We demonstrate through theoretical, empirical, and sociocultural evidence that the concept of impulsivity fails the basic requirements of a psychological construct and should be rejected as such. ...Impulsivity (or impulsiveness) currently holds a central place in psychological theory, research, and clinical practice and is considered a multifaceted concept. However, impulsivity falls short of the theoretical specifications for hypothetical constructs by having meaning that is not compatible with psychometric, neuroscience, and clinical data. Psychometric findings indicate that impulsive traits and behaviors (e.g., response inhibition, delay discounting) are largely uncorrelated and fail to load onto a single, superordinate latent variable. Modern neuroscience has also failed to identify a specific and central neurobehavioral mechanism underlying impulsive behaviors and instead has found separate neurochemical systems and loci that contribute to a variety of impulsivity types. Clinically, these different impulsivity types show diverging and distinct pathways and processes relating to behavioral and psychosocial health. The predictive validity and sensitivity of impulsivity measures to pharmacological, behavioral, and cognitive interventions also vary based on the impulsivity type evaluated and clinical condition examined. Conflation of distinct personality and behavioral mechanisms under a single umbrella of impulsivity ultimately increases the likelihood of misunderstanding at a sociocultural level and facilitates misled hypothesizing and artificial inconsistencies for clinical translation. We strongly recommend that, based on this comprehensive evidence, psychological scientists and neuroscientists reject the language of impulsivity in favor of a specific focus on the several well-defined and empirically supported factors that impulsivity is purported to cover.
Molecules that undergo singlet fission, converting singlet excitons into pairs of triplet excitons, have potential as photovoltaic materials. The possible advantages of endothermic singlet fission ...(enhanced use of photon energy and larger triplet energies for coupling with common absorbers) motivated us to assess the role of exciton delocalization in the activation of this process. Here we report the synthesis of a series of linear perylene oligomers that undergo endothermic singlet fission and have endothermicities in the range 5-10 k
T at room temperature in solution. We study these compounds using transient spectroscopy and modelling to unravel the singlet and triplet dynamics. We show that the minimal number of coupled chromophores needed to undergo endothermic singlet fission is three, which provides sufficient statistical space for triplet excitons to separate and avoid annihilation-and a subsequent fast return to the singlet state. Our data additionally suggest that torsional motion of chromophores about the molecular axis following triplet-pair separation contributes to the increase in entropy, thus lengthening the triplet lifetime in longer oligomers.
Certain organic materials can generate more than one electron-hole pair per absorbed photon, a property that could revolutionize the prospects for solar energy. This process, called singlet fission, ...is one possible “exciton multiplication” scheme that could be useful in a variety of photovoltaic device designs from dye-sensitized solar cells to solar cell bilayers to bulk heterojunctions. For such applications to be possible, however, singlet fission must occur with near perfect efficiency in compounds that also have other requisite properties such as strong visible light absorption and photostability. Many recent investigations of singlet fission have focused on crystalline polyacenes, which have been known for some time to undergo singlet fission. While these materials have promise, limitations in stability, cost, and performance may hinder practical application of polyacene solar cells, while their complex photophysics may limit our fundamental understanding of singlet fission in crystalline polyacenes. In this Account, we describe rationally designed singlet fission chromophores whose excited state dynamics should be fairly simple and whose coupling can be well controlled through the formation of covalent dimers, aggregates, or polycrystalline films. In principle, investigations of these chromophores should provide the clearest connection to theoretical concepts explaining how an excited state evolves from a singlet (S1) into two triplets (TT). Realizing the promise of efficient singlet fission rests with two tasks: (i) producing an ideal molecular energy level structure and (ii) inducing the correct type and strength of chromophore coupling. In this Account, we offer theoretical guidance for achieving (i) and consider more extensively recent results aimed at (ii). For (i), theoretical guidance suggests that, in addition to alternant hydrocarbons like tetracene and pentacene, biradicals (i.e., molecules with two independent radical centers) may also be used as the basis for designing chromophores with low-lying triplet states such that the energy relationship 2E(T1) ≤ E(S1) is satisfied. Although molecules that do not fulfill this condition can also exhibit singlet fission from a higher lying or vibrationally excited singlet state, fast relaxation processes will likely reduce the singlet fission yield and complicate determination of the singlet fission mechanism. For (ii), once an appropriate chromophore has been chosen, the task of coupling two or more of them together must be done carefully. We discuss three pathways by which a dimer could undergo singlet fission: (1) A direct route in which slipped cofacial stacking is favorable under certain conditions. Cofacial stacking is common in molecular crystals, and it is likely not a coincidence that recent reports of efficient singlet fission involve slipped-stacked molecules in polycrystalline thin films. (2) A mediated route in which S1 interacts with (TT) through a virtual radical cation/anion state, which may be important in some situations. (3) A two-step route (i.e., through a real charge transfer intermediate) which others have suggested theoretically. We present data on 1,3-diphenylisobenzofuran (DPIBF) dimers that are consistent with this pathway. Finally, we review potential solar photoconversion efficiency gains utilizing singlet fission in several contexts.
Due to its ability to offset thermalization losses in photoharvesting systems, singlet fission has become a topic of research interest. During singlet fission, a high energy spin-singlet state in an ...organic semiconductor divides its energy to form two lower energy spin-triplet excitations on neighboring chromophores. While key insights into mechanisms leading to singlet fission have been gained recently, developing photostable compounds that undergo quantitative singlet fission remains a key challenge. In this report, we explore triplet exciton production via singlet fission in films of perylenediimides, a class of compounds with a long history of use as industrial dyes and pigments due to their photostability. As singlet fission necessitates electron transfer between neighboring molecules, its rate and yield depend sensitively on their local arrangement. By adding different functional groups at their imide positions, we control how perylenediimides pack in the solid state. We find inducing a long axis displacement of ∼3 Å between neighboring perylenediimides gives a maximal triplet production yield of 178% with a fission rate of ∼245 ps despite the presence of an activation barrier of ∼190 meV. These findings disagree with Marcus theory predictions for the optimal perylenediimide geometry for singlet fission, but do agree with Redfield theory calculations that allow singlet fission to occur via a charge transfer-mediated superexchange mechanism. Unfortunately, triplets produced by singlet fission are found to decay over tens of nanoseconds. Our results highlight that singlet fission materials must be designed to not only produce triplet excitons but to also facilitate their extraction.
The marriage of colloidal semiconductor nanocrystals and functional organic molecules has brought unique opportunities in emerging photonic and optoelectronic applications. Traditional semiconductor ...nanocrystals have been widely demonstrated to initiate efficient triplet energy transfer at the nanocrystal–acene interface. Herein, we report that unlike conventional semiconductor nanocrystals, lead halide perovskite nanocrystals promote an efficient Dexter-like singlet energy transfer to surface-anchored pentacene molecules rather than triplet energy transfer. Subsequently, molecular pentacene triplets are efficiently generated via singlet fission on the nanocrystal surface. Our demonstrated strategy not only unveils the obscure energy dynamics between perovskite nanocrystal and acenes, but also brings important perspectives of utilizing singlet fission throughout the solar spectrum.
Type three secretion is the mechanism of protein secretion found in bacterial flagella and injectisomes. At its centre is the export apparatus (EA), a complex of five membrane proteins through which ...secretion substrates pass the inner membrane. While the complex formed by four of the EA proteins has been well characterised structurally, little is known about the structure of the membrane domain of the largest subunit, FlhA in flagella, SctV in injectisomes. Furthermore, the biologically relevant nonameric assembly of FlhA/SctV has been infrequently observed and differences in conformation of the cytoplasmic portion of FlhA/SctV between open and closed states have been suggested to reflect secretion system specific differences. FlhA has been shown to bind to chaperone-substrate complexes in an open state, but in previous assembled ring structures, SctV is in a closed state. Here, we identify FlhA and SctV homologues that can be recombinantly produced in the oligomeric state and study them using cryo-electron microscopy. The structures of the cytoplasmic domains from both FlhA and SctV are in the open state and we observe a conserved interaction between a short stretch of residues at the N-terminus of the cytoplasmic domain, known as FlhA.sub.L /SctV.sub.L, with a groove on the adjacent protomer's cytoplasmic domain, which stabilises the nonameric ring assembly.
We present a cation-exchange approach for tunable A-site alloys of cesium (Cs+) and formamidinium (FA+) lead triiodide perovskite nanocrystals that enables the formation of compositions spanning the ...complete range of Cs1–x FA x PbI3, unlike thin-film alloys or the direct synthesis of alloyed perovskite nanocrystals. These materials show bright and finely tunable emission in the red and near-infrared range between 650 and 800 nm. The activation energy for the miscibility between Cs+ and FA+ is measured (∼0.65 eV) and is shown to be higher than reported for X-site exchange in lead halide perovskites. We use these alloyed colloidal perovskite quantum dots to fabricate photovoltaic devices. In addition to the expanded compositional range for Cs1–x FA x PbI3 materials, the quantum dot solar cells exhibit high open-circuit voltage (V OC) with a lower loss than the thin-film perovskite devices of similar compositions.
Analysis of photoluminescence (PL) from chemically treated lead sulfide (PbS) quantum dot (QD) films versus temperature reveals the effects of QD size and ligand binding on the motion of carriers ...between bright and dark trap states. For strongly coupled QDs, the PL exhibits temperature-dependent quenching and shifting consistent with charges residing in a shallow exponential tail of quasi-localized states below the band gap. The depth of the tail varies from 15 to 40 meV, similar to or smaller than exponential band tail widths measured for polycrystalline Si. The trap state distribution can be manipulated with QD size and surface treatment, and its characterization should provide a clearer picture of charge separation and percolation in disordered QD films than what currently exists.
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