Photoexcited System Research Articles

Metal-organic frameworks for improving wound healing

• Recent advances in MOF materials for wound healing are comprehensively reviewed. • The physiological and pathological mechanisms in wound healing are emphasized. • Various MOF-based strategies for improving wound healing performance are discussed. • The current challenges and opportunities on the further studies are also proposed. Wound healing is the most important restorative process for injury to the skin and other tissues. In the process of wound healing, bacterial infection can easily lead to serious tissue damage. There is an urgent need to discover new drugs or methods to accelerate wound healing. Metal-organic frameworks (MOFs), a type of porous hybrid materials containing metal ions connected by organic ligands, have great potential in improving wound healing. Firstly, this review discusses the pathophysiology of complex wounds, current approaches to achieve healing in patients and the limitations of traditional methods. Then the review emphasizes the application of MOF materials in improving wound healing. MOF materials-based metal ion sustained-release systems can show long-term antibacterial effects or promote angiogenesis in the degradation process of MOFs. As an enzyme-like active material, it can be used to convert hydrogen peroxide into highly toxic hydroxyl groups to improve antibacterial efficiency in wounds. As a controlled drug delivery system for wound healing, it is important for MOFs to load drugs (molecular or gas therapeutic drugs), effectively inhibiting the growth of microorganisms or enhancing angiogenesis in the wound site. Moreover, MOF materials-based photoexcited systems can provide the potential of combining photodynamic therapy and photothermal therapy for bacterial infections. Lastly, the future challenges and directions of MOF materials in wound healing are also discussed.

Nonequilibrium-DMFT based RIXS investigation of the two-orbital Hubbard model

Resonant inelastic X-ray scattering (RIXS) detects various types of high- and low-energy elementary excitations in correlated solids, and this tool will play an increasingly important role in investigations of time-dependent phenomena in photo-excited systems. While theoretical frameworks for the computation of equilibrium RIXS spectra are well established, the development of appropriate methods for nonequilibrium simulations is an active research field. Here, we apply a recently developed nonequilibrium dynamical mean field theory (DMFT) based approach to compute the RIXS response of photo-excited two-orbital Mott insulators. The results demonstrate the feasibility of multi-orbital nonequilibrium RIXS calculations and the sensitivity of the quasi-elastic fluorescence-like features and excitation peaks on the nonequilibrium population of the Hubbard bands.

Intramolecular Magnetic Interaction in a Photogenerated Dual Angular Momentum System in a Terbium-Phthalocyaninato 1:1 Complex.

Intramolecular magnetic interaction between a localized open-shell 4f-electronic system and a photoexcited macrocyclic π-conjugate system in terbium-phthalocyaninnato (Tb-Pc) 1:1 complex was investigated using variable-temperature variable-field magnetic circular dichroism (VTVH MCD) spectroscopy. The 1:1 complex [Tb(Pc)(cyclen)]Cl (Pc2- = phthalocyaninato dianion, cyclen = 1,4,7,10-tetraazacyclododecane) with the capping ligand providing an exact fourfold symmetry showed a significant temperature dependence and a nonlinear field dependence in the MCD intensity of the Pc-centered highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) π-π* transition, while a diamagnetic congener [Y(Pc)(cyclen)]Cl showed a temperature-independent MCD with a linear-field dependence. This indicates that the (4f)8 system of the Tb ion with a total angular momentum J and the photoexcited π-system of the Pc macrocycle with an orbital angular momentum L are magnetically coupled. By numerical simulation using a model where ground doublet state |Jz⟩ = |±6⟩ and excited quartet state |Jz, Lz⟩ = |±6, ±|Lz|⟩ are included, the J-L interaction magnitude ΔJL and the Pc-centered orbital angular momentum |Lz|ℏ were determined to be 1.1 cm-1 and 2.0 ℏ, respectively. From ab initio restricted active space self-consistent field (RASSCF)-restricted active space state interaction (RASSI) calculations on the π-π* excited states of the Tb complex, the magnitude of the J-L interaction was estimated. The comparison between the calculations on the Y and Tb complexes revealed that the ferromagnetic-type coupling occurs between the orbital component in the J of Tb and the L on Pc, supporting the model that we employed for the analysis of the experimental data.

Effects of frustration on the nonequilibrium dynamics of photoexcited lattice systems

We theoretically investigate the effects of the lattice geometry on the nonequilibrium dynamics of photo-excited carriers in a half-filled two-dimensional Hubbard model. Using a nonequilibrium generalization of the dynamical cluster approximation, we compare the relaxation dynamics in lattices which interpolate between the triangular lattice and square lattice configuration and thus reveal the role of the geometric frustration in these strongly correlated nonequilibrium systems. In particular, we show that the cooling effect resulting from the disordering of the spin background is less effective in the triangular case because of the frustration. This manifests itself in a longer relaxation time of the photo-doped population, as measured by the time-resolved photo-emission signal, and a higher effective temperature of the photo-doped carriers in the non-thermal steady state after the intra-Hubbard-band thermalization.

Z-scheme photocatalyst sheets with P-doped twinned Zn0.5Cd0.5S1-x and Bi4NbO8Cl connected by carbon electron mediator for overall water splitting under ambient condition

Cutting edge research within solar energy harvesting focuses on H2 production from photocatalytic overall water splitting (OWS) using artificial two-step photoexcitation system known as Z-scheme. Inspired by natural photosynthesis, Z-scheme imparts a unique vectorial electron transfer from the ingenious arrangement of PS I-PS II coupling connected by an electron mediator. This allows Z-scheme to confer efficient charge isolation and split water into its constituent components, hydrogen (H2) and oxygen (O2), at two different positions with strong redox ability. More recently, particulate Z-scheme photocatalyst sheets have been worth noting as potentially scalable approach for solar water splitting. In this contribution, particulate Z-scheme photocatalyst sheets were developed using P-doped twinned Zn0.5Cd0.5S1-x (d-ZCS-P) as hydrogen evolution photocatalysts (HEP) and Bi4NbO8Cl as oxygen evolution photocatalysts (OEP), which both embedded on N-doped carbon nanotubes (N-CNTs) as carbon conductive film. Further surface modification on photocatalyst sheets through concerted deposition of co-catalyst and protective shell warrants an efficient overall water splitting from pure water, with a solar-to-hydrogen conversion efficiency (STH) of 0.15% under ambient condition. The rational Z-scheme configuration of photocatalyst sheets alleviates the effect of H+ and OH− concentration overpotentials which in turn bolstering the photocatalytic performance and paves a promising way of solar energy augmentation.

NIC-CAGE: An open-source software package for predicting optimal control fields in photo-excited chemical systems

We present an open-source software package, NIC-CAGE (Novel Implementation of Constrained Calculations for Automated Generation of Excitations), for predicting quantum optimal control fields in photo-excited chemical systems. Our approach utilizes newly derived analytic gradients for maximizing the transition probability (based on a norm-conserving Crank–Nicolson propagation scheme) for driving a system from a known initial quantum state to another desired state. The NIC-CAGE code is written in the MATLAB and Python programming environments to aid in its readability and general accessibility to both users and practitioners. Throughout this work, we provide several examples and outputs on a variety of different potentials, propagation times, and user-defined parameters to demonstrate the robustness of the NIC-CAGE software package. As such, the use of this predictive tool by both experimentalists and theorists could lead to further advances in both understanding and controlling the dynamics of photo-excited systems. Program summaryProgram Title: NIC-CAGECPC Library link to program files:http://dx.doi.org/10.17632/82jcpk5svt.1Licensing provisions: GNU General Public License 3Programming language: MATLAB or PythonSupplementary material: Comparisons of propagated wavefunctions obtained from analytical π pulses vs wavefunctions resulting from numerically optimized electric fields predicted by the NIC-CAGE programNature of problem: The NIC-CAGE software package utilizes analytic Crank–Nicolson gradients to compute optimized (and constrained) electric fields that can drive a system from a known initial vibrational eigenstate to a specified final quantum state with a large (≈1) transition probability.Solution method: Analytic gradients, Crank–Nicolson propagation, and gradient ascent optimization

Nonthermal Dynamics of Dielectric Functions in a Resonantly Bonded Photoexcited Material

AbstractA fundamental question of whether it is thermal or nonthermal has often been argued, when the system undergoes an ultrafast phase change by femtosecond laser photoexcitation. One of the difficulties therein is how to consider the thermalization of a photoexcited system, in which the “lattice” and “electron” systems are decoupled from an equilibrium state just after photoexcitation. This work shows a methodology to identify a thermalization‐time domain of a photoexcited semiconductor, where it can be detected by monitoring the time‐resolved photoabsorption broadband spectrum and comparing that to thermally induced spectrum changes, by which the electron–lattice thermalization time is evaluated to be 12 ps for PbTe. Further analyzing the dynamic dielectric functions substantiates that thermalization time is related to the recovery time of the resonant bonding. Such a crucial determination of thermalization time would eliminate an uncertainty of “thermal or nonthermal” in ultrafast phase change of the resonantly bonded materials.

Covalent heterodyads of synthetic chlorophyll derivatives linked with linear rigid substituents at the 20-positions constructing photoexcited energy transfer systems

Chlorophyll and bacteriochlorophyll pigments are important porphyrinoids, which play key roles in light absorption, energy migration/transfer, and electron transfer in natural photosynthesis. Dyads of such pigments have attracted much attention from the viewpoints of mimicking photosynthetic light-harvesting and charge-separating systems. In this study, we report the synthesis and optical properties of heterodyads of zinc chlorin and free-base bacteriochlorin covalently linked by linear rigid substituents at the 20-position, which were prepared from naturally occurring chlorophyll-a. A free-base bacteriochlorin moiety accepted the singlet excitation energy from the zinc chlorin in the heterodyads. In the co-aggregate systems of heterodyads with chlorosomal self-assemblies, the latter transfers the singlet excitation energy to the bacteriochlorin moiety of the former, which are useful for artificial chlorosomal supramolecular light-harvesting systems.

Photoenhanced excitonic correlations in a Mott insulator with nonlocal interactions

We investigate the effect of nonlocal interactions on the photo-doped Mott insulating state of the two-dimensional Hubbard model using a nonequilibrium generalization of the dynamical cluster approximation. In particular, we compare the situation where the excitonic states are lying within the continuum of doublon-holon excitations to a set-up where the excitons appear within the Mott gap. In the first case, the creation of nearest-neighbor doublon-holon pairs by excitations across the Mott gap results in enhanced excitonic correlations, but these excitons quickly decay into uncorrelated doublons and holons. In the second case, photo-excitation results in long-lived excitonic states. While in a low-temperature equilibrium state, excitonic features are usually not evident in single-particle observables such as the photoemission spectrum, we show that the photo-excited nonequilibrium system can exhibit in-gap states associated with the excitons. The comparison with exact-diagonalization results for small clusters allows us to identify the signatures of the excitons in the photo-emission spectrum.

Excited-state band mapping and momentum-resolved ultrafast population dynamics in In/Si(111) nanowires investigated with XUV-based time- and angle-resolved photoemission spectroscopy

We investigate the excited state electronic structure of the model phase transition system In/Si(111) using femtosecond time- and angle-resolved photoemission spectroscopy (trARPES) with an XUV laser source at 500 kHz . Excited state band mapping is used to characterize the normally unoccupied electronic structure above the Fermi level in both structural phases of indium nanowires on Si(111): the metallic (4x1) and the gapped (8x2) phases. The extracted band positions are compared with the band structure calculated using density functional theory (DFT) within both the LDA and GW approximations. While good overall agreement is found between the GW calculated band structure and experiment, deviations in specific momentum regions may indicate the importance of excitonic effects not accounted for at this level of approximation. To probe the dynamics of these excited states, their momentum-resolved transient population dynamics are extracted. The transient intensities are then simulated by a spectral function determined by a state population employing a transient elevated electronic temperature as determined experimentally. This allows the momentum-resolved population dynamics to be quantitatively reproduced, revealing important insights into the transfer of energy from the electronic system to the lattice. In particular, a comparison between the magnitude and relaxation time of the transient electronic temperature observed by trARPES with those of the lattice as probed in previous ultrafast electron diffraction studies imply a highly non-thermal phonon distribution at the surface following photo-excitation. This suggests the energy from the excited electronic system is initially transferred to high energy optical phonon modes followed by cooling and thermalization of the photo-excited system by much slower phonon-phonon coupling.

Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations.

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D1 surface with little excess energy at the equilibrium neutral geometry. These D1 ions are preferably formed because in this case D1 and D0 switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D1 surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D1/D0 conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D0 becomes populated with molecules that are energized mainly along the cis-trans isomerization coordinate, with the kinetic energy above the cis-trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D0 surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole-lone pair interaction that drives the molecule along the cis-trans isomerization coordinate to facilitate the transition between the involved electronic states.

Selective Stabilization of the Spin States of a Magnetically Anisotropic Dysprosium Ion Induced by Photo-Excitation of the Associated Cyclic π-Conjugated System.

The presence of a new electronic interaction, which couples a 4 f-electronic system with a total angular momentum J and a photoexcited cyclic π-conjugated system with an orbital angular momentum L, in the bis(phthalocyaninato)dysprosium single-molecule magnet ([DyPc2 ]- ) is reported. Two π-π* excited states in the visible spectral region of the [DyPc2 ]- complex, which are denoted here as QL and QH , showed significantly different temperature and field dependences of the magnetic circular dichroism (MCD) A-term intensity. This phenomenon not only indicates the presence of a "J-L" interaction, but also that the interaction generates two different preferred orientations of the J-L pair, either parallel (for the QH band) or antiparallel (for the QL band), depending on the excitation energy. We have constructed a theoretical model that reproduces the temperature and field dependences, and quantitatively evaluated the J-L interaction.

Magneto-transport characteristics of a 2D electron system driven to negative magneto-conductivity by microwave photoexcitation

The magneto-transport characteristics of the negative conductivity/resistivity state in the microwave photo-excited two-dimensional electron system (2DES) is examined through a numerical solution of the associated boundary value problem. The results suggest, surprisingly, that a bare negative diagonal conductivity/resistivity state in the 2DES under photo-excitation should yield a positive diagonal resistance along with a sign reversal in the Hall voltage.

Unveiling the excited state energy transfer pathways in peridinin-chlorophyll a-protein by ultrafast multi-pulse transient absorption spectroscopy

Time-resolved multi-pulse methods were applied to investigate the excited state dynamics, the interstate couplings, and the excited state energy transfer pathways between the light-harvesting pigments in peridinin-chlorophyll a-protein (PCP). The utilized pump-dump-probe techniques are based on perturbation of the regular PCP energy transfer pathway. The PCP complexes were initially excited with an ultrashort pulse, resonant to the S0→S2 transition of the carotenoid peridinin. A portion of the peridinin-based emissive intramolecular charge transfer (ICT) state was then depopulated by applying an ultrashort NIR pulse that perturbed the interaction between S1 and ICT states and the energy flow from the carotenoids to the chlorophylls. The presented data indicate that the peridinin S1 and ICT states are spectrally distinct and coexist in an excited state equilibrium in the PCP complex. Moreover, numeric analysis of the experimental data asserts ICT→Chl-a as the main energy transfer pathway in the photoexcited PCP systems.

Long-lived magnetoexcitons in 2D-fermion system

The paper addresses the experimental technique that, when applied to a 2D-electron system in the integer quantum Hall regime with filling factor ν = 2 (the Hall insulating state), allows resonant excitation of magnetoexcitons, their detection, control of an ensemble of long-lived triplet excitons and investigation of their radiationless decay related to exciton spin relaxation into the ground state. The technique proposed enables independent control of photoexcited electrons and Fermi-holes using photoinduced resonance reflection spectra as well as estimate with a reasonable degree of accuracy the resulting density of photoinduced electron-hole pairs bound into magnetoexcitons. The mere existence of triplet excitons was directly established by inelastic light scattering spectra which were analyzed to determine the value of singlet-triplet exciton splitting. It was found that the lifetimes of triplet excitons conditioned by electron spin relaxation in highly perfect GaAs/AlGaAs heterostructures with highly mobile 2D electrons are extremely long exceeding 100 μs at T < 1 K. The paper presents a qualitative explanation of the long-spin relaxation lifetimes which are unprecedented for translation-invariant 2D systems. This enabled us to create sufficiently high concentrations of triplet magnetoexcitons, electrically neutral excitations following Bose–Einstein statistics, in a Fermi electron system and investigate their collective properties. At sufficiently high densities of triplet magnetoexcitons and low temperatures, T < 1 K, the degenerate magnetofermionic system exhibits condensation of the triplet magnetoexcitons into a qualitatively new collective state with unusual properties which occurs in the space of generalized moments (magnetic translation vectors). The occurrence of a condensed phase is accompanied with a significant decrease in the viscosity of the photoexcited system, which is responsible for electron spin transport at macroscopic distances, as well as with the effects of threshold enhancement of the system response to the external action of the electromagnetic field and emergence of a new intensive radiative recombination channel.

The new high field photoexcitation muon spectrometer at the ISIS pulsed neutron and muon source.

A high power pulsed laser system has been installed on the high magnetic field muon spectrometer (HiFi) at the International Science Information Service pulsed neutron and muon source, situated at the STFC Rutherford Appleton Laboratory in the UK. The upgrade enables one to perform light-pump muon-probe experiments under a high magnetic field, which opens new applications of muon spin spectroscopy. In this report we give an overview of the principle of the HiFi laser system and describe the newly developed techniques and devices that enable precisely controlled photoexcitation of samples in the muon instrument. A demonstration experiment illustrates the potential of this unique combination of the photoexcited system and avoided level crossing technique.

Modulating the band gap of a boron nitride bilayer with an external electric field for photocatalyst

By virtue of first principle calculations, we propose an approach to reduce the band gap of layered semiconductors through the application of external electric fields for photocatalysis. As a typical example, the band gap of a boron nitride (BN) bilayer was reduced in the range from 4.45 eV to 0.3 eV by varying the external electric field strength. More interestingly, it is found that the uppermost valence band and the lowest conduction band are dominated by the N-pz and B-pz from different layers of the BN sheet, which suggests a wonderful photoexcited electron and hole separation system for photocatalysis. Our results imply that the strong external electric field can present an abrupt polarized surface.

Using Nonequilibrium Dynamics to Probe Competing Orders in a Mott-Peierls System.

Competition between ordered phases, and their associated phase transitions, are significant in the study of strongly correlated systems. Here, we examine one aspect, the nonequilibrium dynamics of a photoexcited Mott-Peierls system, using an effective Peierls-Hubbard model and exact diagonalization. Near a transition where spin and charge become strongly intertwined, we observe antiphase dynamics and a coupling-strength-dependent suppression or enhancement in the static structure factors. The renormalized bosonic excitations coupled to a particular photoexcited electron can be extracted, which provides an approach for characterizing the underlying bosonic modes. The results from this analysis for different electronic momenta show an uneven softening due to a stronger coupling near k_{F}. This behavior reflects the strong link between the fermionic momenta, the coupling vertices, and ultimately, the bosonic susceptibilities when multiple phases compete for the ground state of the system.

Magnetic order dynamics in optically excited multiferroicTbMnO3

We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 +- 1.1 ps, which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wavevector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wavevector on these time scales.

Mechanistic Study of a Photocatalyzed C-S Bond Formation Involving Alkyl/Aryl Thiosulfate.

This study presents thioether construction involving alkyl/aryl thiosulfates and diazonium salt catalyzed by visible-light-excited [Ru(bpy)3 Cl2 ] at room temperature in 44-86 % yield. Electron paramagnetic resonance studies found that thiosulfate radical formation was promoted by K2 CO3 . Conversely, radicals generated from BnSH or BnSSBn (Bn=benzyl) were clearly suppressed, demonstrating the special property of thiosulfate in this system. Transient absorption spectra confirmed the electron-transfer process between [Ru(bpy)3 Cl2 ] and 4-MeO-phenyl diazonium salt, which occurred with a rate constant of 1.69×10(9) M(-1) s(-1) . The corresponding radical trapping product was confirmed by X-ray diffraction. The full reaction mechanism was determined together with emission quenching data. Furthermore, this system efficiently avoided the over-oxidation of sulfide caused by H2 O in the photoexcited system containing Ru(2+) . Both aryl and heteroaryl diazonium salts with various electronic properties were investigated for synthetic compatibility. Both alkyl- and aryl-substituted thiosulfates could be used as substrates. Notably, pharmaceutical derivatives afforded late-stage sulfuration smoothly under mild conditions.