Force Fields for Macromolecular Assemblies Containing Diketopyrrolopyrrole and Thiophene

Abstract

Utilising a force matching procedure, we parameterise new force fields systematically for large conjugated systems. We model both conjugated polymers and molecular crystals that contain diketopyrrolopyrrole, thiophene, and thieno[3,2-b]thiophene units. These systems have recently been found to have low band gaps, which exhibit high efficiency for photovoltaic devices. The equilibrium structures, forces and energies of the building block chromophores: diketopyrrolopyrrole thiophene, and thieno[3,2b]thiophene computed using our parameters are comparable to those computed using the reference electronic structure method. We assess the suitability of this new force field for electronic property calculations by comparing the electronic excitation properties computed along classical and ab initio molecular dynamics trajectories. For both trajectories, we find similar distributions of TDDFT calculated excitation energies and oscillator strengths for the building block chromophore diketopyrrolopyrrole-thieno[3,2b]thiophene. The structural, dynamical, and electronic properties of the macromolecular assemblies built upon these chromophores are characterised. For both polymers and molecular crystals, pronounced peaks around 0 o or 180 o are observed for the torsions between chromophores under ambient conditions. The high planarity in these systems can promote local ordering and π-π stacking, thereby potentially facilitating charge transport across these materials. For the model conducting polymers, we found that the fluctuations in the density of states per chain per monomer is neglegibly small and does not vary significantly with chains comprising 20 to 40 monomers. Analysis of the electron-hole distributions and the transition density matrices indicates that the delocalised length is approximately four to six monomers, which is in good agreement with other theoretical and experimental studies of different conducting polymers. For the molecular crystals, our investigation of the characteristic timescale of the fluctuation in the excitonic couplings shows that low frequency vibration below 10 cm −1 is observed for the nearest neighbors. These observations are in line with previous studies on other molecular crystals, in which low frequency vibrations are believed to be responsible for the large modulation of the excitonic coupling. Thus, our approach and the new force fields provide a direct route for studying the structure-property relations and the molecular level origins of the high-efficiency of these classes of materials.

Local structuring of diketopyrrolopyrrole (DPP)-based oligomers from molecular dynamics simulations

“…All the equilibrium bond distances and angles of the conjugated part were taken from optimisation of trimer chains where values of the middle monomer were used to account for intermonomer connections at the B3LYP/6-311G** level of theory, the atomic point charges were calculated via CHelpG (charges from electrostatic potentials using a grid-based method) 83 and the torsional potentials between the conjugated fragments were recomputed through scanning the dihedral potential at the MP2/6-311G** level. 84,85 The equilibrium geometry and intermolecular interaction parameters are very similar to those computed by Jiang et al 86 with a force matching procedure and validated with simulations of small molecular crystals. OPLS-AA parameters were used for the alkyl side chains.…”

“…Lennard-Jones (LJ) potential were taken directly from OPLS-AA for all atom types using the geometric mixing rule as well as long range Coulomb interactions between point charges, as common practice for simulation of polymeric semiconductors. 86…”

“…Attempts have been made to develop a specialised force-field (FF) [86][87][88] for this class of materials, however, due to the complexity of the systems, some parametrisation is needed for each new polymer. In this work, simulations were performed on oligomers of three repeat units of each of the monomers.…”

Automated Parameterization of Quantum Mechanically Derived Force Fields for Soft Materials and Complex Fluids: Development and Validation

“…In this framework, classical molecular dynamics (MD) simulations , are among the most popular computational tools to tackle structure–property investigations, as they allow for achieving the required microscopic insight and, at the same time, for adequate sampling of the phase space visited by the system . Nowadays, MD simulations flank and integrate experimental findings on supramolecular features, exploiting a level of detail unattainable in real experiments. − …”

Accounting for Vibronic Features through a Mixed Quantum-Classical Scheme: Structure, Dynamics, and Absorption Spectra of a Perylene Diimide Dye in Solution

“…This might introduce a further obstacle, which can be overcome by refining or fully re-parameterizing the FF, based on accurate Quantum Mechanical (QM) data, as recently reported by several groups. [44][45][46][47][48][49] Once the reliability of the classical MD trajectories has been validated, a very convenient method to compute spectra of complex and flexible systems in explicit environments is performing a Classical Ensemble Average of Vertical Excitations (CEA-VE). It essentially consists in performing a sufficiently long and accurate MD trajectory, from which a number of uncorrelated snapshots, large enough to ensure convergence, is extracted, and successively computing vertical transition energies and intensities for each frame.…”