1,721,080 research outputs found
Structure and Electrical Bistability of a New Class of Diphenyl-bithiophenes: A Combined Theoretical and Experimental Study
No full textThe more stable conformers of recently synthesized diphenyl-bithiophene (DPBT) derivatives, a new class of relatively flexible conjugated molecules displaying electrical bistability, are investigated with the help of density functional theory calculations and by single crystal X-ray diffraction. The electronic structures of the neutral and positively charged species are computed for the most relevant isomeric forms along with intramolecular reorganization energies associated with charging. Two major mechanisms, charge injection at the interface and bulk charge transport, are considered to rationalize the observed electrical bistability and the efficiency of the electrical phenomenon for different DPBT species, in terms of computed molecular parameters. It is suggested that bistability is governed by an interplay of the two charge transport processes, with the OFF state being determined by activated hole injection
Multi-length-scale relationships between the polymer molecular structure and charge transport: the case of poly-naphthalene diimide bithiophene
No full textCharge transport in organic polymer semiconductors is a complex phenomenon affected by structural and electronic properties ranging over different length scales, from the molecular one up to the macro-scale. Charge carriers show markers of spatial localization (polarons), and drift for distances from a few to 100 mm in typical field-effect devices. Being sensitive to such different length scales, field-effect mobility is evidently a figure of merit that averages local properties at the molecular scale, over distances orders of magnitude larger. Understanding charge transfer processes at each length scale is consequently of paramount importance. To fulfill this aim, a multi-length-scale approach, encompassing experimental and theoretical modeling investigations, has to be built. Here we critically revise a series of experimental and theoretical tools that can contribute to develop a consistent multi-scale investigation methodology. We consider them within the study of an exemplary, good electron transporting naphthalene-diimide bi-thiophene copolymer, which has represented a breakthrough for the class of n-type polymers since its disclosure in 2009
A computational investigation on singlet and triplet exciton couplings in acene molecular crystals
No full textQuantum chemical calculations (DFT, TDDFT and ZINDO/S) of singlet and triplet exciton couplings are presented and discussed for some acene derivatives (such as anthracene, tetracene, 9,10-di(phenyl) anthracene and 9,10-bis(phenylethynyl) anthracene). An accurate excited state single molecule characterization has been carried out followed by an analysis of the inter-molecular excitonic interactions, taking place in the crystalline phase. These have been correlated to exciton coupling terms obtaining guidelines for the choice of molecular materials with large exciton couplings. Such organic systems are likely to show multiexciton processes such as singlet fission (SF) and triplet-triplet annihilation (TTA) which are useful in energy conversion phenomena to be exploited in photonic and optoelectronic devices
Modeling ultrafast exciton deactivation in oligothiophenes via nonadiabatic dynamics
No full textUltrafast excited-state processes play a key role in organic electronics and photovoltaics, governing the way of how excitons can relax and separate. Through the use of nonadiabatic excited-state dynamics, relaxation processes were investigated at the sub-picosecond timescale in thiophene and oligothiophenes (nT, n = 2, 3, 4), prototype oligomers for efficient pi-electron conjugated polymers adopted in photovoltaics. For thiophene, TDDFT and TDA nonadiabatic excited-state dynamics revealed ultrafast nonradiative relaxation processes through ring opening and ring puckering, bringing the system to an S-1/S-0 conical intersection seam. The computed relaxation time is 110 fs, matching well the experimental one (similar to 105 fs). In oligothiophenes (n = 2-4), high-energy (hot) excitations were considered. Exciton relaxation through the manifold of excited states to the lowest excited state is predicted to occur within similar to 150-200 fs, involving bond stretching, ring puckering, and torsional oscillations. For the longer oligomer (4T), the ultrafast relaxation process leads to exciton localization over three thiophene rings in 150 fs. These data agree with the self-localization mechanism (similar to 100-200 fs) observed for poly(3-hexylthiophene) (P3HT) and shed light on the complex exciton relaxation dynamics occurring in pi-conjugated oligomers of potential interest for optoelectronic applications
Quantum-Chemical Insights into the Prediction of Charge Transport Parameters for a Naphthalenetetracarboxydiimide-Based Copolymer with Enhanced Electron Mobility
No full textTheoretical modeling has been applied to study the charge transport (CT) parameters of a high-electron-mobility (n-type) naphthalenetetracarboxydiimide copolymer that was recently synthesized and tested for organic field-effect transistor applications. To understand the physicochemical characteristics of such a material, the intra- and intermolecular CT properties of holes and electrons were investigated using different DFT functionals, evidencing the need of range-separated hybrid functionals to predict key parameters such as the hole and electron reorganization energies. Our calculations revealed clear differences between hole- and electron-charging processes, providing fundamental elements for the rationalization of their transport
Addressing the Elusive Polaronic Nature of Multiple Redox States in a π-Conjugated Ladder-Type Polymer
Full text linkPoly(benzimidazole–benzophenanthroline) (BBL) is a ladder-type conjugated polymer showing remarkable charge transport properties. Upon doping it displays various conductive regimes, leading to two insulator-to-conductor transitions. Such transitions are never fully characterized, limiting understanding of its charged states. Open issues are: i) the electron/hole polaron relaxations, ii) the structure–function relationships of multiple redox states and their connection with the conductive regimes, and iii) the role of protonation. Such knowledge-gaps are tackled via a comprehensive computational investigation of multiple redox species. Polarons show polyradicaloid character, as revealed by combining broken-symmetry density functional theory, fragment orbital density, and multireference analysis. Electron/hole polaron relaxations occur on the polymer chain, the former localizing on the benzophenanthroline moieties, the latter on the benzimidazole units. Modeling of multiple charged species, up to one electron per repeat unit (1 eru), reveals a complex scenario of quasidegenerate states each featuring different spin multiplicity. Four redox states are responsible for the BBL insulator-to-conductor transitions. The two high conductive states refer to the electron polaron (0.25 eru) and the redox species with 0.75 eru. The insulating regimes refer to the bipolaron (0.50 eru) and the redox state with 1 eru. Protonation is modeled, revealing polaron-like features in the spectroscopic properties
Unveiling the Role of Hot Charge-Transfer States in Molecular Aggregates via Nonadiabatic Dynamics
No full textExciton dynamics governs energy transfer and charge generation in organic functional materials. We investigate high-energy nonadiabatic excited-state dynamics for a bithiophene dimer to describe time-dependent excitonic effects in molecular aggregates. We show that the lowest excited states are populated on the subpicosecond time scale. These states are localized and unproductive in terms of charge separation. Productive high-energy charge-transfer (CT) states are populated within 50 fs during exciton deactivation, but they are short-lived (similar to 100 fs) and quickly transfer their population to lower states. Our simulations offer molecular-level insights into ultrafast photoinduced charge separation potentially triggered by hot CT states in solid-state organic materials. Design rules are suggested to increase hot exciton lifetimes, favoring the population of CT states as gateways for direct charge generation. These rules may boost the CT quantum yield by depleting unproductive recombination channels
Atomistic Simulations of P(NDI2OD-T2) Morphologies: From Single Chain to Condensed Phases
Full text linkWe investigate theoretically the structure, crystallinity, and solubility of a high-mobility n-type semiconducting copolymer, P(NDI2OD-T2), and we propose a set of new force field parameters. The force field is reparametrized against density functional theory (DFT) calculations, with the aim to reproduce the correct torsional angles that govern the polymer chain flexibility and morphology. We simulate P(NDI2OD-T2) oligomers in different environments, namely, in vacuo, in the bulk phase, and in liquid toluene and chloronaphthalene solution. The choice of these solvents is motivated by the fact that they induce different kinds of molecular preaggregates during the casting procedures, resulting in variable device performances. Our results are in good agreement with the available experimental data; the polymer bulk structure, in which the chains are quite planar, is correcly reproduced, yet the isolated chains are flexible enough to fold in vacuo. We also calculate the solubility of P(NDI2OD-T2) in toluene and chloronaphthalene, predicting a much better solubility of the polymer in the latter, also in accordance to experimental observations. Different morphologies and dynamics of the oligomers in the two solvents have been observed. The proposed parameters make it possible to obtain the description of P(NDI2OD-T2) in different environments and can serve as a basis for extensive studies of this polymer semiconductor, such as, for example, the dynamics of aggregation in solvent
Modulation of the electronic structure of polyconjugated organic molecules by geometry relaxation: A discussion based on local Raman parameters
No full textThe Raman response of some polyconjugated materials is analyzed on the basis of local Raman parameters determined by Density Functional Theory calculations and discussed in the frame of the Effective Conjugation Coordinate Theory. This approach allows to explain Raman spectra even when a deviation from the common behaviour is observed, thus providing a key for the interpretation of the main spectroscopic features in terms of structural parameters. The examples reported demonstrate that any change of the parameter, describing the average degree of CC bond alternation along a conjugated sequence, is reflected by a remarkable modulation of the Raman spectrum, which indeed shows the signature of the molecular and electronic structure resulting from different chemical substitutions, conformation changes, chain length and environment effect
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