Semiconductor nanocrystals (NCs) offer actualized and prospective utility in optoelectronic technologies, yet key aspects of their thermal and vibrational behaviors remain unresolved. Compared with bulk crystals, heat dissipation involving NCs can differ substantially owing to pervasiveness of interfacial scattering, phonon confinement, and relaxed phonon momentum selection rules, as well as influences of ligands. Here, in addition to transient absorption, we use femtosecond stimulated Raman spectroscopy to track nonequilibrium optical-phonon dynamics in InP NCs capped with myristic acid ligands and compare the same particles coated with a ZnS shell. We identify distinct phonon decay pathways in ligand-capped versus ZnS-shelled NCs. Increasing the excitation density and introducing the ZnS shell measurably modify optical-phonon lifetimes, whereas phonon formation times remain largely unchanged. In addition, we observe phonon mode softening in ligand-capped nanocrystals, consistent with lattice expansion. Together, these results demonstrate that core-shell structures can strongly govern nanocrystal thermal dissipation pathways and should be considered a key design parameter for optoelectronic operation.

The generation and dissipation of heat in nanocrystals upon optical excitation is a process that may either limit or enhance their performance in certain applications. Using transient absorption spectroscopy, we tracked the flow of heat from plasmonic tin-doped indium oxide (ITO) nanocrystals to surface-adsorbed perylenediimide (PDI) molecules upon excitation of the ITO near-infrared plasmon resonance. We rationalize that the derivative line shapes observed in the transient absorption bleach features are the result of thermal transfer from the ITO core to surface ligands based on temperature-dependent static absorption studies of the PDI molecules adsorbed on plasmonic ITO. Through a series of pump power-dependent measurements, we demonstrate that the PDI heating time is largely fluence-independent at the powers measured, while the overall recovery time increases. Elucidating thermal transfer rates at the nanocrystal and organic ligand interface is key for applications such as plasmon-mediated photocatalysis, where heating may obstruct hot carrier transfer processes.

Coupling exciton and plasmon excitations to form polaritons are of great interest for manipulating energy transfer at the nanoscale via the formation of hybrid light-matter states. In this study, we successfully couple gold tetrahedral nanoparticles with the J-aggregate forming dye 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) to form a strongly coupled colloidal polariton system with a Rabi splitting energy of ∼206 meV. These gold tetrahedra exhibit coherent phonon modes upon photoexcitation, which produce transient oscillations of the LSPR energy for isolated tetrahedra. Transient absorption measurements of the polariton system were performed and show how these coherent phonon modes influence the polariton states' extinction. We found that the oscillation period increases by 0.5 ± 0.14 ps upon surface deposition of TDBC dye, demonstrating LSPR sensitivity to the refractive index environment. Shifts in the plasmon resonance due to the coherent phonon modes transiently alters LSPR alignment with the J-aggregate exciton peak, resulting in shifts of the hybrid polariton states' oscillator strength.

Advances of ultrafast experimental methods enable electronic and nuclear motions in real time to be captured. We explore ultrafast functional structural dynamics in photoexcited states in transition metal complexes with strongly coupled electronic and nuclear motions beyond the Born-Oppenheimer Approximation by identifying key excited-state reaction coordinates and tracking nuclear motions along excited-state trajectories. We will present the following examples. 1) Real-time Capture of Nuclear Motions Influencing Photoinduced Electron Transfer. - This study investigates the dynamics of CVWP during photoinduced electron transfer (PET) in a donor-acceptor complex comprising a cyclometalated Pt(II) dimer as the donor and naphthalene diimide (NDI) as the acceptor. Ultrafast broadband transient absorption (BBTA) spectroscopy shows that certain CVWP motions modulate NDI radical anion formation and are temporally correlated with PET. These CVWPs are identified as vibronic coherences along PET reaction coordinates, involving vibrational modes related to dihedral angles and bond lengths the ligand and NDI. The results demonstrate that the vibrations play a crucial role in shaping efficient PET pathways. 2)Jahn-Teller Distortion Controls Electron Transfer in Photoexcited Cu(I) Donor-Acceptor Systems - This study reveals how JTD influences PET dynamics in Cu(I)-based donor–acceptor dyads using ultrafast BBTA spectroscopy. Ligand substitutions tune the extent of the JTD, leading to distinct charge separation (CS) times. CVWP dynamics uncovers ballistic PET and non-equilibrium coupling between JTD and electron flow during CS. Excited-state potential energy surface (PES) calculations indicate that JTD facilitates spin-vibronic coupling at conical intersections, thereby accelerating PET and shaping the reaction coordinate for efficient charge separation. Our results provide insight into designing photoactive TMCs by harnessing structural dynamics to control electron transfer. 3) Tracking Photoinduced Charge Redistribution in a Cu(I) Diimine Donor-Bridge-Acceptor System with Time-Resolved Infrared Spectroscopy - We investigate photoinduced electron transfer (PET) in a covalently linked donor–bridge–acceptor (D-B-A) system, where [Cu(I)-bis(1,10-phenanthroline)] + acts as an electron donor, and anthraquinone, tethered to one of the phenanthroline ligands via a vibrationally active ethyne bridge, behaves as an electron acceptor. Visible transient absorption spectroscopy revealed the dynamic processes occurring in the excited state, including PET to the acceptor species. 4) Ultrafast Hydrogen-bonding Interactions between a Photoexcited Cu-anthraquinone Donor-acceptor Dyad and Protic Solvents- We have designed a new Cu(I)-anthraquinone framework (CuEthyneAnQ) to serve as a model for studying hydrogen-bonding effects in charge-accumulating photocatalysts. These insights highlight the potential of Cu-based donor-acceptor complexes and mixed-solvent systems to provide valuable guidelines for designing more efficient photocatalytic systems.

The Jahn-Teller distortion (JTD) is a defining structural response to electronic excitation in Cu(i)-based transition metal complexes, yet its role in photoinduced electron transfer (PET) remains largely unexplored. Here, we demonstrate that the JTD governs charge separation (CS) through a vibronically controlled conical intersection in heteroleptic Cu(i) bisphenanthroline-naphthalene diimide (CuHETPHEN-NDI) donor-acceptor dyads. Using 20-fs broadband transient absorption spectroscopy combined with coherent vibrational wavepacket (CVWP) analysis and quantum chemical calculations, we directly track nuclear motions that steer the system from the metal-to-ligand charge-transfer (1MLCT) state to the CS state. Steric bulkiness of the pendant groups at the 2,9-positions of the phenanthroline ligand systematically slows both JTD and CS. Short-time Fourier transformation and Fourier filtering analyses identify two key vibrational signatures: a low-frequency breathing mode (∼100 cm-1) via a bond distance change between Cu and ligated Ns (Cu-N) that modulates the NDI anion absorption and acts as a vibronic coupling coordinate, and a higher-frequency mode (∼313 cm-1) that evolves along the PET trajectory. Normal mode analysis and potential energy surface calculations show that the JTD brings the 1MLCT and CS states into degeneracy, while the Cu-N breathing motion dynamically modulates donor-acceptor electronic coupling to enable ultrafast nonadiabatic electron transfer. Steric hindrance exerted by the groups at the 2,9 positions of the phenanthroline ligands suppresses this vibronic coupling, leading to faster CVWP decoherence for the 313 cm-1 mode and slower CS. These findings unravel JTD-controlled vibronic coupling at conical intersection as a governing factor for CS and provide insight into designing Cu-based photosensitizers by harnessing structural dynamics to control PET.

Understanding and controlling ultrafast excitons and trion dynamics in monolayer transition metal dichalcogenides (TMDs) is critical for optoelectronic and photonic applications. These dynamics depend strongly on carrier density, but most studies use fluence-dependence to modulate electron-hole populations rather than direct electrostatic gating that is more relevant to optoelectronic devices. We utilize electrolyte gating to tune the ground-state carrier density of monolayer MoS2 and probe excitonic dynamics using transient absorption spectroscopy, thereby modifying absorption through bandgap renormalization, screening, phase-space filling, and exciton-trion crossover. Spectral deconvolution reveals distinct but coupled exciton and trion dynamics. Excitons form independently of voltage but relax faster with increasing carrier density, consistent with Auger-like processes, while trions decay more rapidly through multiple channels. At higher voltages, trion relaxation shifts from a subpicosecond cooling to slower trapping-assisted processes. Our results provide missing insights into the interplay between excitons and trions in monolayer MoS2, relevant for TMD-based optoelectronics.

Three supramolecular square assemblies featuring Re(CO)3Cl corners and monodentate pyridyl linkers [4,4'-bipyridine, (1a), pyrazine (2a), and 1,2-di(4-pyridyl)ethylene (3a), where a denotes the native state] were synthesized and studied via infrared spectroelectrochemistry (IR-SEC). IR-SEC revealed that the length of the bridging ligand and degree of electronic coupling in rhenium squares greatly impact the reduction pathway in both inert and CO2 atmospheres; notably, the squares are capable of interacting with CO2 after only ligand reduction, thereby avoiding the need for an additional reductive event to generate the rhenium anion. This mechanism differs from that of mononuclear complexes, highlighting how unique redox properties can be endowed by electronic coupling in supramolecular systems.

Coinage-metal chalcogenide clusters are widely studied for their attractive photoluminescence properties. Copper chalcogenides are especially promising, but are often confined to solid-state investigations due to their limited solution stability and the difficulty of synthesizing stable, well-defined clusters. Here, we investigate copper-sulfur clusters incorporating a small number of Cu atoms to elucidate fundamental atomic interactions, ground- and excited-state characteristics, and photophysical behavior in both solid and solution. We have synthesized the Cu6(4,6-dimethyl-2-mercaptopyrimidine)6 cluster in both neutral and charged states, Cu6 and Cu6-2H 2+, respectively, by selective ligand protonation. The molecular structures are determined using single-crystal X-ray diffraction, while Cu K-edge X-ray absorption spectroscopy is used to probe Cu electronic structure differences arising from the ligand modification. Steady-state and pump-probe optical spectroscopy is used to investigate photophysical properties, interpreted using density functional theory methods. Both clusters exhibit good stability in the solid state and in solution and show characteristic near-infrared emission with microsecond lifetimes. Overall, the Cu6S6 clusters display favorable charge-transfer characteristics and show potential for further use in driving photochemical transformations.

Cu(I)-bisdiimine complexes, [Cu(NN)2]+ (where NN represents a diimine such as phenanthroline or bipyridine derivatives), are considered promising photosensitizers for various photochemical applications. However, their effectiveness is subject to several key challenges. In particular, controlling steric strain around the copper(I) center by introducing appropriate substituents (R) at the α-position of the nitrogen atoms is crucial for optimizing the excited-state properties of the complex. In brief, increasing the size of R leads to longer emission lifetimes and higher quantum yields. Additionally, the energy of the singlet excited state rises with increasing steric bulk, enhancing photoinduced reactivity. However, excessive steric strain from bulky substituents can significantly destabilize the coordination sphere. To balance complex stability with increased steric bulk around the metal center, we have developed two novel nonsymmetrical ligands featuring branched alkyl chains and benzyl groups at the α-position of the nitrogen atoms. Our findings demonstrate that intramolecular π-stacking interactions between the benzyl group and the opposing phenanthroline ligand contribute to stabilizing the coordination sphere. Furthermore, the flexibility of the benzyl group reinforces the tetrahedral geometry around copper(I), resulting in an increased singlet excited-state energy compared to benchmark complexes. Notably, we show that this enhancement in excited-state energy translates into greater excited-state reactivity.

Metal-organic frameworks (MOFs) are an excellent platform for photochemical CO2 reduction into valuable chemicals. Herein, we report the synthesis and photocatalytic behavior of Ru@MOF-808, a Zr-based MOF that was post-synthetically modified with a Ru-polypyridyl complex. The post-synthetic modification was achieved using solvent-assisted incorporation of bipyridine-carboxylate ligands onto the nodes of the MOF-808, followed by the coordination of the Ru(II)-terpiridine moiety. A thorough characterization including 1H-NMR, diffuse reflectance UV/Vis spectroscopy, X-ray absorption spectroscopy and gas adsorption studies, combined with DFT calculations, provide strong support for efficient incorporation of the molecular Ru-complex at the loading of one Ru center per node. In the presence of a strong sacrificial reductant BIH(1,3-Dimethyl-2-phenylbenzimidazoline), Ru@MOF-808 was found to catalyze photochemical reduction of CO2 into a mixture of CO and formate ion. When compared to the homogeneous model catalyst Ru(tpy)(bpy)₂⁺, Ru@MOF-808 was found to exhibit higher formate yields. To explain these formate enhancements, we propose a mechanism that involves the CO2 capture at the MOF nodes to form Zr-bicarbonate species, which further react in a hydride transfer reaction with photo-generated Ru-H donor, thereby outperforms molecular catalyst in HCOO- production. Overall, the results presented in this work indicate the potential of Zr-based MOFs in integrating CO2 capture with its photochemical conversion to desired products.

The functionality of biomacromolecules, such as proteins and nucleic acids, is intricately linked to their three-dimensional (3D) structures, which are dictated by the 1D sequences of amino acids or nucleotides. The process by which a 1D sequence folds into a functional 3D structure has been a longstanding challenge in molecular biology. While significant progress has been made in understanding the intrinsic forces that govern this transformation, our comprehension remains limited regarding how external factors, including pH, temperature, metal ion coordination, and concentration, influence the formation of 3D structures through interactions with the macromolecular environment.

Understanding electron density migration along excited-state pathways in photochemical systems is critical for optimizing solar energy conversion processes. In this study, we investigate photoinduced electron transfer (PET) in a covalently linked donor–bridge–acceptor (D-B-A) system, where [Cu(I)-bis(1,10-phenanthroline)]+ acts as an electron donor, and anthraquinone, tethered to one of the phenanthroline ligands via a vibrationally active ethyne bridge, behaves as an electron acceptor. Visible transient absorption spectroscopy revealed the dynamic processes occurring in the excited state, including PET to the acceptor species. This was indicated by the spectral features of the anthraquinone radical anion that appeared on a timescale of 30 ps in polar solvents. Time-resolved infrared (TRIR) spectroscopy of the alkyne vibration (CC stretch) of the ethyne bridge provided insight into electronic structural changes in the metal-to-ligand charge transfer (MLCT) state and along the PET reaction coordinate. The observed spectral shift and enhanced transition dipole moment of the CC stretch demonstrated that there was already partial delocalization to the anthraquinone acceptor following MLCT excitation, verified by DFT calculations. An additional excited-state TRIR signal unrelated to the vibrational mode highlighted delocalization between the phenanthroline ligands in the MLCT state. This signal decayed and the CC stretch narrowed and shifted towards the ground-state frequency following PET, indicating a degree of localization onto the acceptor species. This study experimentally elucidates charge redistribution during PET in a Cu(I) diimine D-B-A system, yielding important information on the ligand design for optimizing PET reactions.

Understanding protein folding pathways is crucial to deciphering the principles of protein structure and function. Here, the unfolding dynamics of the 35-residue villin headpiece (HP35) and a norleucine-substituted variant (2F4K) using a combination of experimental and computational techniques is investigated. Time-resolved X-ray solution scattering coupled with equilibrium molecular dynamics simulations and Markov state modeling reveals distinct unfolding mechanisms between the two variants: HP35 and 2F4K. Specifically, HP35 exhibits a two-state unfolding process, whereas an intermediate state is identified for the 2F4K mutant. A Markov state model constructed from simulations is used to map atomic-level transitions to experimental observations, providing insights into the role of sequence variations in modulating folding pathways. The findings underscore the importance of integrating experimental and computational approaches to unravel protein unfolding mechanisms between heterogenous structural ensembles.

Inverted p-i-n structure perovskite solar cells (PSCs) have outperformed traditional n-i-p PSCs in recent years. A key advancement is the use of self-assembled monolayers (SAMs) as hole transport layers. One class of widely used SAMs is carbazole-based phosphonic acids. However, it is found that these SAMs lack strong binding with transparent conducting oxides (TCO) and perovskite. The weak binding strength results in suboptimal interfacial adhesion of the buried interface, which limits the device's stability. Here, interfacial binding is enhanced by increasing the dipole moment that creates a strong interfacial electric field that enhances electrostatic interactions at the TCO/perovskite interface, while incorporating tailored functional groups in SAMs to improve chemical anchoring to TCO and binding to perovskite. Specifically, the donor-acceptor SAM molecule 4-(7-(4-(bis(4-methoxyphenyl)amino)-2,5-difluorophenyl)benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (PAFTB) is employed, which features an enhanced dipole moment along with electron-donating and electron-withdrawing functional groups to optimize interfacial interactions. Compared to extensively used [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz), PAFTB enhances total interfacial adhesion by 2.8 times, thereby improving the thermal stability of the layer. Using this approach, PSCs are demonstrated with a certified quasi-steady-state power conversion efficiency of 24.9% and maintain 80% of the initial efficiency after 900 h of maximum power point tracking at 85 °C.

The sulfidation of metal oxides is critical to the creation of catalyst active sites for industrially-relevant reactions, but occurs at high temperatures for refractory oxides like zirconia. Here, we investigate sulfidation of the structurally-well-defined, Zr-based metal-organic framework (MOF) NU-1000 as a model system. Sulfidation with H2S and alkane thiols occurred readily at room temperature after thermal distortion of the Zr6 clusters in NU-1000. The sulfidation occurred via heterolytic S-H bond cleavage over frustrated Lewis-acid/Lewis-base pairs in the form of under-coordinated Zr(IV) sites and a terminal O(-II) site formed during the thermal distortion. In situ synchrotron based structural analysis, spectroscopic characterization, and computational studies provide evidence that thiols react with distorted Zr6 nodes to form stable bridging Zr-S-Zr bonds and a terminal hydroxo. This study provides insight into the surface structures responsible for the sulfidation of high-valent metal oxides and also suggests a promising strategy for introducing S-bearing moieties to otherwise sulfur-resistant MOF nodes.

Semiconductor nanomaterials offer a promising platform to produce optically addressable spins for use in quantum technologies. Here, we examine CsPbBr3 nanospheres, cubes, and rods spanning the zero-dimensional (0D) to three-dimensional (3D) transition to investigate the influence of dimensionality and shape on exciton spin decoherence. Using circularly polarized transient absorption spectroscopy, we find that the spin relaxation rate is independent of the surface to volume ratio and instead follows a dependence on the length of the shortest dimension. Additionally, differences in surface quality and termination appear to have no effect on the spin relaxation rate for measured materials, and instead the spin relaxation rate is most clearly correlated with the exciton binding energy. Finally, decreasing the dimensionality of the nanomaterials decreases the influence of multiexciton interactions on the spin relaxation rate.

The spin-vibronic effect (SVE) accelerates quantum-mechanically forbidden electronic transitions, but experimental manifestations of this phenomenon remain limited. In this contribution, the role of SVE in photoinduced electron transfer (PET) dynamics is probed by coherent vibrational wavepacket (CVWP) motions in Pt(II) dimer-naphthalene diimide donor-acceptor complexes. Metal-metal-to-ligand charge-transfer (MMLCT) excitation of the donor triggers ballistic PET, driving the molecule away from equilibrium. The SVE then directs the PET trajectory from the 1MMLCT excited state to an intermediate ligand-centered triplet state, modulating the CVWP dynamics along the reaction coordinates. Herzberg-Teller type oscillations indicate that the Pt-Pt stretching vibration, arising from the formation of the excited triplet intermediate, serves as the reaction coordinate for charge separation. Our experimental findings advance the understanding of the interplay between electronic, vibrational, and spin degrees of freedom in ultrafast photoactivated processes.

Metal-organic frameworks are an excellent platform for photochemical CO2 reduction into valuable chemicals. Herein, we report the synthesis and photocatalytic behavior of Ru@MOF-808, a Zr-based MOF, modified with a Ru-polypyridyl complex. The postsynthetic modification was achieved using solvent-assisted incorporation of bipyridine-carboxylate ligands onto the nodes of the MOF-808, followed by the coordination of Ru(II)-terpyridine moiety. A thorough characterization including 1H NMR, diffuse reflectance UV/vis, X-ray absorption spectroscopy and gas adsorption studies, combined with DFT calculations, provides strong support for efficient incorporation of the molecular Ru-complex at the loading of one Ru center per node. In the presence of a strong sacrificial reductant BIH, Ru@MOF-808 was found to catalyze the photochemical reduction of CO2 into a mixture of CO and formate ion. When compared to the homogeneous model catalyst Ru(tpy)(bpy)2+, Ru@MOF-808 was found to exhibit higher formate yields. To explain these formate enhancements, we propose a mechanism that involves CO2 capture at the MOF nodes to form Zr-bicarbonate species, which further react in a hydride transfer reaction with photogenerated Ru-H donor, thereby outperforming molecular catalysts in HCOO- production. Overall, the results presented in this work indicate the potential of Zr-based MOFs in integrating CO2 capture with its photochemical conversion to desired products.

The rational design of solar energy catalysts requires a mechanistic understanding of the ultrafast interactions with the solvent environment. We have designed a new Cu(I)–anthraquinone framework (CuEthyneAnQ) to serve as a model for studying hydrogen-bonding effects in charge accumulating photocatalysts. Herein, we report the ground and excited-state characterization of CuEthyneAnQ by electrochemical and ultrafast optical transient absorption (OTA) spectroscopy measurements. Significant stabilization of the AnQ-centered reductions due to hydrogen-bonding was observed by electrochemical measurements in protic solvent mixtures. Analysis of the excited-state photophysics with OTA reveals electron transfer occurring in tens of picoseconds after metal-to-ligand charge transfer excitation, resulting in the charge-separated state of Cu(II)EthyneAnQ·–. Charge recombination occurs in 4 ns in aprotic solvent and extends to 19 ns in protic solvent. In order to examine the influence of hydrogen-bonding on the electron-transfer dynamics, we performed OTA measurements on CuEthyneAnQ in varying aprotic:protic solvent mixtures. We observe three effects that depend on the concentration of the protic solvent: (1) after charge separation, a diffusion-limited hydrogen-bond forms with the reduced AnQ·–; (2) the slowdown in charge recombination with protic solvent addition is due to hydrogen-bond stabilization in accordance with Marcus theory; and (3) a spectral shift occurs in the charge-separated state due to an increasing number of hydrogen-bond interactions. Our results are supported by time-dependent density functional theory calculations with explicit solvent hydrogen-bonding interactions. These insights underscore the potential of Cu-based donor–acceptor complexes and mixed-solvent systems to offer valuable guidelines for the design of more efficient photocatalytic systems.