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WOS:000360272500005; International audience; Singlet excitons in pi-stacked molecular crystals can split into two triplet excitons in a process called singlet fission that opens a route to carrier multiplication in photovoltaics. To resolve controversies about the mechanism of singlet fission, we have developed a first principles nonadiabatic quantum dynamical model that reveals the critical role of molecular stacking symmetry and provides a unified picture of coherent versus thermally activated singlet fission mechanisms in different acenes. The slip-stacked equilibrium packing structure of pentacene derivatives is found to enhance ultrafast singlet fission mediated by a coherent superexchange mechanism via higher-lying charge transfer states. By contrast, the electronic couplings for singlet fission strictly vanish at the C-2h symmetric equilibrium pi stacking of rubrene. In this case, singlet fission is driven by excitations of symmetry-breaking intermolecular vibrations, rationalizing the experimentally observed temperature dependence. Design rules for optimal singlet fission materials therefore need to account for the interplay of molecular pi-stacking symmetry and phonon-induced coherent or thermally activated mechanisms.

A united atom force field is empirically derived by minimizing the difference between experimental and simulated crystal cells and melting temperatures for eight compounds representative of organic electronic materials used in OLEDs and other devices: biphenyl, carbazole, fluorene, 9,9′-(1,3-phenylene)bis(9H-carbazole)-1,3-bis(N-carbazolyl)benzene (mCP), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (pCBP), phenazine, phenylcarbazole, and triphenylamine. The force field is verified against dispersion-corrected DFT calculations and shown to also successfully reproduce the crystal structure for two larger compounds employed as hosts in phosphorescent and thermally activated delayed fluorescence OLEDs: N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD), and 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBI). The good performances of the force field coupled to the large computational savings granted by the united atom approximation make it an ideal choice for the simulation of the morphology of emissive layers for OLED materials in crystalline or glassy phases. This work was supported by the Samsung Advanced Institute of Technology (SAIT)s Global Research Outreach (GRO) Program. The research in Bordeaux has been funded by French National Grant ANR-10-LABX-0042-AMADEus managed by the National Research Agency under the initiative of excellence IdEx Bordeaux programme (Reference ANR-10-IDEX-0003-02). The work in Mons was supported by the Programme d’Excellence de la Région Wallonne (OPTI2MAT Project) and FNRS-FRFC.

We present a Monte Carlo approach to estimate how molecular parameters impact hopping rates and charge mobilities in organic pi-conjugated materials. Our goal is to help in establishing structure-properties relationships. As a first step, our approach is illustrated by considering a model system made of a one-dimensional array of pentacene molecules; we describe the variations of the electron-transfer rates and of the resulting charge mobilities as a function of electric field and of the presence of molecular disorder and traps. The results highlight that there is no direct relationship between the degree of spatial overlap among adjacent molecules and charge mobility.

The supramolecular organization of the n-type P(NDI2OD-T2) polymer (also known as Polyera Activink N2200), featuring two branched octyldodecyl lateral chains, has been investigated by a combination of quantum-chemical and molecular mechanics techniques coupled to two-dimensional X-ray diffraction simulations. The structures exhibit non fully extended alkyl chains due to a compromise between the size of the monomer unit and the nature and length of the alkyl chains. Interestingly, the supramolecular organization of the polymer chains is only weakly affected by the nature of the stereogenic centers of the branched alkyl chains. The size and shape of the monomer unit and the chemical structure of the side chains appear to be the key elements governing the relationship between the supramolecular organization of P(NDI2OD-T2) polymer chains and their charge transport properties and are as important as the fine-tuning of the electronic properties of the molecular subunits in order to guarantee large electron (and hole) mobility. This study opens new perspectives for the rational design of new n-type and/or p-type polymers for further improvements of organic-based device efficiencies.

We investigate the cohesive energy of crystalline coronene by the dispersion-corrected methods DFT-D2, DFT-D3, and DFT-NL. For that purpose, we first employ bulk periodic boundary conditions and carefully analyze next all the interacting pairs of molecules within the crystalline structure. Our calculations reveal the nature and importance of the binding forces in every molecular pair tackled and provide revised estimates of the effects of two- and three-body terms, leading to accurate results in close agreement with experimental (sublimation enthalpies) reference values. Financial support by the “Ministerio de Economía y Competitividad” of Spain and the “European Regional Development Fund” through Project No. CTQ2011-27253. The work in Mons was supported by the “Programme d’Excellence de la Région Wallonne” (OPTI2MAT project) and FNRS-FRFC.

Two thienoacene dimers based on the thieno[3,2-b]thiophene moiety were efficiently synthesized, characterized and evaluated as active hole-transporting layers in organic thin-film field-effect transistors. Both compounds behaved as active p-channel organic semi-conductors showing averaged hole mobility of up to 1.33 cm2 V−1 s−1.

Charge dynamics in organic photovoltaic blends based on novel starshaped molecules are studied by ultrafast visible-IR spectroscopy. Pathways of intra-and intermolecular electron and hole transfer and their recombination are identified and discussed.

We theoretically characterize a series of substituted cycloparaphenylene nanohoops to study the effect of incorporating an electron‐withdrawing group into their cyclic structure. We systematically vary the nature, position, and number of nitrogen‐containing acceptor groups in both neutral (pyridine) and charged forms (pyridinium and methylpyridinium) to provide insights into how this functionalization affects the structural, electronic, and optical properties of these systems. We focus also on the singlet‐triplet energy difference, with low values found, which might pave the way to further applications in the field of devices for light‐emitting applications providing a potential class of TADF‐based emitters. Ministerio de Economía y Competitividad; European Regional Development Fund, Grant/Award Number: CTQ2014-55073-P; E2TP-CYTEMA-SANTANDER; French National Grant, Grant/Award Number: ANR-10-LABX-0042-AMADEus; National Research Agency (IdEx Bordeaux programme),Grant/Award Number: ANR-10-IDEX-0003-02; Programme d’Excellence de la Région Wallonne, Grant/Award Number: OPTI2MAT project; European Union Horizon 2020 research and innovation program, Grant/Award Number: 646176 (EXTMOS project); Consortium des Équipements de Calcul Intensif (CÉCI); Fonds de la Recherche Scientifique de Belgique (F.R.S.-FNRS), Grant/Award Number: 2.5020.11

The Seebeck coefficient of both electrons and holes in a family of ambipolar diketopyrrolopyrrole copolymers is investigated using field‐effect gated measurements. It is found that at high carrier concentrations the magnitude of the Seebeck coefficients is not strongly dependent on the molecular structure of the backbone within this family of polymers. Additionally, the Seebeck coefficients of electrons and holes have very similar magnitudes showing that electrons and holes experience a similar energetic landscape. Subtle differences in the carrier concentration dependence between the polymers are observed, which are interpreted in terms of different degrees of energetic disorder on the basis of molecular dynamics and electronic structure simulations.

Measurements and simulations of several high-mobility conjugated polymers show that their charge transport properties reflect an almost complete lack of disorder in the polymers, despite their amorphous microstructures, resulting from the resilience of the planar polymer backbone conformations to side-chain disorder. So-called 'conjugated polymers' have attracted much interest in recent decades. They are organic macromolecules with covalent-bond-containing backbone structures that combine the flexibility and processibility of plastics with the useful electronic properties of semiconductors. Polymeric materials tend to be naturally disordered however, and such disorder ultimately limits their electronic performance. Deepak Venkateshvaran and colleagues now show that several of the better-performing conjugated polymers are actually behaving electronically as if they were free of disorder, despite their amorphous microstructure. With the aid of simulations, the authors identify the molecular origins of this surprising 'disorder-free' behaviour, and offer guidelines for how this might be engineered into other conjugated polymeric systems. Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25 years, device performance has improved greatly as a wide variety of molecular structures have been studied1. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder2,3,4,5. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor–acceptor copolymer with a near-amorphous microstructure6, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for ‘disorder-free’ conjugated polymers.

The structural organization of three different families of semicrystalline π-conjugated polymers is reported (poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-alkyl-4H-cyclopenta-[2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](cyclopentadithiophene-benzothiadiazole) (CDT-BTZ) and poly(N,N"-bis-2-octyldodecylnaphtalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5–2,2-bithiophene (P(NDI2OD-T2))). These have triggered significant interest for their remarkable charge-transport properties. By performing molecular mechanics/dynamics simulations with carefully re-parameterized force fields, it is illustrated in particular how the supramolecular organization of these conjugated polymers is driven by an interplay between the length and nature of the conjugated monomer unit and the packing of their alkyl side chains, and to what extent it impacts the charge-carrier mobility, as monitored by quantum-chemical calculations of the intermolecular hopping transfer integrals. This Progress Report is concluded by providing generic guidelines for the design of materials with enhanced degrees of supramolecular organization.

Triplet exciton migration in the anthracene single crystal is modeled using a kinetic Monte Carlo scheme based on fully atomistic hopping rates. These account for the coupling of the triplet excitations to both intramolecular vibrations and lattice phonons from first principles. Thermal fluctuations in site energies and excitonic couplings obtained within the fragmented excitation difference approach yield triplet exciton diffusion constants in quantitative agreement with experiment.

Quantum-chemical calculations are combined to a model Frenkel-Holstein Hamiltonian to assess the nature of the lowest electronic excitations in the pentacene crystal. We show that an admixture of charge-transfer excitations into the lowest singlet excited states form the origin of the Davydov splitting and mediate instantaneous singlet exciton fission by direct optical excitation of coherently coupled single and double exciton states, in agreement with recent experiments.

International audience; Polyphenols (synthetically modified or directly provided by human diet) scavenge free radicals by H-atom transfer and may thus decrease noxious effects due to oxidative stress. Free radical scavenging by polyphenols has been widely theoretically studied from the thermodynamic point of view whereas the kinetic point of view has been much less addressed. The present study describes kinetic-based structure-activity relationship for quercetin. This compound is very characteristic of the wide flavonoid subclass of polyphenols. H-atom transfer is a mechanism based on either atom or electron transfer. This is analyzed here by quantum chemical calculations, which support the knowledge acquired from experimental studies. The competition between the different processes is discussed in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom- and electron-transfer efficiency. The role of the catechol moiety and the 3-OH group of quercetin as scavengers of different types of free radicals (CH3OO(*), CH3O(*), (*)OH, and (*)CH2OH) is rationalized. Identifying the exact mechanism and accurately evaluating kinetics is of fundamental importance to understand antioxidant behavior in physiological environments.

Prediction of cohesive energy of crystals is of particular interest in order to understand crystal growth mechanisms for further molecular engineering. In this study, we have aimed at assessing the accuracy of dispersion-corrected calculations (DFT-D2, DFT-D3, and DFT-NL) in reproducing the experimental cohesive energy of the anthracene crystal. Preliminary comparison of the interaction energies calculated at revPBE(0)-D3 and revPBE(0)-NL levels in isolated dimers (taken from the crystalline structure) with benchmark calculations performed at the SCS-MP2 and LPNO-pCCSD1a level enlightens the reliability of these DFT-based methods for which the best accuracy achieved is within 1-2 kJ/mol of the ab initio methods. Interestingly, the evaluation of the cohesive energy reveals that 35%-37% of this energy come from the consideration of a second coordination shell. Three-body interaction energy correction is calculated for revPBE-D3 functional and happens to reduce the cohesive energy of an anthracene nanoaggregate by 7 kJ/mol, while similar results are obtained with revPBE(0)-NL functional. In the end, dispersion-corrected estimates of the cohesive energy show sufficiently good agreement with experiment.

The extraordinary semiconducting properties of conjugated organic materials continue to attract attention across disciplines including materials science, engineering, chemistry, and physics, particularly with application to organic electronics. Such materials are used as active components in light-emitting diodes, field-effect transistors, or photovoltaic cells, as a substitute for (mostly Si-based) inorganic semiconducting materials. Many strategies developed for inorganic semiconductor device building (doping, p-n junctions, etc.) have been attempted, often successfully, with organics, even though the key electronic and photophysical properties of organic thin films are fundamentally different from those of their bulk inorganic counterparts. In particular, organic materials consist of individual units (molecules or conjugated segments) that are coupled by weak intermolecular forces. The flexibility of organic synthesis has allowed the development of more efficient opto-electronic devices including impressive improvements in quantum yields for charge generation in organic solar cells and in light emission in electroluminescent displays. Nonetheless, a number of fundamental questions regarding the working principles of these devices remain that preclude their full optimization. For example, the role of intermolecular interactions in driving the geometric and electronic structures of solid-state conjugated materials, though ubiquitous in organic electronic devices, has long been overlooked, especially when it comes to these interfaces with other (in)organic materials or metals. Because they are soft and in most cases disordered, conjugated organic materials support localized electrons or holes associated with local geometric distortions, also known as polarons, as primary charge carriers. The spatial localization of excess charges in organics together with low dielectric constant (ε) entails very large electrostatic effects. It is therefore not obvious how these strongly interacting electron-hole pairs can potentially escape from their Coulomb well, a process that is at the heart of photoconversion or molecular doping. Yet they do, with near-quantitative yield in some cases. Limited screening by the low dielectric medium in organic materials leads to subtle static and dynamic electronic polarization effects that strongly impact the energy landscape for charges, which offers a rationale for this apparent inconsistency. In this Account, we use different theoretical approaches to predict the energy landscape of charge carriers at the molecular level and review a few case studies highlighting the role of electrostatic interactions in conjugated organic molecules. We describe the pros and cons of different theoretical approaches that provide access to the energy landscape defining the motion of charge carriers. We illustrate the applications of these approaches through selected examples involving OFETs, OLEDs, and solar cells. The three selected examples collectively show that energetic disorder governs device performances and highlights the relevance of theoretical tools to probe energy landscapes in molecular assemblies.

A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm(-1), far smaller than the measured value of 631 cm(-1) and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm(-1) and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.

The synthesis of four new TTF derivatives bearing phthalimides and fluorinated alkyl moieties as potential ambipolar semiconductors is described. The presence of such electron-withdrawing groups permits the stabilization of the energy of HOMO and LUMO orbitals. The solid-state structures of these novel molecules have been characterized by X-ray diffraction techniques. The potential of these materials as hole and electron conductors has been estimated under theoretical considerations by evaluating the position of the frontier energy levels as well as their charge carrier mobilities. Preparation of solution-processed single crystal organic field-effect transistors (OFETs) has resulted in hole mobilities of up to 0.33 cm2 V-1 s -1 for compound 1. On the other hand, electrical time of flight (EToF) measurements on single crystals of compound 3 demonstrated ambipolar transport, reaching very high mobility values around 2.0 cm2 V -1 s-1 for both types of charges. © 2010 American Chemical Society., The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 212311 of the ONE-P project, Marie Curie EST FuMASSEC, DGI, Spain (contracts CTQ2006-06333/BQU and CTQ2010-195011/BQU), the Generalitat de Catalunya, (2009SGR00516) and the program “Juan de la Cierva”(MICINN). We also thank Stefan T. Bromley for his advice regarding the DFT calculations and CESGA for the use of their computational resources. The work in Mons is also supported by the European ONE-P project as well as by the Belgian National Fund for Scientific Research (FNRS). Y.O. and J.C. are FNRS research fellows.

A series of new tetrathiafulvalene (TTF) derivatives bearing dimethoxycarbonyl and phenyl or phthalimidyl groups fused to the TTF core (6 and 15-18) has been synthesized as potential soluble semiconductor materials for organic field-effect transistors (OFETs). The electron-withdrawing substituents lower the energy of the HOMO and LUMO levels and increase the solubility and stability of the semiconducting material. Crystal structures of all new TTF derivatives are also described, and theoretical DFT calculations were carried out to study the potential of the crystals to be used in OFET. In the experimental study, the best performing device exhibited a hole mobility up to 7.5 × 10-3 cm2 V-1 s-1). © 2010 American Chemical Society., EU by the EC FP7 ONE-P large-scale project (no. 212311), Marie Curie EST FuMASSEC, DGI, Spain (Contract Nos.CTQ2006-06333/BQU and CTQ2010-19501/BQU), Generalitat de Catalunya, (2009SGR00516), and the programme “Juan de la Cierva”(MICINN). We also thank Stefan T. Bromley for his advice regarding the DFT calculations and CESGA for the use of their installations. The work in Mons is also supported by the European ONE-P project as well as by the Belgian National Fund for Scientific Research (FNRS). Y.O. and J.C. are FNRS research fellows.

Intramolecular structure and intermolecular packing in crystalline molecular semiconductors should have profound effects on the charge-carrier wave function, but simple drift mobility measurements are not very sensitive to this. Here we show that differences in the Hall resistance of two soluble pentacene derivatives can be explained with different degrees of carrier delocalization being limited by thermal lattice fluctuations. A combination of Hall measurements, optical spectroscopy, and theoretical simulations provides a powerful probe of structure-property relationships at a molecular level.

A great deal of interest has recently focused on host-guest systems consisting of one-dimensional collinear arrays of conjugated molecules encapsulated in the channels of organic or inorganic matrices. Such architectures allow for controlled charge and energy migration processes between the interacting guest molecules and are thus attractive in the field of organic electronics. In this context, we characterize here at a quantum-chemical level the molecular parameters governing charge transport in the hopping regime in 1D arrays built with different types of molecules. We investigate the influence of several parameters (such as the symmetry of the molecule, the presence of terminal substituents, and the molecular size) and define on that basis the molecular features required to maximize the charge carrier mobility within the channels. In particular, we demonstrate that a strong localization of the molecular orbitals in push-pull compounds is generally detrimental to the charge transport properties.

Charge carrier mobility is the key quantity to characterize the charge transport properties in devices. Based on earlier work of Bassler and co‐workers, we set up a Monte‐Carlo approach that allows us to calculate mobility using transfer rates derived from Marcus theory. The parameters entering into the rate expression are evaluated by means of different quantum‐chemical techniques. Our approach is applied here to a model one‐dimensional system made of pentacene molecules as well as to real systems such as crystalline structures and columnar liquid crystal phases.

The quest for new materials is one of the main factors propelling recent advances in organic photovoltaics. Star-shaped small molecules (SSMs) have been proven promising candidates as perspective donor material due to the increase in numbers of excitation pathways caused by the degeneracy of the lowest unoccupied molecular orbital (LUMO) level. In order to unravel the pathways of the initial photon-to-charge conversion, the photovoltaic blends based on three different SSMs with a generic structure of N(phenylenent hiophene-dicyanovinyl-alkyl) 3 ( n = 1–3), and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) acceptor are investigated by ultrafast photoinduced absorption spectroscopy assisted by density functional theory calculations. It is shown that both electron transfer from SSMs to PC 71 BM and hole transfer from PC 71 BM to SSMs are equally signifi cant for generation of long-lived charges. In contrast, intramolecular (intra-SSM) charge separation results in geminate recombination and therefore constitutes a loss channel. Overall, up to 60% of long-lived separated charges are generated at the optimal PC 71 BM concentrations. The obtained results suggest that further improvement of the SSM-based solar cells is feasible via optimization of blend morphology and by suppressing the intra-SSM recombination channel.

The non-covalent interactions in organic molecules are known to drive their self-assembly to form molecular crystals. We compare, in the case of anthracene and against experimental (electronic-only) sublimation energy, how modern quantum-chemical methods are able to calculate this cohesive energy taking into account all the interactions between occurring dimers in both first-and second-shells. These include both O(N 6)- and O(N 5)-scaling methods, Local Pair Natural Orbital-parameterized Coupled-Cluster Single and Double, and Spin-Component-Scaled-Møller-Plesset perturbation theory at second-order, respectively, as well as the most modern family of conceived density functionals: double-hybrid expressions in several variants (B2-PLYP, mPW2-PLYP, PWPB95) with customized dispersion corrections (–D3 and –NL). All-in-all, it is shown that these methods behave very accurately producing errors in the 1–2 kJ/mol range with respect to the experimental value taken into account the experimental uncertainty. These methods are thus confirmed as excellent tools for studying all kinds of interactions in chemical systems. Financial support by the “Ministerio de Economía y Competitividad” of Spain and the “European Regional Development Fund” through projects CTQ2011-27253, CTQ2012-31914, and Consolider-Ingenio CSD2007-00010 in Molecular Nanoscience, and by the Generalitat Valenciana (ISIC 2012/008 and PROMETEO/2012/053) is acknowledged. The work in Mons is supported by the Belgian National Fund for Scientific Research (FNRS). Y.O. is a FNRS Post-doctoral Research Fellow. J.C.S.G. is a FNRS Visiting Professor.

Three conjugated triphenylamine-based poly(azomethine)s were prepared via well-known polycondensation chemistry using cheap and readily available starting materials and the results were contrasted with rrP3HT. Three functionalized diaminetriphenylamines (TPA(X), X ¼ –H, –OMe, –CN) were polymerized in a simple one-step process with 2,3-dihydrothieno[3,4-b][1,4]dioxine-5,7-dicarbaldehyde (ThOx), with water being the only side product. The resulting polymers (TPA(X)ThOx, X ¼ –H, –OMe, –CN) were characterized by GPC, IR and NMR, and show a good thermal stability. The opto-electronic properties could be tuned by changing the functionalization (X ¼ –H, –OMe, –CN) on the triphenylamine moiety. Photovoltaic devices based on TPA(X)ThOx/PCBM (1 : 2) showed power conversion efficiencies in the range of 0.02–0.04%. TRMC measurements showed that the presence of PCBM as an electron acceptor facilitates the formation of free mobile charges after excitation of the polymer. The low device efficiencies are attributed to a low hole-mobility of the polymer in combination with poor active layer morphology.

We report a joint experimental and theoretical study on the electronic structure and the solid-state organisation of bis(naphthoquinone)- tetrathiafulvalene (BNQ-TTF) as a promising ambipolar semiconductor. Accordingly, organic field-effect transistors (OFETs) fabricated with this material show both hole and electron transport for the first time in TTF derivatives. © The Royal Society of Chemistry 2011., We thank the funding from the European Community's Framework Programme (FP7/2007-2013) under grant agreement no. 212311 of the ONE-P project, Marie Curie EST FuMASSEC, DGI, Spain (projects CTQ2006-06333/BQU, CTQ2010-195011/BQU, and HOPE Consolider 2010 CSD2007-00007), the Generalitat de Catalunya (2009SGR00516), the program “Juan de la Cierva” (MICINN) and the CIBER de Bioingeniera, Biomateriales y Nanomedicina (CIBER-BBN), promoted by ISCIII, Spain. We also thank CESGA for the use of their computational resources. The work in Mons is also supported by the Belgian National Fund for Scientific Research (FNRS). Y.O. and J.C. are FNRS research fellows. This research used resources of the Interuniversity Scientific Computing Facility located at the University of Namur, Belgium, which is supported by the F.R.S.-FNRS under Convention No. 2.4617.07.

The generation of mobile charges and their transport across organic layers are commonly the most critical steps affecting the performance of organic-based electronic devices. Charge-transport properties are often described by quantum-chemical calculations which, however, face a challenge when the nanostructure of the material has to be concomitantly addressed together with electronic aspects. We tackle here this challenging task by applying dispersion-corrected Density Functional Theory methods, which allow us not only to give an insight into the molecular packing but also to accurately extract key molecular parameters governing charge transport. When applied to a set of functionalized (chlorinated) tetracene molecules, our approach yields the expected molecular packing, which has motivated its use to predict the packing of other fluorinated or brominated derivatives which are not yet synthesized. The charge mobilities have been calculated on the basis of the determined packing motifs and exhibit significant differences among the derivatives. This work paves the way towards the development of a computational protocol that could be implemented not only for idealized packing motifs or known crystallographic structures but also for self-organizing materials as well as supramolecular and host-guest interactions.

In an effort to gain a fundamental understanding of the electromechanical response in high mobility crystalline organic semiconductors, we have investigated the uniaxial strain–mobility relationships in rubrene and benzothienobenzothiophene crystals. Elastic moduli and Poisson ratios of the materials are evaluated and the strain mobility response of these materials is rationalized using the effective masses and electronic couplings in the framework of hopping and band transport models, giving consistent results. The microscopic origin of the response is investigated in relation to the strain induced variations in the inter- and intra-molecular degrees of freedom. We demonstrate that the strain applied along one of the crystallographic directions in these materials does not only induce mobility variations along the same direction, but also along the other crystallographic directions that are mechanically coupled with large Poisson ratios. A rational design of electronic devices could therefore benefit from the efficient exploitation of this anisotropic strain mobility response in relation to the inherent crystalline anisotropy.

The St Andrews team would like to thank the Leverhulme Trust (RPG-2016-047) for financial support. P.R. acknowledges support from a Marie Skłodowska-Curie Individual Fellowship (MCIF; No. 749557). S.M.S acknowledges support from the Marie Skłodowska-Curie Individual Fellowship, grant 27 agreement no. 838885 (NarrowbandSSL). Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. We acknowledge support from the European Union’s Horizon 2020 research and innovation programme under the ITN TADFlife (GA 812872). Y.O. acknowledges funding by the Fonds de la Recherche Scientifique-FNRS under Grant n° F.4534.21 (MIS-IMAGINE). D.B. is a FNRS Research Director. In this work a new acceptor is used for use in thermally activated delayed fluorescence (TADF) emitters, pyridylbenzimidazole, which when coupled with phenoxazine allows efficient TADF to occur. N-functionalization of the benzimidazole using methyl, phenyl, and tert-butyl groups permits color tuning and suppression of aggregation-caused quenching (ACQ) with minimal impact on the TADF efficiency. The functionalized derivatives support a higher doping of 7 wt% before a fall-off in photoluminescence quantum yields is observed, in contrast with the parent compound, which undergoes ACQ at doping concentrations greater than 1 wt%. Complex conformational dynamics, reflected in the time-resolved decay profile, is found. The singlet−triplet energy gap, ΔEST, is modulated by N-substituents of the benzimidazole and ranges of between 0.22 and 0.32 eV in doped films. Vacuum-deposited organic light-emitting diodes, prepared using three of the four analogs, show maximum external quantum efficiencies, EQEmax, of 23.9%, 22.2%, and 18.6% for BIm(Me)PyPXZ , BIm(Ph)PyPXZ , and BImPyPXZ , respectively, with a correlated and modest efficiency roll-off at 100 cd m–2 of 19% 13%, and 24% of the EQEmax, respectively. Publisher PDF