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Prediction of accurate geometries is a prerequisite for accurate prediction of molecular properties. Impact of Hartree Fock (HF) exchange (a0) on geometry in the framework of DFT is investigated by monitoring dihedral angles, bond length alternations, and torsional energy barriers of 10 dimeric systems against CCSD (ADZ/ATZ) benchmarks. A strong correlation is observed between the fraction of HF exchange, equilibrium dihedral angles, and the potential energy barriers in global hybrids. Full HF exchange is critical to accurately predict the nonplanarity. Lower fractions of (a0)/larger DFT exchange (1‐a0) results in overestimation of torsional energy barriers at 900 and underestimation at 00. Large contributions of (1‐a0) in global hybrid functionals tend to overestimate torsional energy barriers (900) and are biased toward planar geometries. However, inclusion of larger fractions of (a0)/lower (1‐a0) also overestimate the torsional energy barriers in syn‐conformations due to the localization errors associated with HF exchange in global hybrids. Hence, irrespective of the fraction of HF/DFT exchange incorporated, global hybrids fail to accurately predict torsional energy barriers at 00 and 900 simultaneously. Long‐range corrected (LC) functionals, which employ full HF exchange at longer regions, outperform global hybrid functionals in predicting geometries and torsional energy barriers of the dimeric molecules. The distance dependence of (a0) thus provides a balanced fraction of HF exchange as the dihedral torsion varies. Impact of range separation parameter on geometries is marginal in altering the planarity/nonplanarity. However, range separation parameter within 0.20–0.40 bohr−1 predicts more reliable torsional energies and geometries. © 2019 Wiley Periodicals, Inc.

We have investigated the structure property relationship of organic molecules with Donor-Linker-Acceptor (DLA) framework which can be used as precursors of OLED materials employing Density Functional Theory (DFT) and Time dependent DFT calculations. Two types of linkers' thiophene (π-conjugated) and ethylene (non-π-conjugated), carbazole as electron donor and cyano-substituted benzene as electron acceptor in DLA framework are chosen. Donor-Linker-Acceptor network allows the HOMO and LUMO orbitals to be spatially separated reducing the overlap of frontier orbitals decreasing the exchange energy (J) which results in smaller ΔEST (energy gap between excited singlet and triplet states) thus allowing us to realize metal free molecules suitable for OLED applications. Incorporation of donor and acceptor groups in the same moiety helps in reducing the number of layers conventionally used in OLEDS, reducing the cost with more simplified device fabrication. By enhancing the electron donating nature of donor group and increasing the withdrawing nature of acceptor group we observe a decrease in exchange energy, which is further decreased by a non-conjugated linker. Important properties like change in dipole moments, absorption and emission energies corresponding oscillator strengths, transport energy gaps, electron affinities, ionization potentials (vertical and adiabatic), reorganization energies, and exciton binding energies of various substituted molecules are investigated and their role in determining the efficiencies are discussed. In order to account the effects of solvents and their role in altering various properties, the studies are carried out in polar and non-polar solvent phases in addition to the gas phase calculations. Please do not adjust marginsPlease do not adjust margins towards HOMO are found to be 89%, 9% and 2% and for LUMO 2%, 35% and 63% respectively. For 2.a, the contribution of donor, linker and acceptor towards HOMO are found to be 95%, 4% and 1% and for LUMO 1%, 4% and 95% respectively. Figure 3. HOMO, LUMO orbitals of molecules with thiophene linker (1.a-1.e) and ethylene linker (2.a-2.e). Grey, white, blue, red and yellow colours indicate carbon, hydrogen, nitrogen, oxygen and sulphur atoms respectively. Graphical AbstractWe have investigated the structure property relationship of organic molecules with Donor-Linker-Acceptor (DLA) framework which can be used as precursors of OLED materials employing Density Functional Theory (DFT). Two types of linkers thiophene (π-conjugated) and ethylene (non-πconjugated), carbazole as electron donor and cyano-substituted benzene as electron acceptor in DLA framework are chosen. By enhancing the electron donating nature of donor group and increasing the withdrawing nature of acceptor group we observe a decrease in exchange energy, which is further decreased by a non-conjugated linker. Important properties like energy gap between excited singlet and triplet states, change in dipole moments, absorption and emission energies corresponding oscillator strengths, tra...

Accurate determination of ionization potentials (IPs), electron affinities (EAs), fundamental gaps (FGs), and HOMO, LUMO energy levels of organic molecules play an important role in modeling and predicting the efficiencies of organic photovoltaics, OLEDs etc. In this work, we investigate the effects of Hartree Fock (HF) Exchange, correlation energy, and long range corrections in predicting IP and EA in Hybrid Functionals. We observe increase in percentage of HF exchange results in increase of IPs and decrease in EAs. Contrary to the general expectations inclusion of both HF exchange and correlation energy (from the second order perturbation theory MP2) leads to poor prediction. Range separated Hybrid Functionals are found to be more reliable among various DFT Functionals investigated. DFT Functionals predict accurate IPs whereas post HF methods predict accurate EAs. © 2017 Wiley Periodicals, Inc.