Inverse molecular design from first principles: Tailoring organic chromophore spectra for optoelectronic applications

The discovery of molecules with tailored optoelectronic properties, such as specific frequency and intensity of absorption or emission, is a major challenge in creating next-generation organic light-emitting diodes (OLEDs) and photovoltaics. This raises the following question: How can we predict a potential chemical structure from these properties? Approaches that attempt to tackle this inverse design problem include virtual screening, active machine learning, and genetic algorithms. However, these approaches rely on a molecular database or many electronic structure calculations, and significant computational savings could be achieved if there was prior knowledge of (i) whether the optoelectronic properties of a parent molecule could easily be improved and (ii) what morphing operations on a parent molecule could improve these properties. In this Perspective, we address both of these challenges from first principles. We first adapt the Thomas-Reiche-Kuhn sum rule to organic chromophores and show how this indicates how easily the absorption and emission of a molecule can be improved. We then show how by combining electronic structure theory and intensity borrowing perturbation theory we can predict whether or not the proposed morphing operations will achieve the desired spectral alteration, and thereby derive widely applicable design rules. We go on to provide proof-of-concept illustrations of this approach to optimizing the visible absorption of acenes and the emission of radical OLEDs. We believe that this approach can be integrated into genetic algorithms by biasing morphing operations in favor of those that are likely to be successful, leading to faster molecular discovery and greener chemistry

ColabFit Exchange: open-access datasets for data-driven interatomic potentials

Data-driven (DD) interatomic potentials (IPs) trained on large collections of first principles calculations are rapidly becoming essential tools in the fields of computational materials science and chemistry for performing atomic-scale simulations. Despite this, apart from a few notable exceptions, there is a distinct lack of well-organized, public datasets in common formats available for use with IP development. This deficiency precludes the research community from implementing widespread benchmarking, which is essential for gaining insight into model performance and transferability, while also limiting the development of more general, or even universal, IPs. To address this issue, we introduce the ColabFit Exchange, the first database providing open access to a large collection of systematically organized datasets from multiple domains that is especially designed for IP development. The ColabFit Exchange is publicly available at \url{https://colabfit.org/}, providing a web-based interface for exploring, downloading, and contributing datasets. Composed of data collected from the literature or provided by community researchers, the ColabFit Exchange consists of 106 datasets spanning nearly 70,000 unique chemistries, and is intended to continuously grow. In addition to outlining the software framework used for constructing and accessing the ColabFit Exchange, we also provide analyses of data, quantifying the diversity and proposing metrics for assessing the relative quality and atomic environment coverage of different datasets. Finally, we demonstrate an end-to-end IP development pipeline, utilizing datasets from the ColabFit Exchange, fitting tools from the KLIFF software package, and validation tests provided by the OpenKIM framework

Switching between coherent and incoherent singlet fission via solvent-induced symmetry-breaking.

Singlet fission in organic semiconductors causes a singlet exciton to decay into a pair of triplet excitons and holds potential for increasing the efficiency of photovoltaic devices. In this combined experimental and theoretical study, we reveal that a covalent dimer of the organic semiconductor tetracene undergoes activated singlet fission by qualitatively different mechanisms depending on the solvent environment. We show that intramolecular vibrations are an integral part of this mechanism, giving rise to mixing between charge transfer and triplet pair excitations. Both coherent or incoherent singlet fission can occur, depending on transient solvent-induced energetic proximity between the states, giving rise to complex variation of the singlet fission mechanism and timescale in the different environments. Our results suggest a more general principle for controlling the efficiency of photochemical reactions by utilizing transient interactions to tune the energetics of reactant and product states and switch between incoherent and coherent dynamics.This work was supported by the Engineering and Physical Sciences Research Council, UK (grant numbers EP/L015552/1, EP/M025330/1 and EP/M005143/1). A.M.A. acknowledges the support of the Winton Programme for the Physics of Sustainability. S.L. thanks A*STAR Graduate Scholarship support from A*STAR Singapore. T.J.H.H. acknowledges a Research Fellowship from Jesus College, Cambridge. E.G.F. acknowledges financial support from the National Science Foundation Award No. CHE-1555205. J. W. acknowledges financial support from the MOE Tier 3 programme (MOE2014-T3-1-004)

Correspondence Between Perceived Pubertal Development and Hormone Levels in 9-10 Year-Olds From the Adolescent Brain Cognitive Development Study.

Aim: To examine individual variability between perceived physical features and hormones of pubertal maturation in 9-10-year-old children as a function of sociodemographic characteristics. Methods: Cross-sectional metrics of puberty were utilized from the baseline assessment of the Adolescent Brain Cognitive Development (ABCD) Study—a multi-site sample of 9–10 year-olds (n = 11,875)—and included perceived physical features via the pubertal development scale (PDS) and child salivary hormone levels (dehydroepiandrosterone and testosterone in all, and estradiol in females). Multi-level models examined the relationships among sociodemographic measures, physical features, and hormone levels. A group factor analysis (GFA) was implemented to extract latent variables of pubertal maturation that integrated both measures of perceived physical features and hormone levels. Results: PDS summary scores indicated more males (70%) than females (31%) were prepubertal. Perceived physical features and hormone levels were significantly associated with child\u27s weight status and income, such that more mature scores were observed among children that were overweight/obese or from households with low-income. Results from the GFA identified two latent factors that described individual differences in pubertal maturation among both females and males, with factor 1 driven by higher hormone levels, and factor 2 driven by perceived physical maturation. The correspondence between latent factor 1 scores (hormones) and latent factor 2 scores (perceived physical maturation) revealed synchronous and asynchronous relationships between hormones and concomitant physical features in this large young adolescent sample. Conclusions: Sociodemographic measures were associated with both objective hormone and self-report physical measures of pubertal maturation in a large, diverse sample of 9-10 year-olds. The latent variables of pubertal maturation described a complex interplay between perceived physical changes and hormone levels that hallmark sexual maturation, which future studies can examine in relation to trajectories of brain maturation, risk/resilience to substance use, and other mental health outcomes

Quantum Mechanical Studies of Nonadiabatic Systems

Understanding nonadiabatic processes is tantamount to understanding the mechanisms underlying phenomena such as energy transfer in photovoltaic cells and catalysis at metal surfaces. A complete quantum description of such events is unfortunately intractable, but recent simulations show great promise in approximately but accurately modeling nonadiabatic systems. Perhaps the first step towards modeling these systems is the proper description of the relevant electronic states involved. We discuss two specific systems in which these states have been calculated, allowing for accurate dynamical simulations to be carried out. The first system described here is a model for intramolecular singlet fission in bipentacenes. Singlet fission, the process by which photoexcited singlet excitons spontaneously split into two lower energy triplet excitons, has received much attention as a promising avenue towards increasing solar cell efficiency. Recently, the blueprint for controlled synthesis of acene dimers has been utilized to create chromophores exhibiting efficient singlet fission, rivaling that of the best crystalline systems. Examining a specific dimer system (2,2-bipentacene), we show that intramolecular singlet fission proceeds nonadiabtically through an avoided crossing between single and multiexcitonic states. Subsequent dynamic calculations reveal singlet fission occurring on ultrafast timescales in agreement with experiment, supporting the proposed mechanism. Bipentacenes are not only interesting for their singlet fission capabilities, but also for their unusual spectral features in the visible region. Depending on bonding geometries, the spectrum of bipentacenes can be significantly altered (a second absorption peak appears in the visible) from that of the monomer. Despite first being observed in 1948, this spectral feature has not been properly described. We algebraically detail the origin of this spectral perturbation and give simple design principles for oligoacenes backed by intuitive molecular orbital arguments. Finally, we discuss a model for energy transfer between diatomics and metal surfaces. Despite being extensively studied experimentally, an adequate theoretical model, accurate across all experimental regimes, has not emerged. Exploiting the simplicity of a Schmidt decomposition of the single particle states of the Newns Hamiltonian, we show how intuitive local states can be constructed and utilized in dynamic calculations

Selected Columns of the Density Matrix in an Atomic Orbital Basis I: An Intrinsic and Non-iterative Orbital Localization Scheme for the Occupied Space

In this work, we extend the selected columns of the density matrix (SCDM) methodology [J. Chem. Theory Comput. 2015, 11, 1463–1469]a non-iterative and real-space procedure for generating localized occupied orbitals for condensed-phase systemsto the construction of local molecular orbitals (LMOs) in systems described using non-orthogonal atomic orbital (AO) basis sets. In particular, we introduce three different theoretical and algorithmic variants of SCDM (referred to as SCDM-M, SCDM-L, and SCDM-G) that can be used in conjunction with the AO basis sets used in standard quantum chemistry codebases. The SCDM-M and SCDM-L variants are based on a pivoted QR factorization of the Mulliken and Löwdin representations of the density matrix and are tantamount to selecting a well-conditioned set of projected atomic orbitals (PAOs) and projected (symmetrically-) orthogonalized atomic orbitals, respectively, as proto-LMOs that can be orthogonalized to exactly span the occupied space. The SCDM-G variant is based on a real-space (grid) representation of the wavefunction, and therefore has the added flexibility of considering a large number of grid points (or δ functions) when selecting a set of well-conditioned proto-LMOs. A detailed comparative analysis across molecular systems of varying size, dimensionality, and saturation level reveals that the LMOs generated by these three non-iterative/direct SCDM variants are robust, comparable in orbital locality to those produced with the iterative Boys or Pipek–Mezey (PM) localization schemes, and completely agnostic toward any single orbital locality metric. Although all three SCDM variants are based on the density matrix, we find that the character of the generated LMOs can differ significantly between SCDM-M, SCDM-L, and SCDM-G. In this regard, only the grid-based SCDM-G procedure (like PM) generates LMOs that qualitatively preserve σ–π symmetry (in systems such as s-trans alkenes), and are well-aligned with chemical (i.e., Lewis structure) intuition. While the direct and standalone use of SCDM-generated LMOs should suffice for most chemical applications, our findings also suggest that the use of these orbitals as an unbiased and cost-effective (initial) guess also has the potential to improve the convergence of iterative orbital localization schemes, in particular for large-scale and/or pathological molecular systems

Data Supporting "Switching between coherent and incoherent singlet fission via solvent-induced symmetry-breaking"

This data set contains all data necessary to generate the figures in the manuscript. Detailed information on how the data was obtained may be found in the open access manuscript which has been deposited in this repository. Figure 1 - structure and general photo physics of the DT-Mes molecule. Figure 2 - determination of the energy levels of DT-Mes and multiple emissive species. Figure 3 - Transient absorption spectroscopy and excited state evolution. Figure 4 - Transient absorption spectroscopy and varying TT/CT balance. Figure 5 - TT and CT action spectra. Figure 6 - switching between coherent and incoherent singlet fission Figure 7 - multidimensional plot of the surfaces participating in the last part of singlet fission. Figure 8 - summary of singlet fission mechanism

E-cigarette device and liquid characteristics and E-cigarette dependence: A pilot study of pod-based and disposable E-cigarette users.

BACKGROUND: E-cigarette device and liquid characteristics, such as electrical power output and liquid nicotine concentration, determine the rate at which nicotine is emitted from the e-cigarette (i.e., nicotine flux), and thus are likely to influence user nicotine dependence. We hypothesize that nicotine flux would be associated with the e-cigarette dependence scale (EDS) among pod-based and disposable e-cigarette products. METHODS: Data were obtained from online panel participants between 18 and 65 years of age, who had indicated that they were either former or current e-cigarette users and resided within the United States (N=1036). To be included in these analyses, participants had to provide information regarding device type (pod-based or disposable), power (watts), and nicotine concentration (mg/mL), from which we could determine nicotine flux (μg/s) (N=666). To assess the relationship between nicotine flux and EDS, a series of multivariable linear regressions were conducted. Each model was separated by device type and adjusted for by age and past 30-day e-cigarette use. RESULTS: Greater nicotine flux was associated with higher EDS scores among pod-based e-cigarette users (beta = 0.19, SE = 0.09, p-value = 0.043), but not users of disposable e-cigarettes. Neither power nor nicotine concentration were associated with EDS scores among users of either e-cigarette device type. CONCLUSION: Results support the hypothesis that nicotine flux is positively associated with nicotine dependence in a sample of current users of pod-based and disposable e-cigarettes

A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers

Interest in materials that undergo singlet fission (SF) has been catalyzed by the potential to exceed the Shockley–Queisser limit of solar power conversion efficiency. In conventional materials, the mechanism of SF is an intermolecular process (xSF), which is mediated by charge transfer (CT) states and depends sensitively on crystal packing or molecular collisions. In contrast, recently reported covalently coupled pentacenes yield ∼2 triplets per photon absorbed in individual molecules: the hallmark of intramolecular singlet fission (iSF). However, the mechanism of iSF is unclear. Here, using multireference electronic structure calculations and transient absorption spectroscopy, we establish that iSF can occur via a direct coupling mechanism that is independent of CT states. We show that a near-degeneracy in electronic state energies induced by vibronic coupling to intramolecular modes of the covalent dimer allows for strong mixing between the correlated triplet pair state and the local excitonic state, despite weak direct coupling

Tuning Singlet Fission in π‑Bridge‑π Chromophores

We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in these molecules, we find that homoconjugated dimers display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores, since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the nonconjugated dimer, where the iSF occurs with a time constant >10 ns, comparable to the fluorescence lifetime, we used electron spin resonance spectroscopy to unequivocally establish the formation of triplet–triplet multiexcitons and uncoupled triplet excitons through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF