Ultrafast Spectroscopy Research Articles

Superior Photostability of the Unnatural Base 6-Amino-5-nitropyridin-2-ol: A Case Study Using Ultrafast Broadband Fluorescence, Transient Absorption, and Theoretical Computation.

6-Amino-5-nitropyridin-2-ol (Z), a nitroaromatic compound and a base for Hachimoji nucleic acids, holds significant potential in expanding the genetic alphabet, as well as in synthetic biology and biotechnology. Despite its promising applications, the spectral characterization and photoinduced properties of Z have remained largely unexplored until now. This study presents a comprehensive investigation into its excited state dynamics in various solvents, utilizing state-of-the-art ultrafast broadband time-resolved fluorescence and transient absorption spectroscopy, complemented by computational methods. The acquired results provide direct experimental evidence that, upon photoexcitation, Z emits prompt fluorescence from a nearly planar structure in its excited state, independent of solvent properties. This state deactivates nonradiatively within sub-picoseconds through internal conversion with a unitary yield, primarily mediated by the rotation of the nitro group. This unusually rapid deactivation pathway entirely excludes the involvement of long-lived nπ* states, triplet states, and photoproducts, which are commonly observed in most nitroaromatic compounds and natural DNA and RNA bases. Our findings underscore that Z, as an unnatural base, exhibits superior photostability compared to canonical natural bases. This provides valuable insights into the photodynamics of nitroaromatic compounds, which is beneficial for strategic substitution design in environmental and biological applications.

Ultrafast Excitonic Transitions in Enantiopure and Racemic Squaraine Thin Films

ABSTRACTWhile chirality is a prevalent character of numerous biological and synthetic organic molecules, its selective absorption of circularly polarized light, known as circular dichroism (CD), is typically small due to intrinsically weak coupling between magnetic and electric dipoles. However, thin films of aggregated, enantiopure prolinol‐derived squaraine molecules (ProSQ‐C16) exhibit an unusually large excitonic CD signal, although the underlying mechanism is not yet known. In this study, we employ steady‐state and ultrafast transient absorption spectroscopy to investigate the nature and dynamics of excitons in aggregates of enantiopure and racemic ProSQ‐C16 thin films. Highly resembling transient responses of enantiopure thin films under excitations at different photon energies strongly indicate that a single type of aggregate dominates the linear optical response, that is, a strong red‐shifted (J‐like) and weak blue‐shifted (H‐like) absorption band. On the other hand, the transient properties of the racemic thin film deviate from this pattern and remain largely ambiguous. The short lifetime of excited states and coherent oscillations present in the dynamics of the transient absorption signal indicate that the early time dynamics are governed by a transition towards a dark intermediate state, which might arise from intermolecular charge transfer with potential contributions from the coupling of excitons to the vibrations. This non‐radiative relaxation pathway explains the unusually weak fluorescence of the predominately J‐like behaving aggregate. Our findings conclusively show that the chiral aggregate structure has a strong impact on the optical and dynamic response of the excitons and underline the significance of non‐Frenkel exciton states for the optical properties of anilino squaraine dyes.

Ultrafast dynamics evidence of strong coupling superconductivity in LaH10±δ

Recently, tremendous research interest has been aroused in clathrate superhydrides. However, their microscopic properties, especially the superconducting (SC) gap and electron-phonon coupling (EPC) strength, are largely unexplored experimentally. Here, we investigate the time-resolved ultrafast spectroscopy of a superconductor LaH10±δ under an ultrahigh pressure of 165 GPa. By analyzing the ultrafast dynamics of the quasiparticles, we experimentally obtain the SC gap Δ(0) = 53 ± 5 meV, revealing a gap ratio 2Δ(0)/kBTc = 5.6 and a gap parameter ϑ = 1.95. Significantly, we experimentally estimate λ⟨Ω2⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda \\langle {\\Omega }^{2}\\rangle$$\\end{document} = (2.4 ± 0.1) × 104 (meV)2, which corresponds to an EPC strength λ = 2.58 ± 0.11. These results together provide direct experimental evidence that strong EPC is responsible for the near-room-temperature superconductivity in clathrate superhydrides. Our investigation significantly advances the experimental exploration of superhydrides, and contributes to the ultrafast dynamics investigations of quantum materials under high pressures.

Self-Trapped Excitons in 3R ZnIn2S4 with Broken Inversion Symmetry for High-Performance Photodetection.

Exploring novel materials with intrinsic self-trapped excitons (STEs) is crucial for advancing optoelectronic technologies. In this study, 2D 3R-phase ZnIn2S4, featuring broken inversion symmetry, is introduced to investigate intrinsic STEs. This material exhibits a broadband photoluminescence (PL) emission with a full width at half maximum of 164nm and a large Stokes shift of ≈0.6eV, which arises from the distortion of [ZnS4]6- tetrahedral unit induced by the symmetry breaking and strong electron-phonon coupling. The photophysical properties of the STEs exhibit a high Huang-Rhys factor (15.0), rapid STEs formation time (166fs), and extended STEs lifetime (1039ps), as demonstrated by experimental evidence from temperature-dependent PL, Raman spectroscopy, and ultrafast absorption spectroscopy. Additionally, STE-induced photoconductiveeffect is elucidated, indicating that intrinsic STEs in 3R-ZnIn2S4 can provide a synergistic effect that enhances absorption capacity, localization, and lifetime by capturing the self-trapped hole state. Consequently, the 2D 3R-ZnIn2S4 photodetector exhibits remarkable broad-spectrum photosensitivity, including a photo-switching ratio of 11286, response times of less than 0.6ms, responsivity of 15.2AW-1, detectivity of 1.02×10¹¹ Jones, and external quantum efficiency of 5032% under 375nm light. These findings provide new ideas for exploring materials with intrinsic STEs to achieve novel high-performance photodetector applications.

Cation Vacancy-Mediated Ultrafast Hole Transport in CuBi2O4 Photocathodes.

In this study, the ultrafast transport dynamics of the valence band hole states in CuBi2O4 (CBO) photocathodes were investigated by varying the atomic composition and manipulating their p-type character. As a comprehensive ultrafast optical transient absorption spectroscopy (TAS) investigation of compositionally manipulated CBO that combines both ex situ and in situ TAS experiments with photoelectrochemical (PEC) performance tests, the study reveals the polaron formation tendencies of the valence band (VB) holes at cationic vacancy sites. Therefore, it draws a complete picture of the ultrafast hole transport dynamics and provides valuable insights into the hindrance of the photocurrent generated in CBO.

Exploring Ultrafast Carrier Dynamics in Photoelectrochemical Water Splitting Using Transient Absorption Spectroscopy

AbstractHydrogen fuel produced from water splitting using sunlight remains one of the cleanest and most sustainable energy sources. However, photoelectrochemical hydrogen production faces several challenges, including poor sunlight absorption, deficient charge carrier lifetime and transfer rates, and very sluggish kinetics of the water oxidation half‐reaction. To address these challenges, researchers have concentrated on enhancing the efficiency of photoelectrochemical water splitting. A critical factor in this enhancement is a deeper understanding of the fundamental processes involved in photoabsorption and the subsequent oxidation evolution reaction. The advent of ultrafast lasers has enabled the detailed tracking of charge carriers within materials after sunlight absorption, including their separation and migration to the surface for water oxidation. Ultrafast transient absorption spectroscopy is a powerful technique that allows real‐time observation of the behavior of excited states and charge carriers on femtosecond to nanosecond timescales. Recent studies utilizing ultrafast transient absorption spectroscopy are explored to investigate the dynamics of charge carriers in photoelectrochemical water splitting, providing insights into the mechanisms that lead to the design of enhanced photoanodes with improved efficiency.

High-sensitivity pump-probe spectroscopy with a dual-comb laser and a PM-Andi supercontinuum.

Amplifier-based pump-probe systems, while versatile, often suffer from complexity and low measurement speeds, especially when probing samples require low excitation fluences. To address these limitations, we introduce a pump-probe system that leverages a 60-MHz single-cavity dual-comb oscillator and an ultra-low noise supercontinuum. The setup can operate in equivalent time sampling or in programmable optical delay generation modes. We employ this system to study the wavelength-dependent excited-state dynamics of the non-fullerene electron acceptor Y6, a compound of interest in solar cell development, with excitation fluences as low as 1 nJ/cm2, well below the onset of nonlinear exciton annihilation effects. Our measurements reach a shot-noise limited sensitivity in differential transmission of 3.4·10-7. The results demonstrate the system's potential to advance the field of ultrafast spectroscopy.

Revealing Defect Passivation and Charge Extraction by Ultrafast Spectroscopy in Perovskite Solar Cells through a Multifunctional Lewis Base Additive Approach

Revealing Defect Passivation and Charge Extraction by Ultrafast Spectroscopy in Perovskite Solar Cells through a Multifunctional Lewis Base Additive Approach

Ultrafast Time-Domain Spectroscopy Reveals Coherent Vibronic Couplings upon Electronic Excitation in Crystalline Organic Thin Films.

The coherent coupling between electronic excitations and vibrational modes of molecules largely affects the optical and charge transport properties of organic semiconductors and molecular solids. To analyze these couplings by means of ultrafast spectroscopy, highly ordered crystalline films with large domains are particularly suitable because the domains can be addressed individually, hence allowing azimuthal polarization-resolved measurements. Impressive examples of this are highly ordered crystalline thin films of perfluoropentacene (PFP) molecules, which adopt different molecular orientations on different alkali halide substrates. Here, we report polarization-resolved time-domain vibrational spectroscopy with 10 fs time resolution and Raman spectroscopy of crystalline PFP thin films grown on NaF(100) and KCl(100) substrates. Coherent oscillations in the time-resolved spectra reveal vibronic coupling to a high-frequency, 25 fs, in-plane deformation mode that is insensitive to the optical polarization, while the coupling to a lower-frequency, 85 fs, out-of-plane ring bending mode depends significantly on the crystalline and molecular orientation. Comparison with calculated Raman spectra of isolated PFP molecules in vacuo supports this interpretation and indicates a dominant role of solid-state effects in the vibronic properties of these materials. Our results represent a first step toward uncovering the role of anisotropic vibronic couplings for singlet fission processes in crystalline molecular thin films.

Sub-20-fs UV-XUV beamline for ultrafast molecular spectroscopy

We present an ultraviolet (UV) - extreme-ultraviolet (XUV) pump-probe beamline with applications in ultrafast time-resolved photoelectron spectroscopy. The UV pump pulses, tuneable between 255 and 285 nm and with µJ-level energy, are generated by frequency up-conversion between ultrashort visible/infrared pulses and visible narrow-band pulses. Few-femtosecond XUV probe pulses are produced by a high-order harmonic generation source equipped with a state-of-the-art time-delay compensated monochromator. Two-colour UV-XUV sidebands are used for a complete in situ temporal characterization of the pulses, demonstrating a temporal resolution of better than 20 fs. We validate the performances of the beamline through a UV-XUV pump-probe measurement on 1,3-cyclohexadiene, resolving the ultrashort dynamics of the first conical intersection. This instrument opens exciting possibilities for investigating ultrafast UV-induced dynamics of organic molecules in ultrashort time scales.

Spectroscopic Evidence of Ultrafast Topological Phase Transition by Light-Driven Strain.

Enabling reversible control over the topological invariants, transitioning them from nontrivial to trivial states, has fundamental implications for quantum information processing and spintronics. It offers a promising avenue for establishing an efficient on/off switch mechanism for robust and dissipationless spin-currents. While mechanical strain has traditionally been advantageous for such manipulation of topological invariants, it often comes with the drawback of in-plane fractures, rendering it unsuitable for high-speed, time-dependent operations. This study employs ultrafast optical and THz spectroscopy to explore topological phase transitions induced by light-driven strain in Bi2Se3. Bi2Se3 requires substantial strain for Z2 switching. Our observations provide experimental evidence of ultrafast switching behavior, demonstrating a transition from a topological insulator with spin-momentum-locked surfaces to hybridized states and normal insulating phases under ambient conditions. Notably, applying light-induced strong out-of-plane strain effectively suppresses surface-bulk coupling, facilitating the differentiation of surface and bulk conductance even at room temperature─significantly surpassing the Debye temperature. We expect various time-dependent sequences of transient hybridization and manipulation of topological invariant through photoexcitation intensity adjustments. The sudden surface and bulk transport alterations near the transition point enable coherent conductance modulation at hypersound frequencies. Our findings on the potential of light-triggered ultrafast switching of topological invariants hold promise for high-speed topological switching and its related applications.

Exploring Charge Transfer Mechanisms in Schiff Base‐Modified N‐Doped GQDs: Insights from DFT and Pump‐Probe Spectroscopy for Bioimaging Applications

AbstractIn recent developments, graphene quantum dots (GQDs) have emerged as valuable tools for imaging and biosensing. Various modifications on the GQDs with any desired functionality and attachment of organic molecules and/or nanostructures allow tuning their photophysical properties as well as charge transfer dynamics for bioimaging applications. This study focuses on synthesizing and characterizing of polyethyleneimine‐functionalized nitrogen‐doped GQDs (NC1), Schiff base‐ functionalized nitrogen‐doped GQDs (NC2), and silver nanocomposites of these Schiff base‐functionalized nitrogen‐doped GQDs (NC3). We explore their absorption and emission properties to understand their interactions in the ground state. Furthermore, ultrafast transient absorption spectroscopy measurements reveal that the presence of NC3 shortens the excited state lifetime of NC1 due to charge transfer, resulting in reduced fluorescence intensity. Both experimental and DFT results suggest the potential of NC3 for bioimaging and sensing applications, making them promising candidates for phototheranostic purposes.

The development and applications of multidimensional biomolecular spectroscopy illustrated by photosynthetic light harvesting.

The parallel and synergistic developments of atomic resolution structural information, new spectroscopic methods, their underpinning formalism, and the application of sophisticated theoretical methods have led to a step function change in our understanding of photosynthetic light harvesting, the process by which photosynthetic organisms collect solar energy and supply it to their reaction centers to initiate the chemistry of photosynthesis. The new spectroscopic methods, in particular multidimensional spectroscopies, have enabled a transition from recording rates of processes to focusing on mechanism. We discuss two ultrafast spectroscopies - two-dimensional electronic spectroscopy and two-dimensional electronic-vibrational spectroscopy - and illustrate their development through the lens of photosynthetic light harvesting. Both spectroscopies provide enhanced spectral resolution and, in different ways, reveal pathways of energy flow and coherent oscillations which relate to the quantum mechanical mixing of, for example, electronic excitations (excitons) and nuclear motions. The new types of information present in these spectra provoked the application of sophisticated quantum dynamical theories to describe the temporal evolution of the spectra and provide new questions for experimental investigation. While multidimensional spectroscopies have applications in many other areas of science, we feel that the investigation of photosynthetic light harvesting has had the largest influence on the development of spectroscopic and theoretical methods for the study of quantum dynamics in biology, hence the focus of this review. We conclude with key questions for the next decade of this review.

Morphology Optimization by Non-Halogenated and Twisted Volatile Solid Additive for High-Efficiency Organic Solar Cells.

Recently, volatile solid additives have attracted tremendous interest in the field of organic solar cells (OSCs), which can effectively improve device efficiency without sacrificing the reproducibility and stability of the device. However, the structure of reported solid additives is onefold and its working mechanism needs to be further investigated. Herein, a novel non-halogenated and twisted solid additive 1,4-diphenoxybenzene (DPB) is employed to optimize the morphology of the active layer in OSCs. The properties of additive DPB, morphology of active layer, and carrier dynamics behaviors have been systematically investigated through theoretical calculations, in situ and ex situ spectroscopy, grazing-incidence wide-angle X-ray scattering (GIWAXS), and grazing-incidence small-angle X-ray scattering (GISAXS) measurement, as well as ultrafast spectroscopy technology. The results reveal that the twisted additive DPB selectively interacts with acceptor Y6, and thus forms optimized morphology of active layer with increased molecular crystallinity, tight molecular packing, and favorable phase separation. As a result, the optimized devices deliver a remarkable power conversion efficiency (PCE) of 19.04%, which is the highest value for the D18-Cl:N3 system to date. These results demonstrate that non-halogenated and twisted solid additive DPB has broad prospects in the preparation of highly efficient OSCs, providing theoretical and experimental guidance for the development of high-performance solid additives.

UltraStat: Ultrafast Spectroscopy beyond the Fourier Limit Using Bayesian Inference.

The discrete Fourier transform (dFT) plays a central role in many ultrafast experiments, allowing the recovery of spectroscopic observables from time-domain measurements. In resonant experiments when population relaxation and coherence components of the signal coexist, the dFT is usually preceded by multiexponential fitting to remove the large population term. However, this procedure results in errors in both the recovered decay rates and the line shapes of the coherence spectral components. While other methods such as linear prediction singular value decomposition fit both terms simultaneously, they are limited to specific models that may not represent the true signal. These methods do not allow for systematic noise analysis or error estimation and require a priori knowledge of the signal rank. Here, we describe a general approach to parameter estimation in ultrafast spectroscopy─UltraStat─grounded in Bayesian analysis without the limitations set by Fourier theory. Using simulated, but realistic data, we demonstrate in a statistical sense how UltraStat provides accurate parameter estimation in the presence of many experimental constraints: noise, signal truncation, limited photon budget, and nonuniform sampling. UltraStat provides superior resolution compared to the dFT, up to an order of magnitude in cases where the line shapes are well-approximated. In these cases, we establish that primarily noise, not sampling, limits spectral resolution. Moreover, we show that subsampling may reduce the number of acquired points by 90% compared to the Nyquist-Shannon criteria. UltraStat greatly improves parameter estimation by providing statistically bound spectral and dynamics analysis, pushing the limits of ultrafast science.

Photophysics and photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile: ultrafast pump-probe spectroscopy and quantum chemical insight.

The ultrafast processes caused by photoexcitation of (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile were studied by means of transient absorption (TA) pump-probe spectroscopy and verified by quantum chemical calculations. The primary photochemical process was found to be an inner-sphere electron transfer followed by an escape of an •NO3 radical to the bulk solution. The reaction occurs via the dissociative triplet excited LMCT state of the initial complex. Based on the experimental data and quantum chemical calculations, the mechanism of ultrafast photophysical and photochemical processes is proposed.

Achromatic correction for birefringent interferometers that improve Fourier transform spectrometers and hyperspectral imaging

Spectroscopy and hyperspectral imaging are widely used tools for identifying compounds and materials. One optical design is a polarization interferometer that uses birefringent wedges, like a Babinet-Soleil compensator, to create the interferograms that are Fourier transformed to give the spectra. Such designs have lateral spatial offset between the no and ne optical beams, which reduces the interferogram intensity and creates a spatially dependent phase that is problematic for hyperspectral imaging. The lateral separation between the beams is wavelength dependent, created by the achromatic nature of Babinet-Soleil compensators. We introduce a birefringent wedge design for Fourier transform spectroscopy that creates collinear no and ne optical beams for optimal interference and no spatial dependent phase. Our 3-wedge design, which we call a Wisconsin interferometer, improves the signal strength of polarization spectrometers, and eliminates phase shifts in hyperspectral imaging. We anticipate that it will find use in analytical, remote sensing, and ultrafast spectroscopy applications.

Hierarchical Equations of Motion for Quantum Chemical Dynamics: Recent Methodology Developments and Applications.

ConspectusQuantum effects are critical to understanding many chemical dynamical processes in condensed phases, where interactions between molecules and their environment are usually strong and non-Markovian. In this Account, we review recent progress from our group in development and application of the hierarchical equations of motion (HEOM) method, highlighting its ability to address some challenging problems in quantum chemical dynamics.In the HEOM method, the bath degrees of freedom are represented using effective modes from exponential decomposition of the bath correlation function. Complex spectral densities and low temperature simulations often require a larger number of modes, making the simulations very expensive. Recent advances, such as the barycentric spectral decomposition (BSD) technique, can significantly reduce the number of effective modes, allowing to handle complex spectral densities and enabling simulations at very low temperatures, including near-zero temperature dynamics.Another key improvement in the computational efficiency is the use of tensor network methods like matrix product states and hierarchical tensor networks. These techniques allow for efficient HEOM propagation with thousands of effective modes, crucial for simulating large molecular systems interacting with multiple baths. This combination enables simulations of excitation energy transfer (EET) in systems like the Fenna-Matthews-Olson (FMO) complex and even larger systems with experimentally determined spectral densities.The versatility of the HEOM method is demonstrated through applications to a wide range of chemical dynamics problems. Simulations of EET and related ultrafast spectroscopy are first briefly covered. Applications of the HEOM to quantum tunneling effects in proton transfer reactions are then presented. Early works have studied the non-Kramers dependence of the rate constant as a function of bath friction due to deep tunneling and revealed vibrationally nonadiabatic dynamics within the so-called nontraditional view of proton transfer reactions. A recent work on the large kinetic isotope effects in soybean lipoxygenase also indicated that many quantum correction approximations to classical transition-state theory may fall short in describing deep tunneling effects.Charge transport and separation dynamics in organic semiconductors are another area where the HEOM method has been instrumental. We first demonstrate that the HEOM provides a unified description of both band-like and thermally assisted charge carrier transport in organic materials. The effect of non-nearest neighbor transitions is then investigated by combining generalized master equations with exact memory kernels. The HEOM method also enables simulation of charge separation in organic photovoltaics (OPVs) and reveals how factors such as external electric fields, entropy, and charge delocalization influence the charge separation barrier and dynamics.Moreover, HEOM has been applied to investigate hydrogen atom scattering on the Au(111) surface and vibrational energy relaxation at molecule-metal interfaces. These studies provide deeper insights into how electron-hole pair excitations and temporary charge transfer states influence the nuclear motion, offering a new framework for simulating nonadiabatic dynamics on metal surfaces.In summary, the HEOM method has developed into a robust tool for simulating quantum effects in condensed phases. Future developments in algorithm efficiency and computational power will likely expand its applicability to even more complex systems.

Ultrafast transient absorption spectra and kinetics of human blue cone visual pigment at room temperature

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Robust excitonic light emission in 2D tin halide perovskites by weak excited state polaronic effect

2D perovskites hold immense promise in optoelectronics due to their strongly bound electron-hole pairs (i.e., excitons). While exciton polaron from interplay between exciton and lattice has been established in 2D lead-based perovskites, the exciton nature and behavior in the emerging 2D tin-based perovskites remains unclear. By combining spin-resolved ultrafast spectroscopy and sophisticated theoretical calculations, we reveal 2D tin-based perovskites as genuine excitonic semiconductors with weak polaronic screening effect and persistent Coulomb interaction, thanks to weak exciton-phonon coupling. We determine an excited state exciton binding energy of ~0.18 eV in n = 2 tin iodide perovskites, nearly twice of that in lead counterpart, despite of same large value of ~0.2 eV from steady state measurement. This finding emphasizes the pivotal role of excited state polaronic effect in these materials. The robust excitons in 2D tin-based perovskites exhibit excitation power-insensitive, high-efficiency and color-purity emission, rendering them superior for light-emitting applications.