Cocrystal Growth in Organic Semiconductor Thin Films: Simulation of Pentacene, Perfluoropentacene, and Their 1:1 Blend Deposited On Graphite

The understanding of crystal formation in thin films and the precise knowledge of the relation between structure and surface diffusion are two important requirements for the efficient (nano)fabrication of organic electronic devices. Here a computational approach for simulating vapor-phase deposition is employed to obtain and investigate three types of crystalline thin films on graphite. All systems, namely pentacene, perfluoropentacene, and their 1:1 blend, which forms an alternate cocrystal, are constituted by recumbent molecules in accordance with experimental findings. The contributions of intermolecular interactions and of molecular rearrangements occurring during the deposition are analyzed to rationalize the final morphologies. Then, the generated structures are employed to evaluate the energy barriers that prevent molecular diffusion at terraces and step-edges, and to study the reorganization of the films upon high-temperature annealing. The broad agreement with experimental observations and the possibility of evaluating the potential energy surface at the molecular detail render the proposed approach a promising tool to make predictions for other systems

Cost-Effective Force Field Tailored for Solid-Phase Simulations of OLED Materials

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

Dynamic nature of excited states of donor–acceptor TADF materials for OLEDs: how theory can reveal structure–property relationships

Spin statistics greatly limits the efficiency of OLEDs, which might be largely improved upon conversion of triplet into singlet-excited (and thus light-emitting) states via a Thermally Activated Delayed Fluorescence (TADF) process. We theoretically investigate here the combination of some real-life donor (D) and acceptor (A) moieties with the connectivity D–A and D–A–D. We selected phenoxazine (PXZ) and phenylthiazine (PTZ) as electron-donating groups, and 2,5-diphenyl-1,3,4-oxadiazole (OXD), 3,4,5-triphenyl-4H-1,2,4-triazole (TAZ), and 2,5-diphenyl-1,3,4-thiadiazole (TDZ) as their electron-accepting partners. The systematic Tamm–Dancoff Approximation-Density Functional Theory calculations performed allowed us to calculate accurately not only the energy levels of low-lying singlet and triplet-excited states, but also to characterize their Charge-Transfer (CT) or Locally Excited (LE) nature, since the energy difference and the coupling between the 3CT, 3LE, and 1CT states become key to understanding the molecular mechanism involved in this process. We have also studied the role played by the conformational landscape, arising from the thermally accessible range of D–A(–D) torsion angles, in the singlet–triplet energy gap as well as its influence on oscillator strengths. Overall, we rationalize the origin of the higher efficiencies found in real devices for D–A–D molecules, disclosing the underlying structure–property relationships and thus anticipating successful design strategies.This work was partially supported by the Samsung Advanced Institute of Technology (SAIT)’s Global Research Outreach (GRO) Program. In addition, the research in Bordeaux has been funded by the French State grant ANR-10-LABX-0042-AMADEus managed by the French National Research Agency under the initiative of excellence IdEx Bordeaux program (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. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifique 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 no. 1117545. Mónica Moral thanks to the E2TP CYTEMA-Santander Program for their financial support

Computational Design of Thermally Activated Delayed Fluorescence Materials: The Challenges Ahead

Thermally activated delayed fluorescence (TADF) offers promise for all-organic light-emitting diodes with quantum efficiencies competing with those of transition-metal-based phosphorescent devices. While computational efforts have so far largely focused on gas-phase calculations of singlet and triplet excitation energies, the design of TADF materials requires multiple methodological developments targeting among others a quantitative description of electronic excitation energetics, fully accounting for environmental electrostatics and molecular conformational effects, the accurate assessment of the quantum mechanical interactions that trigger the elementary electronic processes involved in TADF, and a robust picture for the dynamics of these fundamental processes. In this Perspective, we describe some recent progress along those lines and highlight the main challenges ahead for modeling, which we hope will be useful to the whole TADF community.The work in Mons was supported by the Belgian National Science Foundation, F.R.S.-FNRS. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by 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 Grant Agreement n1117545. The research in Bologna, Grenoble, and Mons is also through the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 646176 (EXTMOS project)

Structure and Charge Transport Properties of Cycloparaphenylene Monolayers on Graphite

The nanoscale organization of cycloparaphenylene molecules when physisorbed on a graphite surface is theoretically investigated by means of atomistic molecular dynamics simulations employing a tailored and benchmarked force field. The landing of a single molecule is first considered, to progressively deposit more molecules to finally reach the full coverage of the surface. This protocol allows to study, consequently, the mechanism and structural pattern of their self‐aggregation. The interfacial morphologies obtained are then analyzed in terms of the electronic coupling between neighboring molecules, allowing thus to provide information about the associated charge‐transfer phenomena which could take place in these highly organized monomolecular layers.A.J.P.J. and J.C.S.G. acknowledge the project CTQ2014–55073-P from the Spanish Government (MINECO/FEDER) and the project AICO/2018/175 from the Regional Government (GVA/FSE). L.M. acknowledges funding from MIUR–PRIN 2015XJA9NT (Molecular Organization in Organic Thin Films via Computer Simulation of their Fabrication Processes)

Efficient analysis of highly complex nuclear magnetic resonance spectra of flexible solutes in ordered liquids by using molecular dynamics

The NMR spectra of n-pentane as solute in the liquid crystal 5CB are measured at several temperatures in the nematic phase. Atomistic molecular dynamics simulations of this system are carried out to predict the dipolar couplings of the orientationally ordered pentane, and the spectra predicted from these simulations are compared with the NMR experimental ones. The simulation predictions provide an excellent starting point for analysis of the experimental NMR spectra using the covariance matrix adaptation evolutionary strategy. This shows both the power of atomistic simulations for aiding spectral analysis and the success of atomistic molecular dynamics in modeling these anisotropic systems. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4705271

Does supramolecular ordering influence exciton transport in conjugated systems? Insight from atomistic simulations

none6We have developed a theoretical platform for modelling temperature-dependent exciton transport in organic materials, using indenofluorene trimers as a case study. Our atomistic molecular dynamics simulations confirm the experimentally observed occurrence of a liquid crystalline smectic phase at room temperature and predict a phase transition to the isotropic phase between 375 and 400 K. Strikingly, the increased orientational disorder at elevated temperatures barely affects the ability of excitons to be transported over large distances, though disorder influences the directionality of the energy diffusion process. Detailed quantum-chemical calculations show that this result arises from a trade-off between reduced excitonic couplings and increased spectral overlap at high temperatures. Our results suggest that liquid crystalline oligomeric materials could be promising candidates for engineering optoelectronic devices that require stable and controlled electronic properties over a wide range of temperatures and supramolecular arrangements.The contribution has received a cover of the JournalmixedT.A. Papadopoulos; L. Muccioli; S. Athanasopoulos; A.B. Walker; C. Zannoni; D. BeljonneT.A. Papadopoulos; L. Muccioli; S. Athanasopoulos; A.B. Walker; C. Zannoni; D. Beljonn

Energetic fluctuations in amorphous semiconducting polymers: Impact on charge-carrier mobility

We present a computational approach to model hole transport in an amorphous semiconducting fluorene-triphenylamine copolymer (TFB), which is based on the combination of molecular dynamics to predict the morphology of the oligomeric system and Kinetic Monte Carlo (KMC), parameterized with quantum chemistry calculations, to simulate hole transport. Carrying out a systematic comparison with available experimental results, we discuss the role that different transport parameters play in the KMC simulation and in particular the dynamic nature of positional and energetic disorder on the temperature and electric field dependence of charge mobility. It emerges that a semi-quantitative agreement with experiments is found only when the dynamic nature of the disorder is taken into account. This study establishes a clear link between microscopic quantities and macroscopic hole mobility for TFB and provides substantial evidence of the importance of incorporating fluctuations, at the molecular level, to obtain results that are in good agreement with temperature and electric field-dependent experimental mobilities. Our work makes a step forward towards the application of nanoscale theoretical schemes as a tool for predictive material screening

N‐doped cycloparaphenylenes: Tuning electronic properties for applications in thermally activated delayed fluorescence

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: 2.5020.1