Ultrafast Electron Dynamics in Solar Energy Conversion

Ultrafast Charge Separation Driven by Solvation-Coupled Intramolecular Torsion

Photoinduced intramolecular charge separation in a pyrene- and triarylamine-based donor-acceptor dyad was studied by polarization-dependent femtosecond time-resolved transient absorption (TA) spectroscopy in polar solvents. Photoexcitation forms an excited state with charge transfer (CT) character due to the intrinsic electronic coupling between the triarylamine and pyrene groups, resulting in ultrafast charge separation (CS) in polar solvents. TA measurements reveal a correlation between the rate of CS and solvation dynamics, which implies that solvation is involved in the CS reaction. In addition, polarization-dependent TA spectroscopy was devoted to tracking the ultrafast anisotropy evolution of the cationic absorption band, which is attributed to intramolecular torsional motion and is proposed to be coupled to diffusive orientational solvent modes. The results therefore reveal that the evolution of the CT state in the condensed phase is driven by solvation-coupled excited-state structural relaxation. In other words, intramolecular torsional motion is directly confirmed to be involved in the reaction coordinate of the CS reaction in a strongly coupled donor-acceptor dyad.

Modifying Proton Relay into Bioinspired Dye-Based Coordination Polymer for Photocatalytic Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) imparts an energetic advantage over single electron transfer in activating inert substances. Natural PCET enzyme catalysis generally requires tripartite preorganization of proton relay, substrate-bound active center, and redox mediator, making the processes efficient and precluding side reactions. Inspired by this, a heterogeneous photocatalytic PCET system was established to achieve higher PCET driving forces by modifying proton relays into anthraquinone-based anionic coordination polymers. The proximally separated proton relays and photoredox-mediating anthraquinone moiety allowed pre-assembly of inert substrate between them, merging proton and electron into unsaturated bonds by photoreductive PCET, which enhanced reaction kinetics compared with the counter catalyst without proton relay. This photocatalytic PCET method was applied to a broad-scoped reduction of aryl ketones, unsaturated carbonyls, and aromatic compounds. The distinctive regioselectivities for the reduction of isoquinoline derivatives were found to occur on the carbon-ring sides. PCET-generated radical intermediate of quinoline could be trapped by alkene for proton relay-assisted Minisci addition, forming the pharmaceutical aza-acenaphthene scaffold within one step. When using heteroatom(X)-H/C-H compounds as proton-electron donors, this protocol could activate these inert bonds through photooxidative PCET to afford radicals and trap them by electron-deficient unsaturated compounds, furnishing the direct X-H/C-H functionalization.

Protonation of an Imine-linked Covalent Organic Framework for Efficient H2O2 Photosynthesis under Visible Light up to 700 nm

Covalent organic frameworks (COFs) are promising photocatalysts for H2O2 production from water via oxygen reduction reaction (ORR). The design of COFs for efficient H2O2 production indubitably hinges on an in-depth understanding of their ORR mechanisms. In this work, taking an imine-linked COF as an example, we demonstrate that protonation of the functional units such as imine, amine, and triazine, is a highly efficient strategy to upgrade the activity levels for H2O2 synthesis. The protonation not only extends the light absorption of the COF but also provides proton sources that directly participate in H2O2 generation. Notably, the protonation simplifies the reaction pathways of ORR to H2O2, i.e. from an indirect superoxide radical (O 2 • - ${{O}_{2}^{\bullet -}}$ ) mediated route to a direct one-step two-electron route. Theoretical calculations confirm that the protonation favors H2O2 synthesis due to easy access of protons near the reaction sites that removes the energy barrier for generating *OOH intermediate. These findings not only extend the mechanistic insight into H2O2 photosynthesis but also provide a rational guideline for the design and upgradation of efficient COFs.

An Exhaustive Quantum-Classical Study of C6F6+ Using the Newly Formulated Parallel TDDVR Method

We recently implemented our parallelized quantum-classical dynamical approach, known as the Time-Dependent Discrete Variable Representation (TDDVR) method, which is applied to the spectroscopically important hexafluorobenzene (HFBz) radical cation, where several conical intersections exist in their seven lowest excited electronic states (S11B2u, S21E1g, S31B1u, S41E1u, and S51A2u) considering degeneracy among potential energy surfaces (PESs), to demonstrate their various dynamical aspects. This new parallel version shows almost linear scalability with increasing number of computing processors. To get photoelectron (PE) spectra, Mass-Analyzed Threshold Ionization (MATI) spectra, population dynamics, and many other dynamical observables, the first-principles dynamics is applied at the state-of-the-art level to the corresponding Hamiltonian, where the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type interactions are involved in those coupled seven electronic states. The quantum-classical method is used to thoroughly analyze the effects of these couplings on the nuclear dynamics of the involved electronic states, and the findings are compared with those observables obtained from experiments. Intrinsic dynamical properties are explained using the reduced densities of the wave packet (WP) in a coupled electronic manifold. The PE and MATI spectra of HFBz computed using TDDVR are found to be in good agreement with earlier experimental data and other theoretically simulated spectra.

Using Iron L-Edge and Nitrogen K-Edge X-ray Absorption Spectroscopy to Improve the Understanding of the Electronic Structure of Iron Carbene Complexes

Iron-centered N-heterocyclic carbene compounds have attracted much attention in recent years due to their long-lived excited states with charge transfer (CT) character. Understanding the orbital interactions between the metal and ligand orbitals is of great importance for the rational tuning of the transition metal compound properties, e.g., for future photovoltaic and photocatalytic applications. Here, we investigate a series of iron-centered N-heterocyclic carbene complexes with +2, + 3, and +4 oxidation states of the central iron ion using iron L-edge and nitrogen K-edge X-ray absorption spectroscopy (XAS). The experimental Fe L-edge XAS data were simulated and interpreted through restricted-active space (RAS) and multiplet calculations. The experimental N K-edge XAS is simulated and compared with time-dependent density functional theory (TDDFT) calculations. Through the combination of the complementary Fe L-edge and N K-edge XAS, direct probing of the complex interplay of the metal and ligand character orbitals was possible. The σ-donating and π-accepting capabilities of different ligands are compared, evaluated, and discussed. The results show how X-ray spectroscopy, together with advanced modeling, can be a powerful tool for understanding the complex interplay of metal and ligand.

Solar overall water-splitting by a spin-hybrid all-organic semiconductor

Direct solar-to-hydrogen conversion from pure water using all-organic heterogeneous catalysts remains elusive. The challenges are twofold: (i) full-band low-frequent photons in the solar spectrum cannot be harnessed into a unified S1 excited state for water-splitting based on the common Kasha-allowed S0 → S1 excitation; (ii) the H+ → H2 evolution suffers the high overpotential on pristine organic surfaces. Here, we report an organic molecular crystal nanobelt through the self-assembly of spin-one open-shell perylene diimide diradical anions (:PDI2-) and their tautomeric spin-zero closed-shell quinoid isomers (PDI2-). The self-assembled :PDI2-/PDI2- crystal nanobelt alters the spin-dependent excitation evolution, leading to spin-allowed S0S1 → 1(TT) → T1 + T1 singlet fission under visible-light (420 nm~700 nm) and a spin-forbidden S0 → T1 transition under near-infrared (700 nm~1100 nm) within spin-hybrid chromophores. With a triplet-triplet annihilation upconversion, a newly formed S1 excited state on the diradical-quinoid hybrid induces the H+ reduction through a favorable hydrophilic diradical-mediated electron transfer, which enables simultaneous H2 and O2 production from pure water with an average apparent quantum yield over 1.5% under the visible to near-infrared solar spectrum.

Integration of conventional surface science techniques with surface-sensitive azimuthal and polarization dependent femtosecond-resolved sum frequency generation spectroscopy

Experimental insight into the elementary processes underlying charge transfer across interfaces has blossomed with the wide-spread availability of ultra-high vacuum (UHV) setups that allow the preparation and characterization of solid surfaces with well-defined molecular adsorbates over a wide range of temperatures. Within the last 15 years, such insights have extended to charge transfer heterostructures containing solids overlain by one or more atomically thin two dimensional materials. Such systems are of wide potential interest both because they appear to offer a path to separate surface reactivity from bulk chemical properties and because some offer completely novel physics, unrealizable in bulk three dimensional solids. Thick layers of molecular adsorbates or heterostructures of 2D materials generally preclude the use of electrons or atoms as probes. However, with linear photon-in/photon-out techniques, it is often challenging to assign the observed optical response to a particular portion of the interface. We and prior workers have demonstrated that by full characterization of the symmetry of the second order nonlinear optical susceptibility, i.e., the χ(2), in sum frequency generation (SFG) spectroscopy, this problem can be overcome. Here, we describe an UHV system built to allow conventional UHV sample preparation and characterization, femtosecond and polarization resolved SFG spectroscopy, the azimuthal sample rotation necessary to fully describe χ(2) symmetry, and sufficient stability to allow scanning SFG microscopy. We demonstrate these capabilities in proof-of-principle measurements on CO adsorbed on Pt(111) and on the clean Ag(111) surface. Because this setup allows both full characterization of the nonlinear susceptibility and the temperature control and sample preparation/characterization of conventional UHV setups, we expect it to be of great utility in the investigation of both the basic physics and applications of solid, 2D material heterostructures.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Charge Transfer Dynamics in Organic-Inorganic Hybrid Heterostructures-Insights by Vibrational-Sum Frequency Generation Spectroscopy

Organic-inorganic heterostructures play a pivotal role in modern electronic and optoelectronic applications including photodetectors and field effect transistors, as well as in solar energy conversion such as photoelectrodes of dye-sensitized solar cells, photoelectrochemical cells, and in organic photovoltaics. To a large extent, performance of such devices is controlled by charge transfer dynamics at and across (inner) interfaces, e.g., between a wide band gap semiconductor and molecular sensitizers and/or catalysts. Hence, a detailed understanding of the structure-dynamics-function relationship of such functional interfaces is necessary to rationalize possible performance limitations of these materials and devices on a molecular level. Vibrational sum-frequency generation (VSFG) spectroscopy, as an interface-sensitive spectroscopic technique, allows to obtain chemically specific information from interfaces and combines such chemical insights with ultrafast time resolution, when integrated as a spectroscopic probe into a pump-probe scheme. Thus, this minireview discusses the advantages and potential of VSFG spectroscopy for investigating interfacial charge transfer dynamics and structural changes at inner interfaces. A critical perspective of the unique spectroscopic view of otherwise inaccessible interfaces is presented, which we hope opens new opportunities for an improved understanding of function-determining processes in complex materials, and brings together communities who are devoted to designing materials and devices with spectroscopists.

Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer

We report a rhenium diimine photosensitizer equipped with a peripheral disulfide unit on one of the bipyridine ligands, [Re(CO)3(bpy)(S-Sbpy4,4)]+ (1+, bpy = 2,2'-bipyridine, S-Sbpy4,4 = [1,2]dithiino[3,4-c:6,5-c']dipyridine), showing anti-Kasha luminescence. Steady-state and ultrafast time-resolved spectroscopies complemented by nonadiabatic dynamics simulations are used to disclose its excited-state dynamics. The calculations show that after intersystem crossing the complex evolves to two different triplet minima: a (S-Sbpy4,4)-ligand-centered excited state (3LC) lying at lower energy and a metal-to-(bpy)-ligand charge transfer (3MLCT) state at higher energy, with relative yields of 90% and 10%, respectively. The 3LC state involves local excitation of the disulfide group into the antibonding σ* orbital, leading to significant elongation of the S-S bond. Intriguingly, it is the higher-lying 3MLCT state, which is assigned to display luminescence with a lifetime of 270 ns: a signature of anti-Kasha behavior. This assignment is consistent with an energy barrier ≥ 0.6 eV or negligible electronic coupling, preventing reaction toward the 3LC state after the population is trapped in the 3MLCT state. This study represents a striking example on how elusive excited-state dynamics of transition-metal photosensitizers can be deciphered by synergistic experiments and state-of-the-art calculations. Disulfide functionalization lays the foundation of a new design strategy toward harnessing excess energy in a system for possible bimolecular electron or energy transfer reactivity.

Confinement of CsPbBr3 Perovskite Nanocrystals into Extra-large-pore Zeolite for Efficient and Stable Photocatalytic Hydrogen Evolution

Metal halide perovskites (MHPs), renowned for their outstanding optoelectronic properties, hold significant promise as photocatalysts for hydrogen evolution reaction (HER). However, the low stability and insufficient exposure of catalytically active sites of bulky MHPs seriously impair their catalytic efficiency. Herein, we utilized an extra-large-pore zeolite ZEO-1 (JZO) as a host to confine and stabilize the CsPbBr3 nanocrystals (3.4 nm) for boosting hydrogen iodide (HI) splitting. The as-prepared CsPbBr3@ZEO-1 featured sufficiently exposed active sites, superior stability in acidic media, along with intrinsic extra-large pores of ZEO-1 that were favorable for molecule/ion adsorption and diffusion. Most importantly, the unique nanoconfinement effect of ZEO-1 led to the narrowing of the band gap of CsPbBr3, allowing for more efficient light utilization. As a result, the photocatalytic HER rate of the as-prepared CsPbBr3@ZEO-1 photocatalyst was increased to 1734 μmol ⋅ h-1 ⋅ g-1 (CsPbBr3) under visible light irradiation compared with bulk CsPbBr3 (11 μmol ⋅ h-1 ⋅ g-1 (CsPbBr3)), and the long-term durability (36 h) can be achieved. Furthermore, Pt was incorporated with well-dispersed CsPbBr3 nanocrystals into ZEO-1, resulting in a significant enhancement in activity (4826 μmol ⋅ h-1 ⋅ g-1 (CsPbBr3)), surpassing most of the Pt-integrated perovskite-based photocatalysts. Density functional theory (DFT) calculations and charge-carrier dynamics investigation revealed that the dramatically boosted photocatalytic performance of Pt/CsPbBr3@ZEO-1 could be attributed to the promotion of charge separation and transfer, as well as to the substantially lowered energy barrier for HER. This work highlights the advantage of extra-large-pore zeolites as the nanoscale platform to accommodate multiple photoactive components, opening up promising prospects in the design and exploitation of novel zeolite-confined photocatalysts for energy harvesting and storage.

How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron N-Heterocyclic Complexes: A Comparative Study

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1'-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic FeIII-hexa-NHC [Fe(mbmi)3](PF6)3 and heteroleptic FeII-tetra-NHC [Fe(mbmi)2(bpy)](PF6)2 (bpy = 2,2'-bipyridine) complexes. They are compared to the reported FeIII-hexa-NHC [Fe(btz)3](PF6)3 and FeII-tetra-NHC [Fe(btz)2(bpy)](PF6)2 complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)3](PF6)3 and [Fe(mbmi)2(bpy)](PF6)2 are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The FeII/FeIII redox couples of [Fe(mbmi)3](PF6)3 (-0.38 V) and [Fe(mbmi)2(bpy)](PF6)2 (-0.057 V, both vs Fc+/0) are less reducing than [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The two complexes show intense absorption bands in the visible region: [Fe(mbmi)3](PF6)3 at 502 nm (ligand-to-metal charge transfer, 2LMCT) and [Fe(mbmi)2(bpy)](PF6)2 at 410 and 616 nm (metal-to-ligand charge transfer, 3MLCT). Lifetimes of 57.3 ps (2LMCT) for [Fe(mbmi)3](PF6)3 and 7.6 ps (3MLCT) for [Fe(mbmi)2(bpy)](PF6)2 were probed and are somewhat shorter than those for [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. [Fe(mbmi)3](PF6)3 exhibits photoluminescence at 686 nm (2LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10-4, compared to (3 ± 0.5) × 10-4 for [Fe(btz)3](PF6)3.

Interfacial electron transfer of perylenes: Influence of the anchor binding mode

Interfacial electron transfer (IET) through saturated single-linker and dual-linker groups from a perylene chromophore into nanostructured TiO2 films was studied by ultrafast spectroscopy. Perylene chromophores with one and two propanoic acid linker groups in the peri and ortho positions were investigated. In comparison to previously studied perylenes bound via unsaturated acrylic acid linkers, the chromophores with saturated linkers showed bi-exponential IET dynamics. Two distinct transfer times were observed that indicate the presence of two concurrent binding modes. A comparison between ortho- and peri-substituted sensitizers resulted in slower IET dynamics and weaker electronic coupling for ortho substitution. Finally, IET from sensitizers with saturated linker groups is neither promoted nor hindered by a second linker group. This indicates that only one of the two linkers binds covalently to the surface. This study reveals the importance of the anchor-binding mode and design considerations of the linker for regulating IET.

Photocatalysis with atomically thin sheets

Atomically thin sheets (e.g., graphene and monolayer molybdenum disulfide) are ideal optical and reaction platforms. They provide opportunities for deciphering some important and often elusive photocatalytic phenomena related to electronic band structures and photo-charges. In parallel, in such thin sheets, fine tuning of photocatalytic properties can be achieved. These include atomic-level regulation of electronic band structures and atomic-level steering of charge separation and transfer. Herein, we review the physics and chemistry of electronic band structures and photo-charges, as well as their state-of-the-art characterization techniques, before delving into their atomic-level deciphering and mastery on the platform of atomically thin sheets.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes

A key challenge to the effective utilization of solar energy is to promote efficient photoinduced charge transfer, specifically avoiding unproductive, circuitous electron-transfer pathways and optimizing the kinetics of charge separation and recombination. We hypothesize that one way to address this challenge is to develop a fundamental understanding of how to initiate and control directional photoinduced charge transfer, particularly for earth-abundant first-row transition-metal coordination complexes, which typically suffer from relatively short excited-state lifetimes. Here, we report a series of functionalized heteroleptic copper(I)bis(phenanthroline) complexes, which have allowed us to investigate the directionality of intramolecular photoinduced metal-to-ligand charge transfer (MLCT) as a function of the substituent Hammett parameter. Ultrafast transient absorption suggests a complicated interplay of MLCT localization and solvent interaction with the Cu(II) center of the MLCT state. This work provides a set of design principles for directional charge transfer in earth-abundant complexes and can be used to efficiently design pathways for connecting the molecular modules to catalysts or electrodes and integration into systems for light-driven catalysis.

Time-Linear Quantum Transport Simulations with Correlated Nonequilibrium Green's Functions

We present a time-linear scaling method to simulate open and correlated quantum systems out of equilibrium. The method inherits from many-body perturbation theory the possibility to choose selectively the most relevant scattering processes in the dynamics, thereby paving the way to the real-time characterization of correlated ultrafast phenomena in quantum transport. The open system dynamics is described in terms of an "embedding correlator" from which the time-dependent current can be calculated using the Meir-Wingreen formula. We show how to efficiently implement our approach through a simple grafting into recently proposed time-linear Green's function methods for closed systems. Electron-electron and electron-phonon interactions can be treated on equal footing while preserving all fundamental conservation laws.

Why Does Single-Atom Photocatalysis Work Better Than Conventional Photocatalysis? A Study on Ultrafast Excited Carrier and Structure Dynamics

With the introduction of single atoms in photocatalysis, a small change in the electronic and geometric structure of the substrate can result in higher energy conversion efficiency, whereas the underlying microscopic dynamics are rarely illustrated. Here, employing real-time time-dependent density functional theory, we explore the ultrafast electronic and structural dynamics of single-atom photocatalysts (SAPCs) in water splitting at the microscopic scale. The results demonstrate that a single-atom Pt loaded on graphitic carbon nitride greatly promotes photogenerated carriers compared to traditional photocatalysts, and effectively separates the excited electrons from holes, prolonging the lifetime of the excited carriers. The flexible oxidation state (Pt²⁺, Pt⁰, or Pt³⁺) renders the single atom as an active site to adsorb the reactant and to catalyze the reactions as a charge transfer bridge at different stages during the photoreaction process. Our results offer deep insights into the single-atom photocatalytic reactions and benefit the design of high-efficiency SAPCs.

Unraveling an Ultrafast Electron Transport Mechanism in a Photocatalytic "Micromachine" for Their Potential Light Harvesting Applications

Following the seminal discovery of Richard Feynman, several micromachines have been made that are capable of several applications, such as solar energy harvesting, remediation of environmental pollution, etc. Here we have synthesized a nanohybrid combining TiO₂ nanoparticle and light harvesting robust organic molecule RK1 (2-cyano-3-(4-(7-(5-(4-(diphenylamino)phenyl)-4-octylthiophen-2-yl)benzo[c][1,2,5] thiadiazol-4-yl)phenyl) acrylic acid) as a model micromachine having solar light harvesting ability potential for application in photocatalysis, preparation of solar active devices, etc. Detailed structural characterization, including High Resolution Transmission Electronic Microscopy (HRTEM) and Fourier-transform infrared spectroscopy (FTIR), has been performed on the nanohybrid. We have studied the excited-state ultrafast dynamics of the efficient push-pull dye RK1 in solution, on mesoporous semiconductor nanoparticles, and in insulator nanoparticles by streak camera (resolution of the order of 500 fs). The dynamics of such photosensitizers in polar solvents have been reported, and it has been observed that completely different dynamics occur when they are attached to the surface of the semiconductor/insulator nanosurface. A femtosecond-resolved fast electron transfer has been reported when photosensitizer RK1 has been attached to the surface of the semiconductor nanoparticle, which in turn plays a crucial role in the development of an efficient light harvesting material. The generation of reactive oxygen species as a result of femtosecond-resolved photoinduced electron injection in the aqueous medium is also investigated in order to explore the possibility of redox-active micromachines, which are found to be crucial for efficient and enhanced photocatalysis.

A multi-fragment real-time extension of projected density matrix embedding theory: Non-equilibrium electron dynamics in extended systems

In this work, we derive a multi-fragment real-time extension of the projected density matrix embedding theory (pDMET) designed to treat non-equilibrium electron dynamics in strongly correlated systems. As in the previously developed static pDMET, the real time pDMET partitions the total system into many fragments; the coupling between each fragment and the rest of the system is treated through a compact representation of the environment in terms of a quantum bath. The real-time pDMET involves simultaneously propagating the wavefunctions for each separate fragment–bath embedding system along with an auxiliary mean-field wavefunction of the total system. The equations of motion are derived by (i) projecting the time-dependent Schrödinger equation in the fragment and bath space associated with each separate fragment and by (ii) enforcing the pDMET matching conditions between the global 1-particle reduced density matrix (1-RDM) obtained from the fragment calculations and the mean-field 1-RDM at all points in time. The accuracy of the method is benchmarked through comparisons to time-dependent density-matrix renormalization group and time-dependent Hartree–Fock (TDHF) theory; the methods were applied to a one- and two-dimensional single-impurity Anderson model and multi-impurity Anderson models with ordered and disordered distributions of the impurities. The results demonstrate a large improvement over TDHF and rapid convergence to the exact dynamics with an increase in fragment size. Our results demonstrate that the real-time pDMET is a promising and flexible method that balances accuracy and efficiency to simulate the non-equilibrium electron dynamics in heterogeneous systems of large size.

Coherent Vibronic Wavepackets Show Structure-Directed Charge Flow in Host-Guest Donor-Acceptor Complexes

Designing and controlling charge transfer (CT) pathways in organic semiconductors are important for solar energy applications. To be useful, a photogenerated, Coulombically bound CT exciton must further separate into free charge carriers; direct observations of the detailed CT relaxation pathways, however, are lacking. Here, photoinduced CT and relaxation dynamics in three host-guest complexes, where a perylene (Per) electron donor guest is incorporated into two symmetric and one asymmetric extended viologen cyclophane acceptor hosts, are presented. The central ring in the extended viologen is either p-phenylene (ExV²⁺) or electron-rich 2,5-dimethoxy-p-phenylene (ExMeOV²⁺), resulting in two symmetric cyclophanes with unsubstituted or methoxy-substituted central rings, ExBox⁴⁺ and ExMeOBox⁴⁺, respectively, and an asymmetric cyclophane with one of the central viologen rings being methoxylated ExMeOVBox⁴⁺. Upon photoexcitation, the asymmetric host-guest ExMeOVBox⁴⁺ ⊃ Per complex exhibits directional CT toward the energetically unfavorable methoxylated side due to structural restrictions that facilitate strong interactions between the Per donor and the ExMeOV²⁺ side. The CT state relaxation pathways are probed using ultrafast optical spectroscopy by focusing on coherent vibronic wavepackets, which are used to identify CT relaxations along charge localization and vibronic decoherence coordinates. Specific low- and high-frequency nuclear motions are direct indicators of a delocalized CT state and the degree of CT character. Our results show that the CT pathway can be controlled by subtle chemical modifications of the acceptor host in addition to illustrating how coherent vibronic wavepackets can be used to probe the nature and time evolution of the CT states.

Phase Change Thermal Storage Materials for Interdisciplinary Applications

Functional phase change materials (PCMs) capable of reversibly storing and releasing tremendous thermal energy during the isothermal phase change process have recently received tremendous attention in interdisciplinary applications. The smart integration of PCMs with functional supporting materials enables multiple cutting-edge interdisciplinary applications, including optical, electrical, magnetic, acoustic, medical, mechanical, and catalytic disciplines etc. Herein, we systematically discuss thermal storage mechanism, thermal transfer mechanism, and energy conversion mechanism, and summarize the state-of-the-art advances in interdisciplinary applications of PCMs. In particular, the applications of PCMs in acoustic, mechanical, and catalytic disciplines are still in their infancy. Simultaneously, in-depth insights into the correlations between microscopic structures and thermophysical properties of composite PCMs are revealed. Finally, current challenges and future prospects are also highlighted according to the up-to-date interdisciplinary applications of PCMs. This review aims to arouse broad research interest in the interdisciplinary community and provide constructive references for exploring next generation advanced multifunctional PCMs for interdisciplinary applications, thereby facilitating their major breakthroughs in both fundamental researches and commercial applications.

Bridge-Mediated Metal-to-Metal Electron and Hole Transfer in a Supermolecular Dinuclear Complex: A Computational Study Using Quantum Electron-Nuclear Dynamics

Bimetallic electron donor-acceptor complexes can facilitate electron and energy transfer with excellent structural control through synthetic design. In this work, we investigate the photochemical dynamics in a Ru-Cu bimetallic complex after photoexcitation of the Ru-centered charge transfer state. The physical underpinnings of the metal-to-metal directional charge transfer process are unraveled via analyses of the quantum electronic dynamics and electron-nuclear trajectories. The effects of molecular vibrations in the photoexcited state on the charge transfer processes are also analyzed.

Active Learning Exploration of Transition-Metal Complexes to Discover Method-Insensitive and Synthetically Accessible Chromophores

Transition-metal chromophores with earth-abundant transition metals are an important design target for their applications in lighting and nontoxic bioimaging, but their design is challenged by the scarcity of complexes that simultaneously have well-defined ground states and optimal target absorption energies in the visible region. Machine learning (ML) accelerated discovery could overcome such challenges by enabling the screening of a larger space but is limited by the fidelity of the data used in ML model training, which is typically from a single approximate density functional. To address this limitation, we search for consensus in predictions among 23 density functional approximations across multiple rungs of "Jacob's ladder". To accelerate the discovery of complexes with absorption energies in the visible region while minimizing the effect of low-lying excited states, we use two-dimensional (2D)efficient global optimization to sample candidate low-spin chromophores from multimillion complex spaces. Despite the scarcity (i.e., ∼0.01%) of potential chromophores in this large chemical space, we identify candidates with high likelihood (i.e., >10%) of computational validation as the ML models improve during active learning, representing a 1000-fold acceleration in discovery. Absorption spectra of promising chromophores from time-dependent density functional theory verify that 2/3 of candidates have the desired excited-state properties. The observation that constituent ligands from our leads have demonstrated interesting optical properties in the literature exemplifies the effectiveness of our construction of a realistic design space and active learning approach.

Ultrafast Charge Separation Driven by Torsional Motion in Orthogonal Boron Dipyrromethene Dimer

In this work, the photoinduced charge separation (CS) via symmetry breaking in an orthogonal meso-β-linked boron dipyrromethene (BODIPY) dimer was investigated by polarized transient absorption spectroscopy. The time constant about 0.76 ps of the CS reaction determined in dimethyl sulfoxide is much faster than the solvation dynamics. The observed transient anisotropy of the BODIPY anion band implies that both hole and electron transfers occur with similar probabilities. The bidirectional charge transfer processes suggest that the locally excited state is weakly coupled to the polar solvent, and the solvation coupled excited-state structural relaxation within the BODIPY monomeric unit is rather limited. In combination with the electronic excitation analysis based on time-dependent density-functional theory calculations, we deduced that the CS in the orthogonal BODIPY dimer is enabled via the torsional motion associated with covalently connected BODIPY units, promoting the electronic coupling, and irrelevant to the dynamic solvent relaxation.

Femtosecond transient absorption spectroscopy investigation into the electron transfer mechanism in photocatalysis

Femtosecond transient absorption spectroscopy (fs-TAS) is a powerful technique for monitoring the electron transfer kinetics in photocatalysis. Several important works have successfully elucidated the electron transfer mechanism in heterojunction photocatalysts (HPs) using fs-TAS measurements, and thus a timely summary of recent advances is essential. This feature article starts with a thorough interpretation of the operating principle of fs-TAS equipment, and the fundamentals of the fs-TAS spectra. Subsequently, the applications of fs-TAS in analyzing the dynamics of photogenerated carriers in semiconductor/metal HPs, semiconductor/carbon HPs, semiconductor/semiconductor HPs, and multicomponent HPs are discussed in sequence. Finally, the significance of fs-TAS in revealing the ultrafast interfacial electron transfer process in HPs is summarized, and further research on the applications of fs-TAS in photocatalysis is proposed. This feature article will provide deep insight into the mechanism of the enhanced photocatalytic performance of HPs from the perspective of electron transfer kinetics.

Controlling Fragmentation of the Acetylene Cation in the Vacuum Ultraviolet via Transient Molecular Alignment

An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the consequence of changes in the electronic response with molecular orientation relative to the ionizing field. We show that stable C2H2+ cations are mainly produced when the molecules are parallel or nearly parallel to the FEL polarization, while the hydrogen fragmentation channel (C2H2+ → C2H+ + H) predominates when the molecule is perpendicular to that direction, thus allowing one to distinguish between the two photochemical processes. The experimental findings are supported by state-of-the art theoretical calculations.

Spectroscopic and kinetic characterization of photogenerated charge carriers in photocatalysts

The catastrophic consequences of increased power consumption, such as drastically rising CO₂ levels, natural disasters, environmental pollution and dependence on fossil fuels supplied by countries with totalitarian regimes, illustrate the urge to develop sustainable technologies for energy generation. Photocatalysis presents eco-friendly means for fuels production via solar-to-chemical energy conversion. The conversion efficiency of a photocatalyst critically depends on charge carrier processes taking place in the ultrafast time regime. Transient absorption spectroscopy (TAS) serves as a perfect tool to track those processes. The spectral and kinetic characterization of charge carriers is indispensable for the elucidation of photocatalytic mechanisms and for the development of new materials. Hence, in this review, we will first present the basics of TAS and subsequently discuss the procedure required for the interpretation of the transient absorption spectra and transient kinetics. The discussion will include specific examples for charge carrier processes occurring in conventional and plasmonic semiconductors.

Dip-and-pull ambient pressure photoelectron spectroscopy as a spectroelectrochemistry tool for probing molecular redox processes

Ambient pressure x-ray photoelectron spectroscopy (APXPS) can provide a compelling platform for studying an analyte's oxidation and reduction reactions in solutions. This paper presents proof-of-principle operando measurements of a model organometallic complex, iron hexacyanide, in an aqueous solution using the dip-and-pull technique. The data demonstrates that the electrochemically active liquid meniscuses on the working electrodes can undergo controlled redox reactions which were observed using APXPS. A detailed discussion of several critical experimental considerations is included as guidance for anyone undertaking comparable experiments.

A quantitative model of charge injection by ruthenium chromophores connecting femtosecond to continuous irradiance conditions

A kinetic framework for the ultrafast photophysics of tris(2,2-bipyridine)ruthenium(II) phosphonated and methyl-phosphonated derivatives is used as a basis for modeling charge injection by ruthenium dyes into a semiconductor substrate. By including the effects of light scattering, dye diffusion, and adsorption kinetics during sample preparation and the optical response of oxidized dyes, quantitative agreement with multiple transient absorption datasets is achieved on timescales spanning femtoseconds to nanoseconds. In particular, quantitative agreement with important spectroscopic handles-the decay of an excited state absorption signal component associated with charge injection in the UV region of the spectrum and the dynamical redshift of a ∼500 nm isosbestic point-validates our kinetic model. Pseudo-first-order rate coefficients for charge injection are estimated in this work, with an order of magnitude ranging from 10¹¹ to 10¹² s^-1. The model makes the minimalist assumption that all excited states of a particular dye have the same charge injection coefficient, an assumption that would benefit from additional theoretical and experimental exploration. We have adapted this kinetic model to predict charge injection under continuous solar irradiation and find that as many as 68 electron transfer events per dye per second take place, significantly more than prior estimates in the literature.

Improved Ultrafast Carrier Relaxation and Charge Transfer Dynamics in CuI Films and Their Heterojunctions via Sn Doping

CuI is one of the promising hole transport materials for perovskite solar cells. However, its tendency to form defects is currently limiting its use for device applications. Here, we report the successful improvement of CuI through Sn doping and the direct measurement of the carrier relaxation and interfacial charge-transfer processes in Sn-doped CuI films and their heterostructures. Femtosecond-transient absorption (fs-TA) measurements reveal that Sn doping effectively passivates the trap states within the bandgap of CuI. The I-V characteristics of heterostructures demonstrate drastic improvement in transport characteristics upon Sn doping. Fs-TA measurements further confirm that the CuSnI/ZnO heterojunction has a type-II configuration with ultrafast charge transfer (<280 fs). The charge transfer time of a CuI/ZnO heterostructure is ∼2.8 times slower than that of the CuSnI/ZnO heterostructure, indicating that Sn doping suppresses the interfacial states that retard the charge transfer. These results elucidate the effect of Sn doping on the performance of CuI-based heterostructures.

Intermediate Complex-Mediated Interfacial Electron Transfer in a Radical Dianion/TiO2 Dye-Sensitized Photocatalytic System

We present a mechanistic study of a PTCDA^2-/TiO₂ dye-sensitized photocatalytic system, in which the stable radical dianion PTCDA^2- is formed via a two-step consecutive photoinduced electron transfer from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride). Photoexcitation of PTCDA^2- brings forth an interesting behavior known as vibrationally excited-state-selective, visible-light photocatalytic hydrogen evolution reaction (HER). In conjunction with the information gleaned from optical spectroscopy and ultrafast dynamics, we reveal that an intermediate complex (IC) state with a lifetime of ∼12 ps exists in the vicinity of a certain vibrationally excited state of PTCDA^2-. Such a unique IC state mediates the interfacial electron transfer (IET) channel from the specific excited state of PTCDA^2- to the conduction band continuum of TiO₂. As an outcome, the effective IC-mediated IET process in this photocatalytic system leads to a remarkable HER rate that reaches ∼4660 μmol g^-1 h^-1.

Cooperative Coupling of H2 O2 Production and Organic Synthesis over a Floatable Polystyrene-Sphere-Supported TiO2 /Bi2 O3 S-Scheme Photocatalyst

Cooperative coupling of photocatalytic H₂ O₂ production with organic synthesis has an expansive perspective in converting solar energy into storable chemical energy. However, traditional powder photocatalysts suffer from severe agglomeration, limited light absorption, poor gas reactant accessibility, and reusable difficulty, which greatly hinders their large-scale application. Herein, floatable composite photocatalysts are synthesized by immobilizing hydrophobic TiO₂ and Bi₂ O₃ on lightweight polystyrene (PS) spheres via hydrothermal and photodeposition methods. The floatable photocatalysts are not only solar transparent, but also upgrade the contact between reactants and photocatalysts. Thus, the floatable step-scheme (S-scheme) TiO₂ /Bi₂ O₃ photocatalyst exhibits a drastically enhanced H₂ O₂ yield of 1.15 mm h^-1 and decent furfuryl alcohol conversion to furoic acid synchronously. Furthermore, the S-scheme mechanism and dynamics are systematically investigated by in situ irradiated X-ray photoelectron spectroscopy and femtosecond transient absorption spectrum analyses. In situ Fourier transform infrared spectroscopy and density functional theory calculations reveal the mechanism of furoic acid evolution. The ingenious design of floatable photocatalysts not only furnishes insight into maximizing photocatalytic reaction kinetics but also provides a new route for highly efficient heterogeneous catalysis.

KiMoPack: A python Package for Kinetic Modeling of the Chemical Mechanism

Herein, we present KiMoPack, an analysis tool for the kinetic modeling of transient spectroscopic data. KiMoPack enables a state-of-the-art analysis routine including data preprocessing and standard fitting (global analysis), as well as fitting of complex (target) kinetic models, interactive viewing of (fit) results, and multiexperiment analysis via user accessible functions and a graphical user interface (GUI) enhanced interface. To facilitate its use, this paper guides the user through typical operations covering a wide range of analysis tasks, establishes a typical workflow and is bridging the gap between ease of use for less experienced users and introducing the advanced interfaces for experienced users. KiMoPack is open source and provides a comprehensive front-end for preprocessing, fitting and plotting of 2-dimensional data that simplifies the access to a powerful python-based data-processing system and forms the foundation for a well documented, reliable, and reproducible data analysis.

Electron Transfer at Quantum Dot–Metal Oxide Interfaces for Solar Energy Conversion

Electron transfer at a donor–acceptor quantum dot–metal oxide interface is a process fundamentally relevant to solar energy conversion architectures as, e.g., sensitized solar cells and solar fuels schemes. As kinetic competition at these technologically relevant interfaces largely determines device performance, this Review surveys several aspects linking electron transfer dynamics and device efficiency; this correlation is done for systems aiming for efficiencies up to and above the ∼33% efficiency limit set by Shockley and Queisser for single gap devices. Furthermore, we critically comment on common pitfalls associated with the interpretation of kinetic data obtained from current methodologies and experimental approaches, and finally, we highlight works that, to our judgment, have contributed to a better understanding of the fundamentals governing electron transfer at quantum dot–metal oxide interfaces.

Short Excited-State Lifetimes Mediate Charge-Recombination Losses in Organic Solar Cell Blends with Low Charge-Transfer Driving Force

A blend of a low-optical-gap diketopyrrolopyrrole polymer and a fullerene derivative, with near-zero driving force for electron transfer, is investigated. Using femtosecond transient absorption and electroabsorption spectroscopy, the charge transfer (CT) and recombination dynamics as well as the early-time transport are quantified. Electron transfer is ultrafast, consistent with a Marcus-Levich-Jortner description. However, significant charge recombination and unusually short excited (S₁ ) and CT state lifetimes (≈14 ps) are observed. At low S₁ -CT offset, a short S₁ lifetime mediates charge recombination because: i) back-transfer from the CT to the S₁ state followed by S₁ recombination occurs and ii) additional S₁ -CT hybridization decreases the CT lifetime. Both effects are confirmed by density functional theory calculations. In addition, relatively slow (tens of picoseconds) dissociation of charges from the CT state is observed, due to low local charge mobility. Simulations using a four-state kinetic model entailing the effects of energetic disorder reveal that the free charge yield can be increased from the observed 12% to 60% by increasing the S₁ and CT lifetimes to 150 ps. Alternatively, decreasing the interfacial CT state disorder while increasing bulk disorder of free charges enhances the yield to 65% in spite of the short lifetimes.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d⁶ valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L^CNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L^CNC)₂](PF₆), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(L^CNC)₂](PF₆) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L^CNC)₂](PF₆), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Unraveling Charge-Separation Mechanisms in Photocatalyst Particles by Spatially Resolved Surface Photovoltage Techniques

The photocatalytic conversion of solar energy offers a potential route to renewable energy, and its efficiency relies on effective charge separation in nanostructured photocatalysts. Understanding the charge-separation mechanism is key to improving the photocatalytic performance and this has now been enabled by advances in the spatially resolved surface photovoltage (SRSPV) method. In this Review we highlight progress made by SRSPV in mapping charge distributions at the nanoscale and determining the driving forces of charge separation in heterogeneous photocatalyst particles. We discuss how charge separation arising from a built-in electric field, diffusion, and trapping can be exploited and optimized through photocatalyst design. We also highlight the importance of asymmetric engineering of photocatalysts for effective charge separation. Finally, we provide an outlook on further opportunities that arise from leveraging these insights to guide the rational design of photocatalysts and advance the imaging technique to expand the knowledge of charge separation.

Emerging S-Scheme Photocatalyst

Photocatalysis is a green technology to use ubiquitous and intermittent sunlight. The emerging S-scheme heterojunction has demonstrated its superiority in photocatalysis. This article covers the state-of-the-art progress and provides new insights into its general designing criteria. It starts with the challenges confronted by single photocatalyst from the perspective of energy dissipation by borrowing the common behaviors in the dye molecule. Subsequently, other problems faced by single photocatalyst are summarized. Then a viable solution for these problems is the construction of heterojunctions. To overcome the problems and mistakes of type-II and Z-scheme heterojunctions, S-scheme heterojunction is proposed and the underlying reaction mechanism is summarized. Afterward, the design principles for S-scheme heterojunction are proposed and four types of S-scheme heterojunctions are suggested. Following this, direct characterization techniques for testifying the charge transfer in S-scheme heterojunction are presented. Finally, different photocatalytic applications of S-scheme heterojunctions are summarized. Specifically, this work endeavors to clarify the critical understanding on curved Fermi level in S-scheme heterojunction interface, which can help strengthen and advance the fundamental theories of photocatalysis. Moreover, the current challenges and prospects of the S-scheme heterojunction photocatalyst are critically discussed.

Porous organic polymers for light-driven organic transformations

As a new generation of porous materials, porous organic polymers (POPs), have recently emerged as a powerful platform of heterogeneous photocatalysis. POPs are constructed using extensive organic synthesis methodologies, with various functional organic units being connected via high-energy covalent bonds. This review systematically presents the recent advances in POPs for visible-light driven organic transformations. Herein, we firstly summarize the common construction strategies for POP-based photocatalysts based on two major approaches: pre-design and post-modification; secondly, we categorize and summarize the synthesis methods and organic reaction types for constructing various types of POPs. We then classify and introduce the specific reactions of current light-driven POP-mediated organic transformations. Finally, we outline the current state of development and the problems faced in light-driven organic transformations by POPs, and we present some perspectives to motivate the reader to explore solutions to these problems and confront the present challenges in the development process.

Metallic Copper-Containing Composite Photocatalysts: Fundamental, Materials Design, and Photoredox Applications

Semiconductor photocatalysis has long been regarded as a potential solution to tackle the energy and environmental challenges since the first discovery of water splitting by TiO₂ almost 50 years ago. The past few years have seen a tremendous flurry of research interest in the modification of semiconductors because of their shortcomings in the aspects of solar harvesting, electron-hole pairs separation, and utilization of photogenerated carriers. Among the various strategies, the introduction of metallic copper into the photocatalysis system can not only enhance the absorption of sunlight and the separation efficiency of photogenerated electrons and holes, but also increase the adsorption ability of substrate and the number of active sites, so as to realize the high solar to chemical energy conversion efficiency. This review focuses on the rational design of copper-based composites and their applications in photoredox catalysis. First, the preparation methods of metallic copper-containing composites are discussed. Then, the applications of different types of copper-based composites in the photocatalytic removal of pollutants, splitting of water to hydrogen production, reduction of carbon dioxide, and conversion of organic matter are introduced. Finally, the opportunities and challenges in the design and synthesis of copper-based composites and their applications in the photocatalysis are prospected.

Quantifying the Steric Effect on Metal-Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering

Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L₃ edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)₂]²⁺ and [Fe(btbip)₂]²⁺, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe-carbene bond length due to steric effects modify the metal-ligand bonding, including σ/π donation and π back-donation. We find that σ donation is significantly stronger in [Fe(bmip)₂]²⁺, whereas the π back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)₂]²⁺ lead to approximately 1 eV destabilization of the quintet metal-centered ⁵T_2g excited state compared to [Fe(btbip)₂]²⁺, providing an explanation for the absence of a photoinduced ⁵T_2g population and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)₂]²⁺. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.

Ag nanoplatelets as efficient photosensitizers for TiO2 nanorods

The lifetime for injecting hot electrons generated in Ag nanoplatelets to nearby TiO₂ nanorods was measured with ultrafast transient IR absorption to be 13.1 ± 1.5 fs, which is comparable to values previously reported for much smaller spherical Ag nanoparticles. Although it was shown that the injection rate decreases as the particle size increases, this observation can be explained by the facts that (1) the platelet has a much larger surface to bulk ratio and (2) the platelet affords a much larger surface area for direct contact with the semiconductor. These two factors facilitate strong Ag-TiO₂ coupling (as indicated by the observed broadened surface plasmon resonance band of Ag) and can explain why Ag nanoplatelets have been found to be more efficient than much smaller Ag nanoparticles as photosensitizers for photocatalytic functions. The fast injection rate, together with a stronger optical absorption in comparison with Au and dye molecules, make Ag nanoplatelets a preferred photosensitizer for wide bandgap semiconductors.

Intruder Peak-Free Transient Inner-Shell Spectra Using Real-Time Simulations

Real-time methods are convenient for simulating core-level absorption spectra but suffer from nonphysical intruder peaks when using atom-centered basis sets. In transient absorption spectra, these peaks exhibit highly nonphysical time-dependent modulations in their energies and oscillator strengths. In this paper, we address the origins of these intruder peaks and propose a straightforward and effective solution based on a filtered dipole operator. In combination with real-time time-dependent density functional theory (RT-TDDFT), we demonstrate how to compute intruder-free attosecond transient X-ray absorption spectra for the aminophenol (C₆H₇NO) oxygen and nitrogen K-edges and the α-quartz (SiO₂) silicon L-edge. Without filtering, the computed spectra are qualitatively wrong. This procedure is suitable for both static and transient inner-shell spectroscopy studies and can easily be implemented in a range of real-time methodologies.