Early Folding Dynamics of i-Motif DNA Revealed by pH-Jump Time-Resolved X-ray Solution Scattering.

The i-motif is a pH-responsive cytosine-rich oligonucleotide sequence that forms, under acidic conditions, a quadruplex structure. This tunable structural switching has made the i-motif a useful platform for designing pH-responsive nanomaterials. Despite the widespread application of i-motif DNA constructs as biomolecular switches, the mechanism of i-motif folding on the atomic scale has yet to be established.

Amidination of ligands for chemical and field-effect passivation stabilizes perovskite solar cells.

Surface passivation has driven the rapid increase in the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, state-of-the-art surface passivation techniques rely on ammonium ligands that suffer deprotonation under light and thermal stress. We developed a library of amidinium ligands, of interest for their resonance effect-enhanced N-H bonds that may resist deprotonation, to increase the thermal stability of passivation layers on perovskite surfaces.

Unexpected Photodriven Linker-to-Node Hole Transfer in a Zirconium-Based Metal-Organic Framework.

Zr(μ-O)(μ-OH) node cores are indispensable building blocks for almost all zirconium-based metal-organic frameworks. Consistent with the insulating nature of zirconia, they are generally considered electronically inert. Contrasting this viewpoint, we present spectral measurements and calculations indicating that emission from photoexcited NU-601, a six-connected Zr-based MOF, comes from both linker-centric locally excited and linker-to-node charge-transfer (CT) states.

Coherent Phonon Dynamics in Plasmonic Gold Tetrahedral Nanoparticle Ensembles.

Coherent phonon modes supported by plasmonic nanoparticles offer prospective applications in chemical and biological sensing. Whereas the characterization of these phonon modes often requires single-particle measurements, synthetic routes to narrow size distributions of nanoparticles permit ensemble investigations. Recently, the synthesis of highly monodisperse gold tetrahedral nanoparticles with tunable edge lengths and corner sharpnesses has been developed.

From Coherence to Function: Exploring the Connection in Chemical Systems.

ConspectusThe role of quantum mechanical coherences or coherent superposition states in excited state processes has received considerable attention in the last two decades largely due to advancements in ultrafast laser spectroscopy. These coherence effects hold promise for enhancing the efficiency and robustness of functionally relevant processes, even when confronted with energy disorder and environmental fluctuations. Understanding coherence deeply drives us to unravel mechanisms and dynamics controlled by order and synchronization at a quantum mechanical level, envisioning optical control of coherence to enhance functions or create new ones in molecular and material systems.

Real-time capture of nuclear motions influencing photoinduced electron transfer.

Although vibronic coupling phenomena have been recognized in the excite state dynamics of transition metal complexes, its impact on photoinduced electron transfer (PET) remains largely unexplored. This study investigates coherent wavepacket (CWP) dynamics during PET processes in a covalently linked electron donor-acceptor complex featuring a cyclometalated Pt(ii) dimer as the donor and naphthalene diimide (NDI) as the acceptors. Upon photoexciting the Pt(ii) dimer electron donor, ultrafast broadband transient absorption spectroscopy revealed direct modulation of NDI radical anion formation through certain CWP motions and correlated temporal evolutions of the amplitudes for these CWPs with the NDI radical anion formation.

Unlocking the unfolded structure of ubiquitin: Combining time-resolved x-ray solution scattering and molecular dynamics to generate unfolded ensembles.

The unfolding dynamics of ubiquitin were studied using a combination of x-ray solution scattering (XSS) and molecular dynamics (MD) simulations. The kinetic analysis of the XSS ubiquitin signals showed that the protein unfolds through a two-state process, independent of the presence of destabilizing salts. In order to characterize the ensemble of unfolded states in atomic detail, the experimental XSS results were used as a constraint in the MD simulations through the incorporation of x-ray scattering derived potential to drive the folded ubiquitin structure toward sampling unfolded states consistent with the XSS signals.

Two-dimensional perovskitoids enhance stability in perovskite solar cells.

Two-dimensional (2D) and three-dimensional (3D) perovskite heterostructures have played a key role in advancing the performance of perovskite solar cells. However, the migration of cations between 2D and 3D layers results in the disruption of octahedral networks, leading to degradation in performance over time. We hypothesized that perovskitoids, with robust organic-inorganic networks enabled by edge- and face-sharing, could impede ion migration.

Intraband Cooling and Auger Recombination in Weakly to Strongly Quantum-Confined CsPbBr Perovskite Nanocrystals.

Semiconductor nanocrystals (NCs) with size-tuned energy gaps present unique and desirable properties for optoelectronic applications. Recent synthetic advancements offer routes to spheroidal CsPbBr perovskite NCs in the strong quantum confinement regime with narrow size dispersion. Using tunable femtosecond laser pulses, we examine intraband carrier relaxation using transient absorption spectroscopy and show that, across the transition from weak to strong confinement, hot carrier lifetime increases compared to larger bulk-like particles.

Structural Evolution and Photoluminescence Quenching across the FASnIBr ( = 0-3) Perovskites.

One of the primary methods for band gap tuning in metal halide perovskites has been halide (I/Br) mixing. Despite widespread usage of this type of chemical substitution in perovskite photovoltaics, there is still little understanding of the structural impacts of halide alloying, with the assumption being the formation of ideal solid solutions. The FASnIBr ( = 0-3) family of compounds provides the first example where the assumption breaks down, as the composition space is broken into two unique regimes ( = 0-2.

Deciphering Charge Transfer Processes in Transition Metal Complexes from the Perspective of Ultrafast Electronic and Nuclear Motions.

Chemical transformations in charge transfer states result from the interplay between electronic dynamics and nuclear reorganization along excited-state trajectories. Here, we investigate the ultrafast structural dynamics following photoinduced electron transfer from the metal-metal-to-ligand charge transfer state of an electron donor, a Pt dimer complex, to a covalently linked electron acceptor group using ultrafast time-resolved wide-angle X-ray scattering and optical transient absorption spectroscopy methods to disentangle the interdependence of the excited-state electronic and nuclear dynamics. Following photoexcitation, Pt-Pt bond formation and contraction takes up to 1 ps, much slower than the corresponding process in analogous complexes without electron acceptor groups.

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes.

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO, HO, and O to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales.

Photodriven electron-transfer dynamics in a series of heteroleptic Cu(I)-anthraquinone dyads.

Solar fuels catalysis is a promising route to efficiently harvesting, storing, and utilizing abundant solar energy. To achieve this promise, however, molecular systems must be designed with sustainable components that can balance numerous photophysical and chemical processes. To that end, we report on the structural and photophysical characterization of a series of Cu(I)-anthraquinone-based electron donor-acceptor dyads.

Orientational analysis of atomic pair correlations in nanocrystalline indium oxide thin films.

The application of grazing-incidence total X-ray scattering (GITXS) for pair distribution function (PDF) analysis using >50 keV X-rays from synchrotron light sources has created new opportunities for structural characterization of supported thin films with high resolution. Compared with grazing-incidence wide-angle X-ray scattering, which is only useful for highly ordered materials, GITXS/PDFs expand such analysis to largely disordered or nanostructured materials by examining the atomic pair correlations dependent on the direction relative to the surface of the supporting substrate. A characterization of nanocrystalline InO-derived thin films is presented here with in-plane-isotropic and out-of-plane-anisotropic orientational ordering of the atomic structure, each synthesized using different techniques.

A Nanocavitation Approach to Understanding Water Capture, Water Release, and Framework Physical Stability in Hierarchically Porous MOFs.

Chemically stable metal-organic frameworks (MOFs) featuring interconnected hierarchical pores have proven to be promising for a remarkable variety of applications. Nevertheless, the framework's susceptibility to capillary-force-induced pore collapse, especially during water evacuation, has often limited practical applications. Methodologies capable of predicting the relative magnitudes of these forces as functions of the pore size, chemical composition of the pore walls, and fluid loading would be valuable for resolution of the pore collapse problem.

Bimolecularly passivated interface enables efficient and stable inverted perovskite solar cells.

Compared with the n-i-p structure, inverted (p-i-n) perovskite solar cells (PSCs) promise increased operating stability, but these photovoltaic cells often exhibit lower power conversion efficiencies (PCEs) because of nonradiative recombination losses, particularly at the perovskite/C interface. We passivated surface defects and enabled reflection of minority carriers from the interface into the bulk using two types of functional molecules. We used sulfur-modified methylthio molecules to passivate surface defects and suppress recombination through strong coordination and hydrogen bonding, along with diammonium molecules to repel minority carriers and reduce contact-induced interface recombination achieved through field-effect passivation.

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.

Spin-vibronic coherence drives singlet-triplet conversion.

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism.

Photoreactive Carbon Dioxide Capture by a Zirconium-Nanographene Metal-Organic Framework.

The mechanism of photochemical CO reduction to formate by PCN-136, a Zr-based metal-organic framework (MOF) that incorporates light-harvesting nanographene ligands, has been investigated using steady-state and time-resolved spectroscopy and density functional theory (DFT) calculations. The catalysis was found to proceed via a "photoreactive capture" mechanism, where Zr-based nodes serve to capture CO in the form of Zr-bicarbonates, while the nanographene ligands have a dual role of absorbing light and storing one-electron equivalents for catalysis. We also find that the process occurs via a "two-for-one" route, where a single photon initiates a cascade of electron/hydrogen atom transfers from the sacrificial donor to the CO-bound MOF.

Revealing Excited-State Trajectories on Potential Energy Surfaces with Atomic Resolution in Real Time.

Photoexcited molecular trajectories on potential energy surfaces (PESs) prior to thermalization are intimately connected to the photochemical reaction outcome. The excited-state trajectories of a diplatinum complex featuring photo-activated metal-metal σ-bond formation and associated Pt-Pt stretching motions were detected in real time using femtosecond wide-angle X-ray solution scattering. The observed motions correspond well with coherent vibrational wavepacket motions detected by femtosecond optical transient absorption.

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.

Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores.

Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups.