We show herein that photoinduced charge transfer from CdSe quantum dots (QDs) to surface-bound methyl viologen (MV2+) acceptors is mediated by a vibronically coherent, nonadiabatic mechanism. Broadband multidimensional electronic spectra and an analysis of coherences show that a mixed QD-MV charge-transfer (CT) state is populated on the <50 fs time scale after optical preparation of the X3 (1P3/2-1Pe) state, well prior to the appearance of the one-electron photoreduced ground state (MV+•). A partial redistribution of charge from the core of the QD to the acceptor is revealed by excited-state coherences of an out-of-plane vibrational mode local to MV2+ and of a low-frequency mode mixing a MV2+ mode with the longitudinal optical (LO) phonon of the QD core. The ultrafast damping of these coherences indicates that excited-state wavepackets travel from the optically prepared, Franck-Condon structure through a conical intersection to reach the CT state. These results suggest that vibronically coherent processes generating CT intermediates can be exploited to improve the efficiency of QD-based solar cells and photocatalysts.

Chemical interface damping (CID) provides an important indicator of plasmon-induced direct charge transfer, but the role of molecular redox states in controlling this process remains poorly understood. Here, we investigate CID in individual methyl viologen (MV)-modified gold nanorods using single-particle hyper-spectroelectrochemistry. Correlated single-particle scattering measurements show a pronounced plasmon resonance energy (Eres) dependence and strong CID under reduction potentials, while both the Eres dependence and CID magnitude are weaker under oxidation potentials. Our model suggests that resonant charge transfer channels are energetically allowed for both MV redox states, but their distinct redox-dependent responses originate from differences in the broadening of adsorbate-derived interfacial states, which govern both the magnitude and the Eres dependence of CID. These results illustrate that electrochemical redox control can tune plasmon-induced direct charge transfer by modulating not only energetic alignment but also by broadening interfacial states resulting from Au-adsorbate coupling. This work highlights redox-dependent interfacial state engineering as a promising strategy for enhancing plasmon-induced direct charge transfer chemistry.

The Cr(III) coordination complex serves as an archetypical 3d transition metal system for probing ultrafast excited-state dynamics with spin conversion due to its intrinsic intersystem crossing (ISC) pathway, ^4T_2g→ ^2E_g, upon photoexcitation. Here we conduct ab initio molecular dynamics simulations in the ^4T_2g state of a model Cr(III) coordination complex, followed by analyses of multireference electronic structure properties. Across 50 trajectories, the compound retains a persistent Jahn-Teller distortion in the excited state, while exhibiting prominent symmetric metal-ligand bond stretching vibrations with frequencies of 219 cm^-1 and 465 cm^-1. State-averaged complete active space self-consistent field (SA-CASSCF) calculations obtain two corresponding normal modes at 225 cm^-1 and 487 cm^-1 with symmetric stretching character. The lower-frequency twisting/scissoring mode strongly modulates the ^4T_2g/^2E_g energy gap, periodically zeroing the energy gap, whereas spin-orbit coupling is essentially invariant to vibrational motion (≈ 60-80 cm^-1). Furthermore, calculations of single-point excited-state absorption from ^2E_g to a higher ligand-to-metal charge-transfer (LMCT) state indicate that the coherences previously observed in transient absorption spectra arise from nuclear motion on the ^2E_g surface. These results provide insights into the correlation between vibrational motion and electronic transitions, which can facilitate rational molecular design of transition metal complexes with desired excited-state properties by leveraging ligand versatility.

Ligand-to-metal charge-transfer (LMCT) excitation has emerged in recent years as a powerful modality in organic synthesis, namely for the generation of heteroatom-centered radicals through formal metal-ligand bond homolysis from the LMCT excited state. However, the exploitation of alternative LMCT excited state processes has been extremely limited. Here, we describe a general strategy for tuning the reaction course from LMCT excited states of titanium alkoxides. This reactivity paradigm has been exploited for tandem β-scission/Giese addition reactions of both scission-amenable and scission-recalcitrant alcohols under divergent reaction pathways of metal-ligand bond homolysis and excited state β-scission through judicious choice of electronically tuned Ti catalysts. Through intramolecular competition studies, catalyst-controlled scission is shown to facilitate a rate enhancement of up to 103-fold over the intrinsic scission of free alkoxyl radicals, highlighting the impact of accessing the excited state scission paradigm. Computations support the relevance of a scission-promoting LMCT excited state with stereoelectronically aligned alkoxyl radical cation character to enable direct, selective β-scission.

We present herein a multidimensional electronic spectroscopy study (2DES and 3DES) of vibronic coherences in CdSe quantum dots (QDs) showing that mid-frequency vibrations of the surface-capping oleate ligands promote hot-carrier cooling on the <50 fs time scale via a vibrationally coherent mechanism. Vibronic progressions in oscillation maps assigned to stimulated Raman coherences indicate that the LO phonon of the QD core is mixed with vibrations of the alkylcarboxylate moiety of the oleate ligands. Excited-state vibronic coherences, including a 375 cm-1 vibration assigned to a bending or wagging motion of the alkylcarboxylate (CCO) or carboxylate (OCO) group and a 126 cm-1 vibration assigned to a mixed, core-ligand mode, are rapidly damped on the same time scale as the nonradiative relaxation to the band edge and photoluminescence states. The results support the hypothesis that the rapidly damped vibrations serve as branching modes in a coherent nonadiabatic mechanism for hot-carrier cooling. The 375 cm-1 vibration may be acting as a tuning mode for the CIs along the relaxation pathway to the band-edge state because it modulates the π-electron donation properties of the alkylcarboxylate moiety of the oleate ligand.

Charge-transfer-to-solvent (CTTS) excitations provide a chemically central route to generating hydrated electrons and initiating redox chemistry in solution, yet the earliest stage of CTTS—the formation of the excited state itself—is usually treated as instantaneous. Here we present a time-domain perspective of how CTTS character builds up during core-level photoexcitation of an aqueous metal ion. Using time-dependent configuration interaction, we simulate the coherent evolution of a dense manifold of core-excited states and track the ultrafast flow of electronic charge from the initially localized site into solvent-supported final states. We find that the dynamics evolves from a few-state, oscillatory behavior to effectively irreversible delocalization, as the charge disperses among many coupled configurations, providing a microscopic mechanism for the early-time emergence of CTTS character. Our results offer a transparent real-time interpretation of what core-level spectroscopies and core-hole-clock-type measurements can probe in solutions, outlining experimental signatures for probing the build-up of CTTS states on the core-hole-lifetime timescale. Looking ahead, attosecond and sub-femtosecond X-ray pump–probe approaches at X-ray free-electron lasers provide a realistic route to directly time-resolve the core-excited CTTS wave packets in solution.

This Perspective is part of a Special Topic that explored the maturity of nonadiabatic molecular dynamics for predicting photochemical processes. In 2023, a prediction challenge was issued to the community of computational photochemists to simulate the photochemistry of cyclobutanone, photoexcited at 200 nm, and the resulting time-resolved mega-electronvolt ultrafast electron diffraction (MeV-UED) signal. The challenge attracted 15 theoretical predictions from more than 70 researchers, employing a wide range of strategies for electronic structure and nonadiabatic molecular dynamics to predict the time-resolved MeV-UED signal before the experiment had been conducted at SLAC (Stanford, USA). The MeV-UED instrument at Shanghai Jiao Tong University was also used to provide a second independent time-resolved MeV-UED signal for the photochemistry of cyclobutanone. This Perspective discusses the various approaches and strategies used by the participants to predict the photochemistry of cyclobutanone. This work also summarizes the strengths and weaknesses of various methods used for photoexcitation, electronic structure, nonadiabatic dynamics, and calculation of observables, as agreed by the participants during a CECAM workshop dedicated to the results of the challenge and organized in Lausanne in April 2025. This Perspective also collects all the predicted time-resolved MeV-UED signals into a single figure, together with the experimental signal. The challenge (i) demonstrated the qualitative predictive power of nonadiabatic molecular dynamics and (ii) underscored the impact of electronic-structure theory on the outcome of the excited-state dynamics and the need for its careful benchmarking. This effort allowed the community to share practical strategies to perform nonadiabatic dynamics (discussed in the present Perspective) and constitutes a "calibration" exercise for computational photochemistry.

Using the results from global modeling of the broadband two-dimensional electronic spectrum and an analysis of the concurrent damping of excited-state vibronic coherences, we show herein that a coherent nonadiabatic mechanism converts the electronic excitation energy of the X3 (1P3/2-1Pe) exciton state to ligand vibrations on the <50 fs time scale in oleate- and hexadecylamine-capped CdSe quantum dots. A comparison of the rates for the two ligands suggests that this process is promoted by mid-frequency vibrations of the ligands due to modulation of their electron-donating tendency. An intramolecular vibrational redistribution process then follows on the ∼200 fs time scale with both ligands, which accompanies thermalization in the band-edge X1 (1S3/2-1Se) state and dephasing of the spectator ligand vibrations. These findings suggest that charge-separated intermediates associated with photoinduced charge transfer or triplet-triplet excitation energy transfer will be produced with retention of phase coherence in the vibrations that modulate the charge-transfer character of surface-bound organic acceptors.

Time-dependent electronic structure methods provide an efficient, accurate, and robust alternative to traditional time-independent methods for computing both linear and nonlinear optical properties. With this in mind, we have developed the real-time Tamm-Dancoff approximation (RT-TDA). This approach models electron dynamics by propagating the linear-response time-dependent density functional theory (LR-TDDFT) amplitudes within the Tamm-Dancoff approximation (TDA) and adiabatic approximation. Because the electronic structure is propagated in real-time in a many-electron basis, RT-TDA overcomes known limitations of adiabatic Kohn-Sham RT-TDDFT for describing dynamics in intense fields. Acceleration by graphics processing units (GPUs) enables simulations of larger molecules and on longer time scales. To demonstrate the utility of our approach, we present the calculations of the linear absorption spectrum of a large organic molecule (120 heavy atoms), Rabi oscillations, and nonlinear 2-photon absorption, in which we observe the AC Stark effect.

The physico-chemical properties of 'bottom-up' carbon dots synthesized from small molecules feature both generalities, such as sp2-networked carbon and core-surface energy transfer, and heterogeneities, due to the unpredictable location of heteroatoms and often non-crystalline structure. Here we focus our review on three aspects of these systems: (1) coupling characterization with bottom-up synthesis to identify and remove confounding byproducts such as small molecules or hydrogen-rich polymers; (2) single-particle characterization to obtain unambiguous information on carbon dots and highlight the distribution of properties around the ensemble average; (3) electronic structure of carbon dots and how it can help elucidate the origin of important properties such as optical absorption and fluorescence from a heterogeneous ensemble of carbon dots.

Leveraging matrix sparsity has proven to be a fruitful strategy for accelerating quantum chemical calculations. Here, we present the hierarchical SOS-MP2 algorithm, which uses hierarchical matrix (H2) compression of the electron repulsion integral (ERI) tensor to reduce both time and space complexity. This approach is based on the atomic orbital Laplace transform MP2 calculations, leveraging the data-sparsity of the ERI tensor and the element-wise sparsity of the energy-weighted density matrices. The H2 representation approximates the ERI tensor in a block low-rank form, taking advantage of the inherent low-rank nature of the repulsion integrals between distant sets of atoms. The resulting algorithm enables the calculation of the Coulomb-like term of the MP2 energy with a theoretical time complexity of O(N2⁡log⁡N) and a space complexity of O(N2⁡log⁡N), where N denotes the number of basis functions. Numerical tests show asymptotic time and space complexities better than O(N2) for both linear alkanes and three-dimensional water clusters.

Experimental observations of vibronic coherences in electronically excited colloidal semiconductor nanocrystals offer a window into the ultrafast dynamics of hot carrier cooling. In previous work, we showed that, in amine-passivated quantum dots (QDs), these coherences arise during relaxation through a cascade of conical intersections between electronically excited states. Here, we demonstrate the generality of this framework by application to QDs with surface-bound carboxylate ligands. A model involving a similar cascade of conical intersections accurately reproduces the frequencies of vibronic coherences observed with broadband multidimensional spectroscopy. The impact of ligands on the relaxation dynamics is attributed to two distinct mechanisms involving either electronic or vibrational coupling between the core and ligands. Compared to the amine-passivated QDs studied previously, the electronic coupling mechanism is less prominent in carboxylate-passivated QDs. Furthermore, comparison of acetate and formate ligands reveals that truncating the ligand alkyl chains alters the relaxation behavior predicted by the model.

Carbon dots (CDs) are renowned for their bright and tunable photoluminescence (PL), stability, and biocompatibility, yet it remains challenging to link their heterogeneous structures to their spectroscopic properties. This study utilizes density functional theory (DFT) and time-dependent DFT (TD-DFT) to systematically investigate how the spectroscopic properties of complex CDs with multiple layers and multiple defects are determined by their structures and compositions. Calculations reveal that strongly oxidizing defects, such as carbonyl and carbonyl acetate, significantly redshift absorption and emission spectra. In contrast, less oxidizing defects, such as hydroxyl, behave as spectators with minimal impact on absorption and emission, except when they interact strongly with more oxidizing defects. We find that not only the excitation energy but also the excitation character itself is impacted by the presence of specific defects, and the pH-dependence of the spectroscopic properties can be attributed to their protonation state-dependent excitation character. We show that the twisting, sliding, and linker-mediated folding of surface-functionalized layers in CDs markedly alter excitation energies and characters, offering a molecular explanation for experimentally observed emission intermittency and polarization fluctuations. These insights provide strategies for optimizing CDs for various applications, including bioimaging, photocatalysis, and optoelectronic devices.

Electronic coherences can be leveraged to control molecular dynamics, but such control is limited by ultrafast decoherence driven by coupling between electronic excitations and molecular vibrations. With the goal of understanding and controlling electronic coherence in molecules, we introduce a first-principles approach that enables direct simulation of the creation and decay of electronic coherences in molecules. Simulations of long-lived experimentally-observed coherences created upon multiphoton excitation of thiophene reveal coherent electronic motions within a dense manifold of Rydberg states, enabled by their relatively parallel potential energy surfaces.

Carbon dots are remarkable nanomaterials with many applications, but the sources of their emission are still uncertain. Carbon dots exhibit complex behaviors such as excitation-dependent emission due to their heterogeneous composition and structure. Most studies have been carried out on the ensemble level, where sample heterogeneity remains hidden. Understanding the complex emission of carbon dots requires single-particle measurements. Here, we determined that for red-emitting carbon dots made from two bottom-up precursors, there is a significant population of dots with more than one emitting moiety. Polarization-resolved, single-dot emission microscopy revealed subpopulations of carbon dots based on their emission intensity and polarization. For the multichromophoric carbon dots, we found an average of about four emitters. Single-particle spectroscopy, acquired in parallel to the emission trajectories, and molecular dynamics simulations furthermore established that the countable chromophores in the carbon dots are chemically similar, considering the rather narrow room-temperature emission line width and the absence of significant spectral diffusion.

The Ehrenfest with collapse-to-a-block (TAB) molecular dynamics approach was recently introduced to allow accurate simulation of nonadiabatic dynamics on many electronic states. Previous benchmarking work has demonstrated it to be highly accurate for modelling dynamics in one-dimensional analytical models, but nonadiabatic dynamics often involves conical intersections, which are inherently two-dimensional. In this report, we assess the performance of TAB on two-dimensional models of cascades of conical intersections in dense manifolds of states. Several variants of TAB are considered, including TAB-w, which is based on the assumption of a Gaussian rather than exponential decay of the coherence, and TAB-DMS, which incorporates an efficient collapse procedure based on approximate eigenstates. Upon comparison to numerically exact quantum dynamics simulations, it is found that all TAB variants provide a suitable description of the dynamical passage through a cascade of conical intersections. The TAB-w approach is found to provide a somewhat more accurate description of population dynamics than the original TAB method, with final absolute population errors $ \leq 0.013 $ <= 0.013 in all cases. Even when only four approximate eigenstates are computed, the use of approximate eigenstates was found to introduce minimal additional error (absolute population error $ \leq 0.018 $ <= 0.018 in all models).

The ligands of metal nanoclusters can be used to control their properties and reactivity, but a framework guiding their use remains elusive. Hammett studies of Au-8(PPh3)(7)(2+) and Au-9(PPh3)(8)(3+) nanoclusters with para- and meta-methyl and -methoxy groups indicate that resonance effects, not inductive effects, yield quantitative shifts of the HOMO-LUMO transitions involving orbitals local to the cluster core. Individual ligand exchanges reveal that these shifts are caused by only four of seven ligands, inconsistent with inductive effects. Quantum chemical calculations predict no trend in Au atom charges with respect to Hammett parameter but do predict bond length trends expected for a resonance structure that includes the Au atoms. Computed orbitals show contributions from specific para-OMe oxygen lone pairs to the HOMO, indicating delocalization from the core to specific ligands. These results suggest that resonance structures could be drawn including Au and ligands, guiding efforts to modulate nanocluster electronic structure and energy transfer.

We introduce an interface between PySpawn, a simulation package to run ab initio multiple spawning (AIMS) nonadiabatic dynamics, and OpenMolcas, a software package to perform multiconfigurational perturbations theory (CASPT2) electronic structure calculations. Our interface allows us to exploit all the functionalities of the two codes: the modular and efficient Python implementation of the AIMS algorithm and the extensive analysis tools offered by PySpawn, with the cutting-edge implementation of CASPT2 equations in OpenMolcas, including the recently introduced analytical gradients and different flavors. Both are fully open-source and free of charge, making the following implementation unique in the current plethora of software for nonadiabatic dynamics. This represents an important step toward a wider application of AIMS-based nonadiabatic dynamics combined with high-accuracy excited-state calculations. The importance and the need for such an implementation are demonstrated by application to the ultrafast relaxation of fulvene from S1 to S0, which is drastically affected by the potential energy surface on which the nuclear wavepacket is propagated. Additionally, the decay is influenced by the CASPT2 flavor adopted, posing interesting questions in the choice of one over the other and opening the door to deeper studies on the effect of CASPT2 formulations in nonadiabatic dynamics.

Transient absorption spectroscopy (TAS) is among the most common ultrafast photochemical experiments, but its interpretation remains challenging. In this work, we present an efficient and robust method for simulating TAS signals from first principles. Excited-state absorption and stimulated emission (SE) signals are computed using time-dependent complete active space configuration interaction (TD-CASCI) simulations, leveraging the robustness of time-domain simulation to minimize electronic structure failure. We demonstrate our approach by simulating the TAS signal of 1′-hydroxy-2′-acetonapthone (HAN) from ab initio multiple spawning nonadiabatic molecular dynamics simulations. Our results are compared to gas-phase TAS data recorded from both jet-cooled (T ∼ 40 K) and hot (∼403 K) molecules via cavity-enhanced TAS (CE-TAS). Decomposition of the computed spectrum allows us to assign a rise in the SE signal to excited-state proton transfer and the ultimate decay of the signal to relaxation through a twisted conical intersection. The total cost of computing the observable signal (∼1700 graphics processing unit hours for ∼4 ns of electron dynamics) was markedly less than that of performing the ab initio multiple spawning calculations used to compute the underlying nonadiabatic dynamics.