Femtosecond Time-resolved Spectroscopy Research Articles

Insights into Ultrafast Relaxation Dynamics of Electronically Excited Furfural and 5-Methylfurfural.

The ultrafast relaxation dynamics of furfural and 5-methylfurfural following excitation in the ultraviolet range is investigated using the femtosecond time-resolved photoelectron spectroscopy method. Specifically, the pump wavelength-dependent decay dynamics of electronically excited furfural and 5-methylfurfural is discussed on the basis of a detailed analysis of our measured time-resolved photoelectron spectroscopy spectra. Irradiation at all pump wavelengths prepares both furfural and 5-methylfurfural molecules with different vibrational levels in the first optically bright S2 (1ππ*) state, the lifetime of which is measured to be at least hundreds of femtoseconds. Besides the prominent deactivation channels of ring-opening and ring-puckering pathways for the S2(1ππ*) state, we propose that there is a minor decay channel of internal conversion from the initially prepared S2(1ππ*) state to the S1(1nπ*) state. The wavepacket decays out of the Franck-Condon region on the S2(1ππ*) state potential energy surface and bifurcates into different parts somewhere. A small fraction of the wavepacket funnels down to the S1(1nπ*) state via internal conversion. The subsequently populated S1(1nπ*) state contains large vibrational excess energy and decays over a lifetime of 2.5-2.8 ps. One of the deactivation channels of the S1(1nπ*) state is intersystem crossing to the 3ππ* triplet state. In addition, methyl substitution effects on the excited-state dynamics of furfural are also discussed. This experimental study provides new insights into the excitation energy-dependent decay dynamics of photoexcited furfural and 5-methylfurfural.

Influence of Aliphatic versus Aromatic Ligand Passivation on Intersystem Crossing in Au25(SR)18.

The electronic relaxation dynamics of gold monolayer protected clusters (MPCs) are influenced by the hydrocarbon structure of thiolate protecting ligands. Here, we present ligand-dependent electronic relaxation for a series of Au25(SR)18- (SR = SC8H9, SC6H13, SC12H25) MPCs using femtosecond time-resolved transient absorption spectroscopy. Relaxation pathways included a ligand-independent femtosecond internal conversion and a competing ligand-dependent picosecond intersystem crossing process. Intersystem crossing was accelerated for the aliphatic (SC6H13, SC12H25) thiolate MPCs compared to the aromatic (SC8H9) thiolate MPCs. Additionally, a 1.2 THz quadrupolar acoustic mode and a 2.4 THz breathing acoustic mode was identified in each cluster, which indicated that differences in ligand structure did not result in significant structural changes to the metal core of the MPCs. Considering that the difference in relaxation rates did not result from ligand-induced core deformation, the accelerated intersystem crossing was attributed to greater electron-vibrational coupling to Au-S vibrational modes. The results suggested that the organometallic semiring was less rigid for the aliphatic thiolate MPCs due to reduced steric effects, and in turn, increases in nonradiative decay rates were observed. Overall, these results imply that the protecting ligand structure can be used to modify carrier relaxation in MPCs.

(Invited) Charge Separation Processes in Non-Fullerene Acceptors Studied by Time-Resolved IR

The mechanism of charge transfer and charge dissociation processes are investigated by femtosecond time-resolved IR absorption spectroscopy.

Right On Time: Ultrafast Charge Separation Before Hybrid Exciton Formation.

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it wasclearly outpaced by TiO2 in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. It is demonstrated that charge separation indeed occurs efficiently on ultrafast (350fs) timescales, but that electrons are recaptured at the interface on a 100ps timescale and subsequently trapped in a strongly bound (0.7eV) hybrid exciton state with a lifetime exceeding 5µs. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large time frame for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybridsystems.

Ultrafast Events of Photoexcited Iron(III) Chloride for Activation of Benzylic C-H Bonds.

The usage of rare-earth-metal catalysts in the synthesis of organic compounds is widespread in chemical industries but is limited owing to its environmental and economic costs. However, recent studies indicate that abundant-earth metals like iron(III) chloride can photocatalyze diverse organic transformations using blue-light LEDs. Still, the underlying mechanism behind such activity is debatable and controversial, especially in the absence of ultrafast spectroscopic results. To address this urgent challenge, we performed femtosecond time-resolved electronic absorption spectroscopy experiments of iron(III) chloride in selected organic solvents relevant to its photocatalytic applications. Our results show that the long-lived species [Fe(II) ← Cl•]* is primarily responsible for both oxidizing the organic substrate and reducing molecular oxygen through the diffusion process, leading to the final product and regenerating the photocatalyst rather than the most widely proposed free chloride radical (Cl•). Our study will guide the rational design of efficient earth-abundant photocatalysts.

Real-time observation of two distinctive non-thermalized hot electron dynamics at MXene/molecule interfaces

The photoinduced non-thermalized hot electrons at an interface play a pivotal role in determining plasmonic driven chemical events. However, understanding non-thermalized electron dynamics, which precedes electron thermalization (~125 fs), remains a grand challenge. Herein, we simultaneously captured the dynamics of both molecules and non-thermalized electrons in the MXene/molecule complexes by femtosecond time-resolved spectroscopy. The real-time observation allows for distinguishing non-thermalized and thermalized electron responses. Differing from the thermalized electron/heat transfer, our results reveal two non-thermalized electron dynamical pathways: (i) the non-thermalized electrons directly transfer to attached molecules at an interface within 50 fs; (ii) the non-thermalized electrons scatter at the interface within 125 fs, inducing adsorbed molecules heating. These two distinctive pathways are dependent on the irradiating wavelength and the energy difference between MXene and adsorbed molecules. This research sheds light on the fundamental mechanism and opens opportunities in photocatalysis and interfacial heat transfer theory.

Cypate-sensitized upconversion nanoprobes for intracellular and in-vivo ATP ratiometric detection

As energy currency of life, adenosine triphosphate (ATP) is highly dynamic in living organisms. It is crucial yet challenging to probe intracellular or in-vivo ATP precisely without destruction of the living biological systems. Herein, we have designed novel ATP-responsive ratiometric nanoprobes by employing upconversion nanoparticles (UCNPs) with cypate as antennas. Nuclear magnetic resonance spectroscopy was used to analyze the interfacial interactions among lanthanide nanocrystal, cypate and adenosine phosphates, disclosing the origin of detection specificity. ATP induced interruption of energy transfer pathway in cypate-sensitized UCNPs was further investigated by femtosecond transient absorption and time-resolved photoluminescence spectroscopy. The nanoprobes with dual near-infrared (NIR)-excited ratiometric UCL were successfully applied for quantification of ATP level in living cells under vegetatively growing, starving, and glycolysis-inhibiting states, respectively. Furthermore, the increase of available energy by feeding for zebrafish larvae was confirmed through monitoring the in-vivo ATP variation with the nanoprobes. Such nanoprobes showed great potential in ATP-associated biological research and clinical applications. These findings may gain deep insights into the surface science and photophysics in dye-lanthanide nanocomposites, and open new avenues for the design and bioapplications of lanthanide nanoprobes.

Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy

The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.

Probing the Local Near-Field Intensity of Plasmonic Nanoparticles in the Mid-infrared Spectral Region.

The enhanced local field of gold nanoparticles (AuNPs) in mid-infrared spectral regions is essential for improving the detection sensitivity of vibrational spectroscopy and mediating photochemical reactions. However, it is still challenging to measure its intensity at subnanometer scales. Here, using the NO2 symmetric stretching mode (νNO2) of self-assembled 4-nitrothiophenol (4-NTP) monolayers on AuNPs as a model, we demonstrated that the percentage of excited νNO2 mode, determined by femtosecond time-resolved sum-frequency generation vibrational spectroscopy, allows us to directly detect the local field intensity of the AuNP surface in subnanometer ranges. The local-field intensity is tuned by AuNP diameters. An approximate 17-fold enhancement was observed for the local field on 80 nm AuNPs compared to the Au film. Additionally, the local field can regulate the anharmonicity of the νNO2 mode by synergistic effect with molecular orientation. This work offers a promising approach to probe the local field intensity distribution around plasmonic NP surfaces at subnanometer scales.

Practical Method for Achieving Single-Photon Femtosecond Time-Resolved Spectroscopy: Transient Stimulated Emission.

Recent advances in single-photon femtosecond spectroscopy have highlighted the power of entangled photons in probing the properties of materials, previously inaccessible through semiclassical spectroscopic approaches. In this study, we theoretically propose a new single-photon-based femtosecond time-resolved spectroscopy technique termed single-photon transient stimulated emission (SP-TSE). SP-TSE not only enables the selective investigation of singly excited superposition states but also harnesses the quantum mechanical nature of photons for the efficient data acquisition of transient responses, thereby supporting the feasibility of experimental realization of SP-TSE. The key aspect of SP-TSE is the selective detection of two-photon states produced through stimulated emission using coincidence counting techniques. Our theoretical framework, supported by numerical simulations, demonstrates the efficacy in capturing the pure decoherence dynamics of a model molecular cavity, highlighting its potential to reveal quantum mechanical properties that are difficult to observe with semiclassical femtosecond time-resolved experiments.

Ultrafast Decay Dynamics of the 21ππ* Electronic State of N-Methyl-2-pyridone.

The ultrafast decay dynamics of N-methyl-2-pyridone upon excitation in the near-ultraviolet range of 261.5-227.9 nm is investigated using the femtosecond time-resolved photoelectron spectroscopy method. Irradiation at 261.5 nm prepares N-methyl-2-pyridone molecules with high vibrational levels in the 11ππ* state. The radiation-less decay to the ground state via internal conversion is suggested to be the dominant channel for the 11ππ* state with large vibrational excess energy, which is revealed by a lifetime of 1.6 ± 0.2 ps. As the pump wavelength decreases, we found that irradiation at 238.5 and 227.9 nm results in the population of the 21ππ* state. This is in agreement with the assignment of the vapor-phase UV absorption bands of N-methyl-2-pyridone. On the basis of the detailed analysis of our measured time-resolved photoelectron spectra at all pump wavelengths, we conclude that the 21ππ* state has an ultrashort lifetime of 50 ± 10 fs. In addition, the S1(11ππ*) state is subsequently populated via internal conversion and decays over a lifetime of 680-620 fs. The most probable whole deactivation pathway of the 21ππ* state is discussed. This experimental study provides new insights into the excitation energy-dependent decay dynamics of electronically excited N-methyl-2-pyridone.

Modulation of Photoinduced Phase Segregation and Stress-Driven Nanoscale Cracking in Hybrid Halide Perovskite Solar Cells.

The negative effect of photoinduced halide segregation (PIHS) on the properties of hybrid halide perovskites poses a major obstacle for its future commercial application. Therefore, the in-depth understanding of halide-ion segregation and its causes is an urgent and intractable problem. When PIHS reaches a certain threshold, it will aggravate the deterioration of the film surface morphology and form nanoscale cracks. Herein, the formation mechanism and types of cracks are revealed by exploring the stress distribution in the film. Using the femtosecond time-resolved transient absorption spectroscopy, the ultrafast formation of the iodine rich phase is observed, which appears earlier than the bromine rich phase. In addition, the introduction of organic ligand didodecyldimethylammonium bromide can significantly inhibit PIHS and improve the surface morphology of the film, which can promote the device efficiency from 9.63 to 11.20%. This work provides a novel perspective for the exploration of the PIHS.

Shell thickness influence on the carrier dynamics of InP/ZnS QDs

InP/ZnS core/shell quantum dots (QDs) have been widely applied in many fields due to their excellent luminescence performance. In this study, the correlation between the ZnS shell thickness and the ultrafast carrier dynamics in InP/ZnS QDs was investigated by using femtosecond time-resolved transient absorption spectroscopy and time-resolved photoluminescence techniques. We found that a thicker ZnS shell could confine more carriers to the InP core, thus effectively weakening the interactions between the carriers in the core and the interfacial defect states; this further enhanced the quantum yields and photoluminescence properties in InP/ZnS QDs.

Charge Trapping in Semiconductor Photocatalysts: A Time- and Space-Domain Perspective.

Harnessing solar energy to produce value-added fuels and chemicals through photocatalysis techniques holds promise for establishing a sustainable and environmentally friendly energy economy. The intricate dynamics of photogenerated charge carriers lies at the core of the photocatalysis. The balance between charge trapping and band-edge recombination has a crucial influence on the activity of semiconductor photocatalysts. Consequently, the regulation of traps in photocatalysts becomes the key to optimizing their activities. Nevertheless, our comprehension of charge trapping, compared to that of well-studied charge recombination, remains somewhat limited. This limitation stems from the inherently heterogeneous nature of traps at both temporal and spatial scales, which renders the characterization of charge trapping a formidable challenge. Fortunately, recent advancements in both time-resolved spectroscopy and space-resolved microscopy have paved the way for considerable progress in the investigation and manipulation of charge trapping. In this Perspective, we focus on charge trapping in photocatalysts with the aim of establishing a direct link to their photocatalytic activities. To achieve this, we begin by elucidating the principles of advanced time-resolved spectroscopic techniques such as femtosecond time-resolved transient absorption spectroscopy and space-resolved microscopic methods, such as single-molecule fluorescence microscopy and surface photovoltage microscopy. Additionally, we provide an overview of noteworthy research endeavors dedicated to probing charge trapping using time- and space-resolved techniques. Our attention is then directed toward recent achievements in the manipulation of charge trapping in photocatalysts through defect engineering. Finally, we summarize this Perspective and discuss the future challenges and opportunities that lie ahead in the field.

Intermolecular Protein-Water Coupling Impedes the Coupling Between the Amide A and Amide I Mode in Interfacial Proteins.

The coupling between different vibrational modes in proteins is essential for chemical dynamics and biological functions and is linked to the propagation of conformational changes and pathways of allosteric communication. However, little is known about the influence of intermolecular protein-H2O coupling on the vibrational coupling between amide A (NH) and amide I (C═O) bands. Here, we investigate the NH/CO coupling strength in various peptides with different secondary structures at the lipid cell membrane/H2O interface using femtosecond time-resolved sum frequency generation vibrational spectroscopy (SFG-VS) in which a femtosecond infrared pump is used to excite the amide A band, and SFG-VS is used to probe transient spectral evolution in the amide A and amide I bands. Our results reveal that the NH/CO coupling strength strongly depends on the bandwidth of the amide I mode and the coupling of proteins with water molecules. A large extent of protein-water coupling significantly reduces the delocalization of the amide I mode along the peptide chain and impedes the NH/CO coupling strength. A large NH/CO coupling strength is found to show a strong correlation with the high energy transfer rate found in the light-harvesting proteins of green sulfur bacteria, which may understand the mechanism of energy transfer through a molecular system and assist in controlling vibrational energy transfer by engineering the molecular structures to achieve high energy transfer efficiency.

Solid-State Emissive meso-Aryl/Alkyl-Substituted and Heteroatom-Mixed BOPPY Dyes: Syntheses, Structures, Physicochemical, Redox Properties, and Computational Analysis.

A series of solid-state emissive meso-aryl/alkyl-substituted and heteroatom-mixed bisBF2-anchoring fluorophore incorporating pyrrolyl-pyridylhydrazone (BOPPY) dyes have been developed by a one-pot condensation of ketonized or formylated pyrroles and 2-heterocyclohydrazine as well as the subsequent borylation coordination. Interestingly, the BOPPY dyes with meso-alkyl-substituted groups or oxygen-substituted pyridine moieties exhibit high fluorescence quantum yields (QYs) of up to 79%, the highest solid QY of 74%, and long lifetimes independent of polarity in the available BOPPYs. On the other hand, the BOPPYs with meso-aryl or N-substituted moieties display a high solution QY of up to 93% and slight emission wavelength maxima. However, the S-substituted BOPPY dye exhibited weak fluorescence in all studied solvents, which was attributed to the structural flexibility of the N-C-S bond and different from those BOPPYs with O or N substitution, indicated by quantum calculations. And the significant excited-state structural rearrangement in a polar solvent is further confirmed by femtosecond time-resolved transient absorption spectroscopy. More importantly, those novel and barely fluorescent BOPPYs in acetonitrile show advantageous aggregation-induced enhanced emission and viscosity-dependent activities. These advancements in the photophysical and electrochemical properties of BOPPY dyes offer valuable insights into their further development and potential applications.

The effect of solvent on the formation of low-dimensional metal halides and their self-trapped exciton emission

Low-dimensional organic–inorganic hybrid metal halides, with broadband luminescence, have attracted much attention for optoelectronic applications due to their rich in structural diversity and solution processibility. However, it is still unclear about how the solvent molecules influence the optoelectronic properties of the solution processed low-dimensional metal halides. Here, we prepared five different antimony-based crystal structures, [SbCl6]3- as the metal halide octahedron and 4, 4-difluoropiperidine (DFPD+) as organic cation, by using different solvents: hydrochloric acid (HCl) aqueous solution and four organic solvents (dimethylformamide (DMF), methanol (MeOH), acetonitrile (ACN) and dimethylacetamide (DMAC)). We revealed the relation between their crystal structures and optical properties, and we found the participation of organic molecules in the crystal structure causes significant lattice distortions, which is beneficial for achieving self-trapped exciton (STE) emission. Among them, (DFPD)6SbCl9·2DMAC exhibits a remarkable photoluminescence (PL) quantum yield of approximately 90 %. The STE dynamics in (DFPD)6SbCl9·2DMAC were characterized by femtosecond transient absorption and time-resolved PL spectroscopies. Simultaneously, this study also provides new directions for expanding the application of low-dimensional luminescent metal halides: in addition to UV-LEDs, efficient and rapid detection of methanol or acetonitrile can be achieved using the raw materials, while also realizing the potential application of multi-level optical anti-counterfeiting.

Difference of carrier dynamics in a semiconductor saturable absorber mirror with and without B+ ion-implantation.

Three samples whose growth temperatures were 450°C, 500°C, and 560°C for S E S A M 1, S E S A M 2, and S E S A M 3, respectively, were tested by femto-second time-resolved transient absorption spectroscopy. The results indicate that the carrier dynamics of excited state absorption were dominant, and the lifetimes of carriers trapped by defect levels were about tens of pico-seconds. To further study the influence of carrier dynamics and recovery time of samples by ion-implantation, B + ions of 80 and 130 KeV were implanted into the samples with dose of 1014/c m 2. The modified samples showed a dominance of ultra-fast carrier dynamics of ground-state bleaching and direct recombination, which lasted for hundreds of femto-seconds, over excited state absorption. Additionally, carrier fast trapping was observed to be competitive with the excited state absorption process. After ion-implantation, the carrier dynamics of carrier trapping were enhanced, which contributed to forming an ultra-short laser, while the carrier dynamics of absorption of the excited state were suppressed. The conclusion that defect levels were partially eliminated by B + ion-implantation can be drawn.

Dual Functionality of 6-Methylthioguanine: Synergistic Effects Enhancing the Photolability of DNA Nucleobases.

A small chemical modification of the nucleobase structure can significantly enhance the photoactivity of DNA, which may incur DNA damage, thus holding promising applications in photochemotherapy treatment of cancers or pathogens. However, single substitution confers only limited phototoxicity to DNA. Herein, we combine femtosecond and nanosecond time-resolved spectroscopy with high-level ab initio calculations to disentangle the excited-state dynamics of 6-methylthioguanine (me6-TG) under variable wavelength UVA excitation (310-330 nm). We find that double substitution of nucleobases (thionation and methylation) boosts the photoactivity by introducing more reactive channels. Intriguingly, 1nNπ*, rather than 1nSπ*, acts as the doorway state engendering the formation of the long-lived reactive triplet state in me6-TG. The 1nNπ* induces a low spin-orbit coupling of 8.3 cm-1, which increases the intersystem crossing (ISC) time (2.91 ± 0.14 ns). Despite the slowed ISC, the triplet quantum yield (ΦT) still accounts for a large fraction (0.6 ± 0.1), consistent with the potential energy surface that favors excited-state bifurcation to 1nNπ*min (3.36 ± 0.15 ps) rather than 1ππ*min (5.05 ± 0.26 ps), such that the subsequent ISC to triplet via 1nNπ*min constitutes the main relaxation pathway in me6-TG. Although this ΦT is inferior to its single-substituted predecessor 6-thioguanine (6-TG, 0.8 ± 0.2), the effect of thionation in synergy with methylation opens a unique C-S bond cleavage pathway through crossing to a repulsive 1πσ* state, generating thiyl radicals as highly reactive intermediates that may invoke biological damage. This photodissociation channel is extremely difficult for conventional nucleobases. These findings demonstrate the synergistic effects of double functionality substitution in modulating excited-state dynamics and enhancing the photolabile character of DNA nucleobases, providing inspirations for the rational design of advanced photodynamic and photochemotherapy approaches.

Dynamics of photoexcited 5-bromouracil and 5-bromo-2'-deoxyuridine studied by extreme ultraviolet time-resolved photoelectron spectroscopy in liquid flat jets.

The UV-induced photo-relaxation dynamics of 5-bromouracil (BrU) and 5-bromo-2'-deoxyuridine (BrUrd) in aqueous solution were investigated using femtosecond time-resolved photoelectron spectroscopy with an extreme ultraviolet (XUV) probe in a flat liquid jet. Upon excitation to the 1ππ* state by 4.66 eV UV photons, both molecules exhibited rapid relaxation into lower-lying electronic states followed by decay to the S0 ground state. By employing a 21.7 eV XUV probe pulse, we were able to differentiate the relaxation of the excited state population from the initially excited 1ππ* state to an intermediate electronic state with 100 fs. Computational results identify this intermediate as the 1πσ* excited state, accessed by a 1ππ*/1πσ* conical intersection, and the signal from this intermediate state disappears within ∼200 fs. In contrast to thymine, formation of neither the 1nπ* state nor a long-lived triplet state was observed. Although the 1πσ* state is largely repulsive, prior studies have reported a low quantum yield for dissociation, and we observe weak signals that are consistent with production of hot S0 ground state (for BrUrd) on a time scale of 1.5-2 ps. It thus appears that solvent caging effects limit the dissociation yield in solution.