Chance Lander

The reaction pathway for the thermal and photochemical oxygen dissociation from (TTP)Mo(η ² ‐O ₂ ) ₂ ( 1a ) to produce cis ‐dioxo (TTP)MoO ₂ ( 2a ) is probed experimentally and computationally. The reaction of 1a is initiated photochemically by absorption at the Soret band (445 nm), but no transient intermediate was detected in the microsecond time domain. The photochemical process for the 1a to monoperoxo intermediate (TTP)Mo(O ₂ ) (3a) conversion is addressed via time‐dependent density functional theory (TD‐DFT) and is suggested to occur via excitation of singlet reactant 1a to an excited state singlet with weakened Mo–O ₂ binding, causing O ₂ dissociation to the monoperoxo 3a . DFT studies of the thermal reaction from (POR)Mo(η ² ‐O ₂ ) ₂ ( 1b ) reveal that it dissociates O ₂ via a superoxo‐like transition state (TS) (POR)Mo(η ¹ ‐O ₂ )(η ² ‐O ₂ )*, leading to the monoperoxo complex ( 3b ). 3b , both in the singlet and triplet state, is suggested to convert to the cis ‐dioxomolybdenum(VI) product 2b by a novel electronic reorganization. On the singlet surface, the 3b / 2b conversion proceeds via an asynchronous concerted process with an unsymmetrical TS and an activation energy of 19.5 kcal/mol. On the triplet surface, the 3b/2b conversion proceeds through a symmetrical maximum with an approximate barrier of 16.4 kcal/mol, followed by spin crossover to reach the singlet dioxo product 2b .

ADLumin is a new class of turn‐on chemiluminescent probes that have shown increased luminescent intensity in the presence of amyloid beta () plaques, which are commonly associated with Alzheimer's disease (AD). ADLumin shares an imidazopyrazinone (IPO) moiety with the well‐known Cypridina luciferin, which reacts with O ₂ and produces bioluminescence. While the reaction mechanism for many other luciferins has been studied extensively, the chemiluminescent mechanism for ADLumin molecules has yet to be modeled computationally. In this work, we focus on the exploration of potential reaction mechanism for the dioxetanone formation in ADLumin‐5, which has shown stronger chemiluminescence in the presence of plaques than other ADLumin molecules. Our density functional theory calculations predict thermally accessible energy barriers for three separate reaction paths, which we refer to as A, B, and B′, respectively. These mechanisms differ in binding sites, all with comparable singlet‐triplet minimum energy crossing point (MECP) energies. While paths A and B are similar to those previously studied for the oxidation of Cypridina luciferin, our calculations revealed a bifurcating crossing point connecting path B and an alternative path B′. Overall, our results suggest that path B is the most energetically favorable among the three pathways for ADLumin‐5.

Simulated absorption spectra of 1D, 2D, and 3D Phthalocyanine (Pc)-based organic semiconductor molecular thin film using Frenkel model Hamiltonian with long-range Coulomb couplings, for the study of exciton transport in optoelectronic devices.

ADLumin is a new class of turn-on chemiluminescent probes that have shown increased luminescent intensity in the presence of amyloid beta (Aβ) plaques, which are commonly associated with Alzheimer’s disease (AD). ADLumin shares an imidazopyrazinone (IPO) moiety with the well known Cypridina luciferin, which reacts with O2 and produces bioluminescence. While the reaction mechanism for many other luciferins has been studied extensively, the chemiluminescent mechanism for ADLumin molecules has yet to be modeled computationally. In this work, we focus on the exploration of potential reaction mechanism for the dioxetanone formation in ADLumin-5, which has shown stronger chemiluminescence in the presence of Aβ plaques than other ADLumin molecules. Our density functional theory calculations predict thermally accessible energy barriers for three separate reaction paths, which we refer to as A, B, and B’, respectively. These mechanisms differ in O2 binding sites, all with comparable singlet-triplet minimum energy crossing point (MECP) energies. While paths A and B are similar to those previously studied for the oxidation of Cypridina luciferin, our calculations revealed a bifurcating crossing point connecting path B and an alternative path B’. Overall, our results suggest that path B is the most energetically favorable among the three pathways for ADLumin-5.

Proton-coupled electron transfer (PCET) reaction at the solid-liquid interface is fundamental to energy storage and conversion. Inner-sphere PCET reactions with equimolar amounts of protons and electrons are thermochemically equivalent to H-atom transfer (HAT) reactions; H-atoms bound to the surfaces of electrocatalysts are critical intermediates for reactions of H₂, O₂, and many others. Thus, the thermodynamics of H-atom binding to the electrocatalyst surface has long been considered the ‘catalytic descriptor’ that scales with the overall activity of these reactions, resulting in a volcano-shaped plot (the Sabatier Principle). These binding energies are computed on defect-free, crystalline surfaces, which minimally represent the complex surface structure of the experimentally employed catalysts. This is particularly pronounced for binary materials where each H-atom binding site can have distinct physical/chemical properties and dynamically evolve during the reaction. We describe our recent efforts in electrochemical determination of H-atom binding energy using redox-active, Ti- and Ce-based metal–organic frameworks (MOFs) as model systems. The metal-oxo nodes of these MOFs are structurally well-defined analogs of TiO₂ and CeO₂, two of the most common metal oxides in heterogeneous catalysis. Using cyclic voltammetry (CV) and open-circuit potential (E_OCP) measurements over a wide range of buffers and proton activity, we have successfully quantified the H-atom binding energy on the nodes; we refer to this thermochemical parameter as MO–H bond dissociation free energy (BDFE; M = Ti³⁺ or Ce³⁺). Density functional theory (DFT) calculations quantitatively corroborated the experimentally measured BDFE values using crystallographically determined node structures. Using the above approach, we have further extended this approach to lithium-coupled electron transfer (LCET) reactions, given its relevance to energy storage devices. Implications of these results regarding energy-relevant catalysis will be discussed.

Phthalocyanine (Pc)-based molecular thin films have emerged in recent years as a promising class of organic semiconductor materials for achieving a long exciton coherence length and fast exciton diffusion. However, the dependence of their exciton properties on the dimensionality and temper- ature of the Pc systems, existence of metal ions, and chemical modifications to the Pc molecule is not yet fully understood. As a first step towards a more comprehensive theoretical understand- ing of the excitonic properties of Pc thin films, we model the low-temperature exciton absorption spectra through employing the Frenkel Hamiltonian and incorporating quantum-chemistry-based site energies and exciton-exciton couplings. The predicted exciton absorption spectra of octabutoxy ph- thalocyanine (H2OBPc) was found to be strongly dependent on the dimensionality of the model as well as the distance cutoff for including the monomer-monomer exciton coupling. The best match to experimental low-temperature absorption spectra requires a 2D or 3D model and an inclusion of at least the nearest neighbor and second-nearest neighbor exciton couplings. We also caution that the widely used dipole-dipole coupling approximation could substantially overestimate the coupling of excitonic transition densities on different monomers.

Surface defect-induced photoluminescence blinking and photodarkening are ubiquitous in lead halide perovskite quantum dots. Despite efforts to stabilize the surface by chemically engineering ligand binding moieties, blinking accompanied by photodegradation still poses barriers to implementing perovskite quantum dots in quantum emitters. To date, ligand tail engineering in the solid state has rarely been explored for perovskite quantum dots. We posit that attractive intermolecular interactions between low-steric ligand tails, such as π-π stacking, can promote the formation of a nearly epitaxial ligand layer that significantly reduces the quantum dot surface energy. Here, we show that single CsPbBr₃ quantum dots covered by stacked phenethylammonium ligands exhibit nearly non-blinking single photon emission with high purity (~ 98%) and extraordinary photostability (12 hours continuous operation and saturated excitations), allowing the determination of size-dependent exciton radiative rates and emission line widths of CsPbBr₃ quantum dots at the single particle level.

Surface defect-induced photoluminescence blinking is ubiquitous in lead halide perovskite quantum dots (QDs). Despite efforts to passivate the defects on perovskite QDs by chemically engineering ligand binding moieties, blinking accompanied by photodegradation still poses barriers to studying and implementing quantum-confined perovskite QDs in quantum emitters. We posited that the intermolecular interaction between ligands can affect the QD surface passivation. In the solid state, steric repulsions among bulky ligand tails prevent adequate QD surface ligand coverage. Alternatively, attractive π-π stacking between low-steric phenethylammonium (PEA) ligands promotes the formation of a nearly epitaxial surface ligand layer. Here, we demonstrate that single CsPbBr₃ QDs covered by these PEA ligands are nearly non-blinking, with single photon purity reaching 98%. Moreover, these QDs exhibited no spectral shifting and photodegradations, and they remained blinking-free after 12 hours of continuous operation. Free of interferences from blinking and photodegradation, we present size-dependent exciton radiative rates and emission line widths of single CsPbBr₃ QDs ranging from strongly to weakly confined regimes.

In the last several years, there has been a surge in the development of machine learning potential (MLP) models for describing molecular systems. We are interested in a particular area of this field — the training of system‐specific MLPs for reactive systems — with the goal of using these MLPs to accelerate free energy simulations of chemical and enzyme reactions. To help new members in our labs become familiar with the basic techniques, we have put together a self‐guided Colab tutorial (https://cc-ats.github.io/mlp_tutorial/), which we expect to be also useful to other young researchers in the community. Our tutorial begins with the introduction of simple feedforward neural network (FNN) and kernel‐based (using Gaussian process regression, GPR) models by fitting the two‐dimensional Müller‐Brown potential. Subsequently, two simple descriptors are presented for extracting features of molecular systems: symmetry functions (including the ANI variant) and embedding neural networks (such as DeepPot‐SE). Lastly, these features will be fed into FNN and GPR models to reproduce the energies and forces for the molecular configurations in a Claisen rearrangement reaction.