Experimentally Delineating the Catalytic Effect of a Single Water Molecule in the Photochemical Rearrangement of the Phenylperoxy Radical to the Oxepin-2(5H)-one-5-yl Radical.

Catalysis plays a pivotal role in both chemistry and biology, primarily attributed to its ability to stabilize transition states and lower activation free energies, thereby accelerating reaction rates. While computational studies have contributed valuable mechanistic insights, there remains a scarcity of experimental investigations into transition states. In this work, we embark on an experimental exploration of the catalytic energy lowering associated with transition states in the photorearrangement of the phenylperoxy radical-water complex to the oxepin-2(5H)-one-5-yl radical. Employing matrix isolation spectroscopy, density functional theory, and post-HF computations, we scrutinize the (photo)catalytic impact of a single water molecule on the rearrangement. Our computations indicate that the barrier heights for the water-assisted unimolecular isomerization steps are approximately 2-3 kcal mol-1 lower compared to the uncatalyzed steps. This decrease directly coincides with the energy difference in the required wavelength during the transformation (Δλ = λ546 nm - λ579 nm ≡ 52.4-49.4 = 3.0 kcal mol-1), allowing us to elucidate the differential transition state energy in the photochemical rearrangement of the phenylperoxy radical catalyzed by a single water molecule. Our work highlights the important role of water catalysis and has, among others, implications for understanding the mechanism of organic reactions under atmospheric conditions.

Silicon Radical-Induced CH4 Dissociation for Uniform Graphene Coating on Silica Surface.

Due to the manufacturability of highly well-defined structures and wide-range versatility in its microstructure, SiO2 is an attractive template for synthesizing graphene frameworks with the desired pore structure. However, its intrinsic inertness constrains the graphene formation via methane chemical vapor deposition. This work overcomes this challenge by successfully achieving uniform graphene coating on a trimethylsilyl-modified SiO2 (denote TMS-MPS). Remarkably, the onset temperature for graphene growth dropped to 720 °C for the TMS-MPS, as compared to the 885 °C of the pristine SiO2. This is found to be mainly from the Si radicals formed from the decomposition of the surface TMS groups. Both experimental and computational results suggest a strong catalytic effect of the Si radicals on the CH4 dissociation. The surface engineering of SiO2 templates facilitates the synthesis of high-quality graphene sheets. As a result, the graphene-coated SiO2 composite exhibits a high electrical conductivity of 0.25 S cm-1. Moreover, the removal of the TMP-MPS template has released a graphene framework that replicates the parental TMS-MPS template on both micro- and nano- scales. This study provides tremendous insights into graphene growth chemistries as well as establishes a promising methodology for synthesizing graphene-based materials with pre-designed microstructures and porosity.

Modeling Excited States of Molecular Organic Aggregates for Optoelectronics.

Light-driven phenomena in organic molecular aggregates underpin several mechanisms relevant to optoelectronic applications. Modeling these processes is essential for aiding the design of new materials and optimizing optoelectronic devices. In this review, we cover the use of different atomistic models, excited-state dynamics, and transport approaches for understanding light-activated phenomena in molecular aggregates, including radiative and nonradiative decay pathways. We consider both intra- and intermolecular mechanisms and focus on the role of conical intersections as facilitators of internal conversion. We explore the use of the exciton models for Frenkel and charge transfer states and the electronic structure methods and algorithms commonly applied for excited-state dynamics. Throughout the review, we analyze the approximations employed for the simulation of internal conversion, intersystem crossing, and reverse intersystem crossing rates and analyze the molecular processes behind single fission, triplet-triplet annihilation, Dexter energy transfer, and Förster energy transfer.

Single-Atom Iridium on Hematite Photoanodes for Solar Water Splitting: Catalyst or Spectator?

Single-atom catalysts (SACs) on hematite photoanodes are efficient cocatalysts to boost photoelectrochemical performance. They feature high atom utilization, remarkable activity, and distinct active sites. However, the specific role of SACs on hematite photoanodes is not fully understood yet: Do SACs behave as a catalytic site or as a spectator? By combining spectroscopic experiments and computer simulations, we demonstrate that single-atom iridium (sIr) catalysts on hematite (α-Fe2O3/sIr) photoanodes act as a true catalyst by trapping holes from hematite and providing active sites for the water oxidation reaction. In situ transient absorption spectroscopy showed a reduced number of holes and shortened hole lifetime in the presence of sIr. This was particularly evident on the second timescale, indicative of fast hole transfer and depletion toward water oxidation. Intensity-modulated photocurrent spectroscopy evidenced a faster hole transfer at the α-Fe2O3/sIr/electrolyte interface compared to that at bare α-Fe2O3. Density functional theory calculations revealed the mechanism for water oxidation using sIr as a catalytic center to be the preferred pathway as it displayed a lower onset potential than the Fe sites. X-ray photoelectron spectroscopy demonstrated that sIr introduced a mid-gap of 4d state, key to the fast hole transfer and hole depletion. These combined results provide new insights into the processes controlling solar water oxidation and the role of SACs in enhancing the catalytic performance of semiconductors in photo-assisted reactions.

Photochemical Fingerprinting Is a Sensitive Probe for the Detection of Synthetic Cannabinoid Receptor Agonists; toward Robust Point-of-Care Detection.

With synthetic cannabinoid receptor agonist (SCRA) use still prevalent across Europe and structurally advanced generations emerging, it is imperative that drug detection methods advance in parallel. SCRAs are a chemically diverse and evolving group, which makes rapid detection challenging. We have previously shown that fluorescence spectral fingerprinting (FSF) has the potential to provide rapid assessment of SCRA presence directly from street material with minimal processing and in saliva. Enhancing the sensitivity and discriminatory ability of this approach has high potential to accelerate the delivery of a point-of-care technology that can be used confidently by a range of stakeholders, from medical to prison staff. We demonstrate that a range of structurally distinct SCRAs are photochemically active and give rise to distinct FSFs after irradiation. To explore this in detail, we have synthesized a model series of compounds which mimic specific structural features of AM-694. Our data show that FSFs are sensitive to chemically conservative changes, with evidence that this relates to shifts in the electronic structure and cross-conjugation. Crucially, we find that the photochemical degradation rate is sensitive to individual structures and gives rise to a specific major product, the mechanism and identification of which we elucidate through density-functional theory (DFT) and time-dependent DFT. We test the potential of our hybrid "photochemical fingerprinting" approach to discriminate SCRAs by demonstrating SCRA detection from a simulated smoking apparatus in saliva. Our study shows the potential of tracking photochemical reactivity via FSFs for enhanced discrimination of SCRAs, with successful integration into a portable device.

Multiconfigurational Calculations and Photodynamics Describe Norbornadiene Photochemistry.

Storing solar energy is a vital component of using renewable energy sources to meet the growing demands of the global energy economy. Molecular solar thermal (MOST) energy storage is a promising means to store solar energy with on-demand energy release. The light-induced isomerization reaction of norbornadiene (NBD) to quadricyclane (QC) is of great interest because of the generally high energy storage density (0.97 MJ kg-1) and long thermal reversion lifetime (t1/2,300K = 8346 years). However, the mechanistic details of the ultrafast excited-state [2 + 2]-cycloaddition are largely unknown due to the limitations of experimental techniques in resolving accurate excited-state molecular structures. We now present a full computational study on the excited-state deactivation mechanism of NBD and its dimethyl dicyano derivative (DMDCNBD) in the gas phase. Our multiconfigurational calculations and nonadiabatic molecular dynamics simulations have enumerated the possible pathways with 557 S2 trajectories of NBD for 500 fs and 492 S1 trajectories of DMDCNBD for 800 fs. The simulations predicted the S2 and S1 lifetimes of NBD (62 and 221 fs, respectively) and the S1 lifetime of DMDCNBD (190 fs). The predicted quantum yields of QC and DCQC are 10 and 43%, respectively. Our simulations also show the mechanisms of forming other possible reaction products and their quantum yields.

Performance of point charge embedding schemes for excited states in molecular organic crystals.

Modeling excited state processes in molecular crystals is relevant for several applications. A popular approach for studying excited state molecular crystals is to use cluster models embedded in point charges. In this paper, we compare the performance of several embedding models in predicting excited states and S1-S0 optical gaps for a set of crystals from the X23 molecular crystal database. The performance of atomic charges based on ground or excited states was examined for cluster models, Ewald embedding, and self-consistent approaches. We investigated the impact of various factors, such as the level of theory, basis sets, embedding models, and the level of localization of the excitation. We consider different levels of theory, including time-dependent density functional theory and Tamm-Dancoff approximation (TDA) (DFT functionals: ωB97X-D and PBE0), CC2, complete active space self-consistent field, and CASPT2. We also explore the impact of selection of the QM region, charge leakage, and level of theory for the description of different kinds of excited states. We implemented three schemes based on distance thresholds to overcome overpolarization and charge leakage in molecular crystals. Our findings are compared against experimental data, G0W0-BSE, periodic TDA, and optimally tuned screened range-separated functionals.

Understanding the solid state luminescence and piezochromic properties in polymorphs of an anthracene derivative.

Luminescent molecular crystals have gained significant research interest for optoelectronic applications. However, fully understanding their structural and electronic relationships in the condensed phase and under external stimuli remains a significant challenge. Here, piezochromism in the molecular crystal 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA) is studied using a combination of density functional theory (DFT) and time-dependent TD-DFT. We investigate the effects that molecular packing and geometry have on the electronic and phonon structure and the excited state properties in this archetypal system. We find that the luminescence properties are red-shifted with the transition from a herringbone to a sheet packing arrangement. An almost continuous red-shift in the band gap is found with the application of an external pressure through the enhancement of π-π and CH-π interactions, and is a mechanism in fine tuning an emissive response. The analysis of the phonon structure of the molecular crystal suggests restriction of motion in the herringbone packing arrangement, with motion restricted at higher pressure. This is supported by the Huang-Rhys factors which show a decrease in the reorganisation energy with the application of pressure. Ultimately, a balance between the decrease in reorganisation energies and the increase in exciton coupling will determine whether nonradiative decay is enhanced or decreased with the increase in pressure in these systems.

Competition between ultralong organic phosphorescence and thermally activated delayed fluorescence in dichloro derivatives of 9-benzoylcarbazole.

Optoelectronic materials based on metal-free organic molecules represent a promising alternative to traditional inorganic devices. Significant attention has been devoted to the development of the third generation of OLEDs which are based on the temperature-activated delayed fluorescence (TADF) mechanism. In the last few years, several materials displaying ultra-long organic phosphorescence (UOP) have been designed using strategies such as crystal engineering and halogen functionalisation. Both TADF and UOP are controlled by the population of triplet states and the energy gaps between the singlet and triplet manifolds. In this paper, we explore the competition between TADF and UOP in the molecular crystals of three dichloro derivatives of 9H-carbazol-3-yl(phenyl)methanone. We investigate the excited state mechanisms in solution and the crystalline phase and address the effects of exciton transport and temperature on the rates of direct and reverse intersystem crossing under the Marcus-Levich-Jortner model. We also analyse how the presence of isomeric impurities and the stabilisation of charge transfer states affect these processes. Our simulations explain the different mechanisms observed for the three derivatives and highlight the role of intramolecular rotation and crystal packing in determining the energy gaps. This work contributes to a better understanding of the connection between chemical and crystalline structures that will enable the design of efficient materials.

The carbon chain growth during the onset of CVD graphene formation on γ-Al2O3 is promoted by unsaturated CH2 ends.

Chemical vapor deposition of methane onto a template of alumina (Al2O3) nanoparticles is a prominent synthetic strategy of graphene meso-sponge, a new class of nano porous carbon materials consisting of single-layer graphene walls. However, the elementary steps controlling the early stages of graphene growth on Al2O3 surfaces are still not well understood. In this study, density functional theory calculations provide insights into the initial stages of graphene growth. We have modelled the mechanism of CH4 dissociation on the (111), (110), (100), and (001) γ-Al2O3 surfaces. Subsequently, we have considered the reaction pathway leading to the formation of a C6 ring. The γ-Al2O3(110) and γ-Al2O3(100) are both active for CH4 dissociation, but the (100) surface has higher catalytic activity towards the carbon growth reaction. The overall mechanism involves the formation of the reactive intermediate CH2* that then can couple to form CnH2n* (n = 2-6) intermediates with unsaturated CH2 ends. The formation of these species, which are not bound to the surface-active sites, promotes the sustained carbon growth in a nearly barrierless process. Also, the short distance between terminal carbon atoms leads to strong interactions, which might lead to the high activity between unsaturated CH2* of the hydrocarbon chain. Analysis of the electron localization and geometries of the carbon chains reveals the formation of C-Al-σ bonds with the chain growing towards the vacuum rather than C-Al-π bonds covering the γ-Al2O3(100) surface. This growth behaviour prevents catalyst poisoning during the initial stage of graphene nucleation.

Emission Quenching in Tetraphenylfuran Crystal: Why This Propeller-Shaped Molecule Does Not Emit in the Condensed Phase.

Due to their substantial fluorescence quantum yields in the crystalline phase, propeller-shaped molecules have recently gained significant attention as potential emissive materials for optoelectronic applications. For the family of cyclopentadiene derivatives, light-emission is highly dependent on the nature of heteroatomic substitutions. In this paper, we investigate excited state relaxation pathways in the tetraphenyl-furan molecule (TPF), which in contrast with other molecules in the family, shows emission quenching in the solid-state. For the singlet manifold, our calculations show nonradiative pathways associated with C-O elongation are blocked in both vacuum and the solid state. A fraction of the population can be transferred to the triplet manifold and, subsequently, to the ground state in both phases. This process is expected to be relatively slow due to the small spin-orbit couplings between the relevant singlet-triplet states. Emission quenching in crystalline TPF seems to be in line with more efficient exciton hopping rates. Our simulations help clarify the role of conical intersections, population of the triplet states and crystalline structure in the emissive response of propeller-shaped molecules.

Role of Conical Intersections on the Efficiency of Fluorescent Organic Molecular Crystals.

Organic molecular crystals are attractive materials for luminescent applications because of their promised tunability. However, the link between the chemical structure and emissive behavior is poorly understood because of the numerous interconnected factors which are at play in determining radiative and nonradiative behaviors at the solid-state level. In particular, the decay through conical intersection dominates the nonadiabatic regions of the potential energy surface, and thus, their accessibility is a telling indicator of the luminosity of the material. In this study, we investigate the radiative mechanism for five organic molecular crystals which display a solid-state emission, with a focus on the role of conical intersections in their photomechanisms. The objective is to situate the importance of the accessibility of conical intersections with regards to emissive behavior, taking into account other nonradiative decay channels, namely, vibrational decay, and exciton hopping. We begin by giving a brief overview of the structural patterns of the five systems within a larger pool of 13 crystals for a richer comparison. We observe that because of the prevalence of sheet like and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of a similar order within a 50 meV interval. Next, we focus on three exemplary cases, where we disentangle the role of nonradiative decay mechanisms and show how rotational minimum energy conical intersections in vacuum lead to puckered ones in the crystal, increasing their instability upon crystallization in typical packing motifs. In contrast, molecules with puckered conical intersections in vacuum tend to conserve this trait upon crystallization, and therefore, their quantum yield of fluorescence is determined predominantly by other nonradiative decay mechanisms.

Rotaxane CoII Complexes as Field-Induced Single-Ion Magnets.

Mechanically chelating ligands have untapped potential for the engineering of metal ion properties. Here we demonstrate this principle in the context of CoII -based single-ion magnets. Using multi-frequency EPR, susceptibility and magnetization measurements we found that these complexes show some of the highest zero field splittings reported for five-coordinate CoII complexes to date. The predictable coordination behaviour of the interlocked ligands allowed the magnetic properties of their CoII complexes to be evaluated computationally a priori and our combined experimental and theoretical approach enabled us to rationalize the observed trends. The predictable magnetic behaviour of the rotaxane CoII complexes demonstrates that interlocked ligands offer a new strategy to design metal complexes with interesting functionality.

fromage: A library for the study of molecular crystal excited states at the aggregate scale.

The study of photoexcitations in molecular aggregates faces the twofold problem of the increased computational cost associated with excited states and the complexity of the interactions among the constituent monomers. A mechanistic investigation of these processes requires the analysis of the intermolecular interactions, the effect of the environment, and 3D arrangements or crystal packing on the excited states. A considerable number of techniques have been tailored to navigate these obstacles; however, they are usually restricted to in-house codes and thus require a disproportionate effort to adopt by researchers approaching the field. Herein, we present the FRamewOrk for Molecular AGgregate Excitations (fromage), which implements a collection of such techniques in a Python library complemented with ready-to-use scripts. The program structure is presented and the principal features available to the user are described: geometrical analysis, exciton characterization, and a variety of ONIOM schemes. Each is illustrated by examples of diverse organic molecules in condensed phase settings. The program is available at https://github.com/Crespo-Otero-group/fromage.

Recent Advances and Perspectives on Nonadiabatic Mixed Quantum-Classical Dynamics.

Nonadiabatic mixed quantum-classical (NA-MQC) dynamics methods form a class of computational theoretical approaches in quantum chemistry tailored to investigate the time evolution of nonadiabatic phenomena in molecules and supramolecular assemblies. NA-MQC is characterized by a partition of the molecular system into two subsystems: one to be treated quantum mechanically (usually but not restricted to electrons) and another to be dealt with classically (nuclei). The two subsystems are connected through nonadiabatic couplings terms to enforce self-consistency. A local approximation underlies the classical subsystem, implying that direct dynamics can be simulated, without needing precomputed potential energy surfaces. The NA-MQC split allows reducing computational costs, enabling the treatment of realistic molecular systems in diverse fields. Starting from the three most well-established methods-mean-field Ehrenfest, trajectory surface hopping, and multiple spawning-this review focuses on the NA-MQC dynamics methods and programs developed in the last 10 years. It stresses the relations between approaches and their domains of application. The electronic structure methods most commonly used together with NA-MQC dynamics are reviewed as well. The accuracy and precision of NA-MQC simulations are critically discussed, and general guidelines to choose an adequate method for each application are delivered.

An alternative modular 'click-SNAr-click' approach to develop subcellular localised fluorescent probes to image mobile Zn2+ .

Zn2+ is involved in a number of biological processes and its wide-ranging roles at the subcellular level, especially in specific organelles, have not yet been fully established due to a lack of tools to image it effectively. We report a new and efficient modular double 'click' approach towards a range of sub-cellular localised probes for mobile zinc. Through this methodology, endoplasmic reticulum, mitochondria and lysosome localised probes were successfully prepared which show good fluorescence responses to mobile Zn2+in vitro and in cellulo whilst a non-targeting probe was synthesized as a control. The methodology appears to have wide-utility for the generation of sub-cellular localised probes by incorporating specific organelle targeting vectors for mobile Zn2+ imaging.

Switching the Spin State of Pentafluorophenylnitrene: Isolation of a Singlet Arylnitrene Complex.

The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in radical-type reactions and unwanted rearrangements, which diminish the use of arylnitrenes as intermediates in organic synthesis. While the closed-shell singlet states of arylnitrenes are expected to undergo useful chemical transformations (comparable to the closed-shell singlet states of carbenes), these states are too high in energy to be chemically accessible. When triplet pentafluorophenylnitrene is interacting with the Lewis acid BF3 under the conditions of matrix isolation, a Lewis acid-base complex consisting of the closed-shell singlet state of the nitrene and two molecules of BF3 is formed. Although the closed-shell singlet state of pentafluorophenylnitrene is calculated (CCSD(T)) to lie more than 25 kcal/mol above its triplet ground state, the reaction with BF3 results in switching the spin state from triplet to singlet. The formation of the singlet complex was monitored by IR, UV-vis, and EPR spectroscopy. DFT, CCSD(T), and CASPT2 calculations confirm the experimental findings.

Excited state proton transfer in 2'-hydroxychalcone derivatives.

Fluorophores exhibiting excited-state intramolecular proton transfer (ESIPT) are promising candidates for applications ranging from imaging and probing to laser dyes, optoelectronic devices and molecular logic gates. Recently, ESIPT-active solid-state emitters based on 2'-hydroxychalcone have been synthesized. The compounds are almost non-emissive in solution but emit in the deep red/NIR region when crystalline. Herein, we present a comprehensive theoretical investigation of the gas-phase excited state relaxation pathways in five 2'-hydroxychalcone systems, using a combination of static and non-adiabatic simulations. We identify two competing non-radiative relaxation channels, driven by intramolecular rotation in the enol and keto excited states. Both mechanisms are accessible for the five compounds studied and their relative population depends on the nature of the substituent. The addition of electron-donating substituents greatly increases the propensity of the ESIPT pathway versus rotation in the enol state. The identification of the fundamental relaxation mechanisms is the first step towards understanding the aggregated emission phonomena of these compounds.

Solution-Processable Carbon Nanoelectrodes for Single-Molecule Investigations.

Here we present a solution-based assembly method for producing molecular transport junctions employing metallic single-walled carbon nanotubes as nanoelectrodes. The molecular junction conductance of a series of oligophenyls was successfully measured, highlighting the potential of an all-carbon based approach for the fabrication of solution-processable single-molecule junctions for molecular electronics.

Surface Hopping Dynamics with DFT Excited States.

Nonadiabatic dynamics simulation of electronically-excited states has been a research area of fundamental importance, providing support for spectroscopy, explaining photoinduced processes, and predicting new phenomena in a variety of specialties, from basic physical-chemistry, through molecular biology, to materials engineering. The demands in the field, however, are quickly growing, and the development of surface hopping based on density functional theory (SH/DFT) has been a major advance in the field. In this contribution, the surface hopping approach, the methods for computation of excited states based on DFT, the connection between these methodologies, and their diverse implementations are reviewed. The shortcomings of the methods are critically addressed and a number of case studies from diverse fields are surveyed.