying the ultrafast photoexcited intramolecular charge separation (CS) of 9,9'-bianthryl (BA) and 10-cyano-9,9'-bianthryl (CBA) in a room temperature ionic liquid, N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4), was investigated by femtosecond time-resolved transient absorption spectroscopy. The frequency of the coherent oscillation observed for CBA in nonpolar n-hexane solution (Hex) was 377 cm-1, while this oscillation was undetectable in DemeBF4. For BA in DemeBF4, coherent oscillation with a frequency of 394 cm-1 was observed, which is similar to that for CBA in Hex. CS of CBA occurs in the ultrashort time range of ≤100 fs, while that of BA occurs in a few picosecond range [E. Takeuchi et al., J. Phys. Chem. C 120, 14502-14512 (2016)]. Hence, the oscillation of CBA in Hex and that of BA in DemeBF4 are assigned to the molecular vibration in the locally excited state, while this oscillation dephases instantaneously for CBA in DemeBF4 due to the ultrafast CS and no oscillation was generated in the CS state. This result suggests that the CS reaction is not mediated by a specific intramolecular vibration in the CS state but occurs incoherently through higher levels of multiple vibrational modes.

Enhanced nonlinear optical absorption of WS2 by Ag nanoparticles

The nonlinear absorption and ultrafast dynamics process of WS2 and WS2-Ag nanocomposite were investigated by using nanosecond Z-scan measurements and femtosecond time-resolved transient absorption spectrum, respectively. Z-scan measurements showed that WS2 exhibit saturation absorption (SA) at low excitation intensities, with the increase of incident intensity, a switch from SA to reverse saturation absorption (RSA) occurs, but WS2-Ag nanocomposite exhibit SA at the same intensity. Transient absorption spectrum provided information show that the response time did not change much. The investigation shows that WS2 and WS2-Ag nanocomposite can be used for mode-locker and optical switcher.

Nonlinear absorption properties and carrier dynamics in MoS2/Graphene van der Waals heterostructures

Recently, molybdenum disulfide (MoS2) nanosheets have been used in passively mode-locked laser due to excellent saturable absorption (SA) property. To enhance their optical nonlinearity, some heterostructures such as MoS2/Graphene (MoS2/G) have been designed, but the carrier dynamics in the materials have rarely been reported. Here, the nonlinear response of MoS2 and MoS2/G heterostructures nanosheets with different thickness are studied via nanosecond laser Z-scan and femtosecond time-resolved transient absorption (TA) technique. The results reveal that the ground state bleaching take place under intense laser excitation, which would block the further absorption of incident light, causing a strong SA effect of the materials. With increasing the thickness of MoS2, electron diffusion take place between adjacent layers, resulting in a slow recombination of excited carriers and the enhancement of SA effect in MoS2. In MoS2/G, excited electrons could transfer from MoS2 to graphene and the interlayer carrier diffusion in MoS2 could be enhanced, prolonging the decay time of ground state bleaching effect. The slower carrier relaxation time in MoS2 would lead a stronger SA effect for nanosecond laser pulse. This work could provide an effective strategy for designing saturable absorbers in mode-locked lasers using MoS2 heterostructures.

MoS2-Stratified CdS-Cu2-xS Core-Shell Nanorods for Highly Efficient Photocatalytic Hydrogen Production.

Heterojunction photocatalysts are widely adopted for efficient water splitting, but ion migration can seriously threaten the stability of heterojunctions, as with the well-known low stability of CdS-Cu2-xS due to intrinsic Cu+ ion migration. Here, we utilize Cu+ migration to design a stratified CdS-Cu2-xS/MoS2 photocatalyst, in which CuI@MoS2 (CuI-intercalated within the MoS2 basal plane) is created by Cu+ migration and intercalation to the adjacent MoS2 surface. The epitaxial vertical growth of the CuI@MoS2 nanosheets on the surface of one-dimensional core-shell CdS-Cu2-xS nanorods forms catalytic and protective layers to simultaneously enhance catalytic activity and stability. Charge transfer is verified by kinetics measurements with femtosecond time-resolved transient absorption spectroscopy and direct mapping of the surface charge distribution with a scanning ion conductance microscope. This design strategy demonstrates the potential of utilizing hybridized surface layers as effective catalytic and protective interfaces for photocatalytic hydrogen production.

Wavefunction engineering for efficient photoinduced-electron transfer in CuInS2 quantum dot-sensitized solar cells

The high efficiency of quantum dot-sensitized solar cells (QDSSCs) is a benefit of the highly efficient photoinduced-electron transfer (PET) to external electrodes. Here, we investigated how the surface defects and conduction-band (CB) offsets between core and shell materials affect the PET from CuInS2 quantum dots (QDs) by means of time-resolved femtosecond transient absorption and nanosecond photoluminescence spectroscopy. The transfer of 1S excited electrons from CuInS2 QDs to TiO2 films is demonstrated and we find that the surface-electron trapping can significantly reduce the efficiency of the PET. Though the electron trapping can be suppressed after ZnS surface passivation, the PET decreases significantly to a low efficiency of ∼33% from the type I CuInS2/ZnS core/shell QDs because of their low electron density at the surface of the QDs. The surface-electron density is increased with the strategy of wavefunction engineering by reducing the CB offset, which allows us to achieve a quasi-type II carrier confinement in CuInS2/CdS core/shell QDs. The PET efficiency appears to be as high as ∼95% from the CuInS2/CdS core/shell QDs, which is ascribed to synergistic effects of the surface passivation and enhanced delocalization of the electron wavefunction from the CuInS2 core to the CdS shell. Finally, we demonstrate that these new mechanistic understandings of the PET processes are crucial to improving the efficiency of CuInS2 QDSSCs.

Enhancement mechanism of the saturable absorption effect in reduced graphene oxide decorated with silver nanoparticles

Reduced graphene oxide (rGO) decorated with silver nanoparticles exhibits an enhanced nonlinear absorption effect compared with pure rGO. Using femtosecond time-resolved transient absorption spectroscopy, the enhancement mechanism and carrier dynamics of the composites are experimentally demonstrated. When the material is excited by laser pulses, the excited carriers in the conduction band of graphene will transfer to the d-band of silver before returning to the valence band. As the decay process (∼210 ps) is much longer than that of the relaxation time in pure graphene (∼fs), the bleaching effect of valence band is prolonged, resulting in enhanced saturable absorption effect.

Solvent effects on triplet-triplet annihilation upconversion kinetics of perylene with a Bodipy-phenyl-C60 photosensitizer.

The solvent effect usually plays an important role in triplet-triplet annihilation (TTA) upconversion processes. In this work, we have studied the TTA upconversion kinetics of perylene with Bodipy-phenyl-C60 as the triplet photosensitizer in five solvents, 1,4-dioxane, dichlorobenzene, chlorobenzene, toluene, and tetrahydrofuran (THF). Although no significant solvent effect was observed in steady-state absorption and fluorescence emission spectra, the overall TTA upconversion quantum yields showed a profound dependence on solvent properties, i.e. 4.9% in 1,4-dioxane, 7.1% in dichlorobenzene, 6.7% in chlorobenzene, 4.6% in toluene, and 2.2% in THF (the maximum of 50%). Each elementary reaction step involved in the overall process was analyzed by applying femtosecond and nanosecond time-resolved transient absorption spectroscopy, revealing that the fluorescence emission of perylene was more significantly affected by the solvents in contrast to the other steps. Moreover, an extra intramolecular energy-transfer pathway of Bodipy-phenyl-C60 was found via the formation of charge-separated states in dichlorobenzene, chlorobenzene, and THF solvents, once being excited. These conclusions provide valuable clues to choose the most favorable solvent for the higher TTA upconversion efficiency in related applications.