Annual Review of Physical Chemistry - Current Issue

This is a recollection of my scientific trajectory. When I look back, I consider myself to be very fortunate for being able to do something I love and on topics of my own will. I am not a competitive person and tend to shy away from the limelight. Nonetheless, I survived in my profession and eventually made some modest contributions, which are beyond what I would have expected. We often forget about the human aspect of scientific endeavor. After all, science is done by individuals; humans have emotions and make mistakes. The frustrations of failures, the joys of finding problems and solutions to them, and the passion for fulfilling curiosity are all parts of this endeavor. Throughout the years, many people—mentors, students, postdocs, collaborators, and colleagues—have accompanied me in this exciting and fruitful journey, for which I am deeply grateful and feel very lucky to have them.

The present autobiography recounts the author's education in the liberal arts, physics, and chemistry, and his participation in various developing stages of ab initio quantum chemistry from its beginning around 1950 to the present. His personal history is briefly noted.

Recent studies on ozone photodissociation in the Hartley and Huggins bands have provided new insights into the dissociation dynamics and product state distributions. A Λ-doublet propensity in the photodissociation has been identified through experiment and theory as the origin of the oscillatory O2(a¹Δg) rotational distributions and provides a promising diagnostic for determining the relative contributions of ³A′ and ³A″ states in Huggins band spin-forbidden processes. Recent experiments on spin-forbidden dissociation have provided detailed information about the vibrational and rotational distributions of the O2 products and the branching ratios between the O2 electronic states, serving as a motivation for high-level theory.

Ultrafast excitation of nanoparticles can excite the acoustic vibrational modes of the structure that correlate with the expansion coordinates. These modes are frequently seen in transient absorption experiments on metal nanoparticle samples and occasionally for semiconductors. The aim of this review is to give an overview of the physical chemistry of nanostructure acoustic vibrations. The issues discussed include the excitation mechanism, how to calculate the mode frequencies using continuum mechanics, and the factors that control vibrational damping. Recent results that demonstrate that the high frequencies inherent to the acoustic modes of nanomaterials trigger a viscoelastic response in surrounding liquids are also discussed, as well as vibrational coupling between nanostructures and mode hybridization within the nanostructures. Mode hybridization provides a way of manipulating the lifetimes of the acoustic modes, which is potentially useful for applications such as mass sensing.

Inspired by the success of graphene, two-dimensional (2D) materials have been at the forefront of advanced (opto-)nanoelectronics and energy-related fields owing to their exotic properties like sizable bandgaps, Dirac fermions, quantum spin Hall states, topological edge states, and ballistic charge carrier transport, which hold promise for various electronic device applications. Emerging main group elemental 2D materials, beyond graphene, are of particular interest due to their unique structural characteristics, ease of synthetic exploration, and superior property tunability. In this review, we present recent advances in atomic-scale studies of elemental 2D materials with an emphasis on synthetic strategies and structural properties. We also discuss the challenges and perspectives regarding the integration of elemental 2D materials into various heterostructures.

Investigating protein dynamic structural changes is fundamental for understanding protein function, drug discovery, and disease mechanisms. Traditional studies of protein dynamics often rely on investigations of purified systems, which fail to capture the complexity of the cellular environment. The intracellular milieu imposes distinct physicochemical constraints that affect macromolecular interactions and dynamics in ways not easily replicated in isolated experimental setups. We discuss the use of fluorescence resonance energy transfer, fluorescence anisotropy, and minimal photon flux imaging technologies to address these challenges and directly investigate protein conformational dynamics in mammalian cells. Key findings from the application of these techniques demonstrate their potential to reveal intricate details of protein conformational plasticity. By overcoming the limitations of traditional in vitro methods, these approaches offer a more accurate and comprehensive understanding of protein function and behavior within the complex environment of mammalian cells.

Plasmonic nanomaterials are promising photocatalysts due to their large optical cross sections and facile generation of nanoscale hotspot regions. They have been used to drive a range of photochemical reactions, including H2 dissociation, CO2 reduction, and ammonia synthesis, offering an exciting approach to light-driven chemistry. Deepening our understanding of how energy can be controllably transferred from the plasmonic nanomaterial to proximal reactants should lead to improvements in the efficiency and selectivity in plasmonic photocatalysis. Here we provide a comprehensive overview of plasmonic properties and explore different energy partitioning pathways. We focus on the importance of mapping molecular potential energy landscapes to understand reactivity and describe recent advancements in spectroscopic techniques, such as ultrafast surface-enhanced Raman spectroscopy, electron microscopy, and electrochemistry, that can aid in understanding how plasmonic nanomaterials can be used to shape potential energy surfaces and modify chemical outcomes. Additionally, we explore innovative hybrid plasmonic nanostructures such as antenna–reactor complexes, plasmonic single-atom catalysts, plasmonic picocavities, and chiral plasmonic substrates, all of which show great promise in advancing the field of plasmon-driven chemistry.

Reaction coordinates (RCs) are the few essential coordinates of a protein that control its functional processes, such as allostery, enzymatic reaction, and conformational change. They are critical for understanding protein function and provide optimal enhanced sampling of protein conformational changes and states. Since the pioneering work in the late 1990s, identifying the correct and objectively provable RCs has been a central topic in molecular biophysics and chemical physics. This review summarizes the major advances in identifying RCs over the past 25 years, focusing on methods aimed at finding RCs that meet the rigorous committor criterion, widely accepted as the true RCs. Notably, the newly developed physics-based energy flow theory and generalized work functional method provide a general and rigorous approach for identifying true RCs, revealing their physical nature as the optimal channels of energy flow in biomolecules.

In this review, we explore the electrostatic environment of the interface between a solid and dilute electrolyte solution, with an emphasis on the electric field profiles that these systems produce. We review the theoretical formalism that connects electrostatic potential profiles, electric field profiles, and charge density fields. This formalism has served as the basis for our understanding of interfacial electric fields and their influences on microscopic chemical and physical processes. Comparing various traditional models of interfacial electrostatics to the results of molecular dynamics (MD) simulation yields mutually inconsistent descriptions of the interfacial electric field profile. We present MD simulation results demonstrating that the average electric field profiles experienced by particles at the interface differ from the properties of traditional models and from the fields derived from the mean charge density of atomistic simulations. Furthermore, these experienced electric field profiles are species-dependent. Based on these results, we assert that a single unifying electrostatic potential profile—the gradient of which defines a single unifying electric field profile—cannot correctly predict the electrostatic forces that act on species at the interface.

Photoredox catalysis has emerged as a powerful platform for chemical synthesis, utilizing chromophore excited states as selective energy stores to surmount chemical activation barriers toward making desirable products. Developments in this field have pushed synthetic chemists to design and discover new photocatalysts with novel and impactful photoreactivity but also with uncharacterized excited states and only an approximate mechanistic understanding. This review highlights specific instances in which ultrafast spectroscopies dissected the photophysical and photochemical dynamics of new classes of photoredox catalysts and their photochemical reactions. After briefly introducing the photophysical processes and ultrafast spectroscopic methods central to this topic, the review describes selected recent examples that evoke distinct classes of photoredox catalysts with demonstrated synthetic utility and ultrafast spectroscopic characterization. This review cements the significant role of ultrafast spectroscopy in modern photocatalyzed organic transformations and institutionalizes the developing intersection of synthetic organic chemistry and physical chemistry.

With markedly different reaction conditions compared to the chemistry of the outside atmosphere, indoor air chemistry poses new challenges to the scientific community that require combined experimental and computational efforts. Here, we review molecular dynamics simulations that have contributed to the mechanistic understanding of the complex dynamics of organic compounds at indoor surfaces and their interplay with experiments and indoor air models. We highlight the rich interactions between volatile organic compounds and silica and titanium dioxide surfaces, serving as proxies for glasses and paints, as well as the dynamics of skin oil lipids and their oxidation products, which sensitively affect the quality of indoor air in crowded environments. As the studies we review here are pioneering in the rapidly emerging field of indoor chemistry, we provide suggestions for increasing the potentially important role that molecular simulations can continue to play.

No longer viewed as a passive consequence of cellular activities, membrane curvature—the physical shape of the cell membrane—is now recognized as an active constituent of biological processes. Nanoscale topographies on extracellular matrices or substrate surfaces impart well-defined membrane curvatures on the plasma membrane. This review examines biological events occurring at the nano-bio interface, the physical interface between the cell membrane and surface nanotopography, which activates intracellular signaling by recruiting curvature-sensing proteins. We encompass a wide range of biological processes at the nano-bio interface, including cell adhesion, endocytosis, glycocalyx redistribution, regulation of mechanosensitive ion channels, cell migration, and differentiation. Despite the diversity of processes, we call attention to the critical role of membrane curvature in each process. We particularly highlight studies that elucidate molecular mechanisms involving curvature-sensing proteins with the hope of providing comprehensive insights into this rapidly advancing area of research.

Vibrational spectroscopy and fluorescence spectroscopy have historically been two established but separate fields of molecular spectroscopy. While vibrational spectroscopy provides exquisite chemical information, fluorescence spectroscopy often offers orders of magnitude higher detection sensitivity. However, they each lack the advantages of each other. In recent years, a series of novel nonlinear optical spectroscopy studies have been developed that merge both spectroscopies into a single double-resonance process. These techniques combine the chemical specificity of Raman or infrared (IR) spectroscopy with the superb detection sensitivity and spatial resolution of fluorescence microscopy. Many facets have been explored, including Raman transition versus IR transition, time domain versus frequency domain, and spectroscopy versus microscopy. Notably, single-molecule vibrational spectroscopy has been achieved at room temperature without the need for plasmonics. Even superresolution vibrational imaging beyond the diffraction limit was demonstrated. This review summarizes the growing field of vibrational-encoded fluorescence microscopy, including key technical developments, emerging applications, and future prospects.

The C60 fullerene molecule has been the subject of intense study for four decades, starting with its identification in the mass spectra of carbon soot in 1985. In this review, we focus on the achievement of ultra-high-resolution spectroscopy of gas phase neutral C60, heralded by the observation of quantum state–resolved infrared spectra in 2019. C60 is now the largest and most symmetric molecule for which rovibrational quantum state resolution has been achieved, motivating the use of large molecules for studying complex quantum systems with symmetries and degrees of freedom not readily available in other composite systems. We discuss the theory, challenges, and experimental techniques of high-resolution C60 spectroscopy and recent experimental results probing the structure, dynamics, and interactions of C60 enabled by quantum state resolution.

The inversion of singlet and triplet states is a rare phenomenon, where, in opposition to Hund's first rule, singlet electronic states are stabilized relative to their triplet counterparts. The recent discovery of organic molecules exhibiting this inversion presents exciting new technological opportunities, such as addressing stability issues in organic light-emitting diodes (OLEDs). In this review, we describe fundamental molecular properties that can yield singlet-triplet inversion, generally ascribed to a phenomenon known as dynamic spin polarization. We discuss the systems in which singlet-triplet inversion was theoretically proposed, experimentally verified, and first implemented in an OLED device. We highlight key insights from the extensive computational work being carried out to understand the intricacies of these systems. Finally, we consider the outlook for future inverted singlet-triplet (IST) emitters.

The optical centrifuge was demonstrated in 2000 as a tool for preparing ensembles of molecules in extreme rotational states. Highly rotationally excited molecules, so-called superrotors, are observed as products of photodissociation and molecular collisions, in high-temperature environments in the atmospheres of Earth and exoplanets, and in the interstellar medium. Traditional optical excitation is limited to small changes in rotation, limiting experiments to relatively low rotational states. In this review, I discuss the use of a tunable optical centrifuge to prepare molecules in selected ranges of excited rotational states and investigations of their collisional relaxation using state-resolved polarization-sensitive transient IR probing. I examine the decay dynamics of population, alignment, and translational energy release, focusing on experimental results, and compare them with simulations that overestimate observed relaxation rates. A clear picture of near-resonant and nonresonant energy transfer pathways emerges and establishes the means to distinguish superrotor and bath collision products.

Molecular machines transduce free energy between different forms throughout all living organisms. Unlike their macroscopic counterparts, molecular machines are characterized by stochastic fluctuations, overdamped dynamics, and soft components, and operate far from thermodynamic equilibrium. In addition, information is a relevant free energy resource for molecular machines, leading to new modes of operation for nanoscale engines. Toward the objective of engineering synthetic nanomachines, an important goal is to understand how molecular machines transduce free energy to perform their functions in biological systems. In this review, we discuss the nonequilibrium thermodynamics of free energy transduction within molecular machines, with a focus on quantifying energy and information flows between their components. We review results from theory, modeling, and inference from experiments that shed light on the internal thermodynamics of molecular machines, and ultimately explore what we can learn from considering these interactions.

As a nonlinear optical process, sum frequency generation (SFG) requires noncentrosymmetry across multiple length scales, ranging from individual molecular functional groups to their arrangements in space. This principle makes SFG not only intrinsically sensitive to molecular species at surfaces but also useful for studying 3D structures of crystalline biopolymers in natural materials. Examples of such biopolymers are cellulose, starch, and chitin in the polysaccharide family and collagen, silk, and keratin in the fibrous protein family. These biopolymers are noncentrosymmetric at multiple length scales, with chirality at the molecular scale, unit cell structure at the nanoscale, and crystallite orientation and polarity at the mesoscale; thus, they are SFG active. In this review, we describe how SFG can be used to determine nano- to mesoscale polarity and orientational orders of crystalline biopolymers interspersed in natural materials containing the same or similar biopolymers in amorphous states, which cannot be obtained with other characterization methods.

Friction is a phenomenon that manifests across all spatial and temporal scales, from the molecular to the macroscopic scale. It describes the dissipation of energy from the motion of particles or abstract reaction coordinates and arises in the transition from a detailed molecular-level description to a simplified, coarse-grained model. It has long been understood that time-dependent (non-Markovian) friction effects are critical for describing the dynamics of many systems, but that they are notoriously difficult to evaluate for complex physical, chemical, and biological systems. In recent years, the development of advanced numerical friction extraction techniques and methods to simulate the generalized Langevin equation has enabled exploration of the role of time-dependent friction across all scales. We discuss recent applications of these friction extraction techniques and the growing understanding of the role of friction in complex equilibrium and nonequilibrium dynamic many-body systems.

In this review, we survey the use of extreme ultraviolet absorption spectroscopy to measure electronic and vibrational dynamics in transition metal complexes. Photons in this 30–100 eV energy range probe 3p → 3d transitions for 3d metals and 4f, 5p → 5d transitions in 5d metals, and the resulting spectra are sensitive to the spin state, oxidation state, and ligand field of the metal. Furthermore, the energy of the core level depends on the metal, providing elemental specificity. Use of tabletop high-harmonic sources allows these spectra to be measured with femtosecond to attosecond time resolution in a standard laser laboratory, revealing short-lived states in chromophores and photocatalysts that were unresolved using other techniques.