Electronic couplings for molecular charge transfer: Benchmarking CDFT, FODFT, and FODFTB against high-level ab initio calculations

Abstract

We introduce a database (HAB11) of electronic coupling matrix elements (H(ab)) for electron transfer in 11 π-conjugated organic homo-dimer cations. High-level ab inito calculations at the multireference configuration interaction MRCI+Q level of theory, n-electron valence state perturbation theory NEVPT2, and (spin-component scaled) approximate coupled cluster model (SCS)-CC2 are reported for this database to assess the performance of three DFT methods of decreasing computational cost, including constrained density functional theory (CDFT), fragment-orbital DFT (FODFT), and self-consistent charge density functional tight-binding (FODFTB). We find that the CDFT approach in combination with a modified PBE functional containing 50% Hartree-Fock exchange gives best results for absolute H(ab) values (mean relative unsigned error = 5.3%) and exponential distance decay constants β (4.3%). CDFT in combination with pure PBE overestimates couplings by 38.7% due to a too diffuse excess charge distribution, whereas the economic FODFT and highly cost-effective FODFTB methods underestimate couplings by 37.6% and 42.4%, respectively, due to neglect of interaction between donor and acceptor. The errors are systematic, however, and can be significantly reduced by applying a uniform scaling factor for each method. Applications to dimers outside the database, specifically rotated thiophene dimers and larger acenes up to pentacene, suggests that the same scaling procedure significantly improves the FODFT and FODFTB results for larger π-conjugated systems relevant to organic semiconductors and DNA.

Ecoupling server: A tool to compute and analyze electronic couplings

“…Then, the results were compared with published data 6,49 ( Figure 4) and indicates the EC decays with the distance between donor and acceptor for all these cases. …”

“…Thus, the effective bridge assisted electron coupling is defined as: (5) where HDA is the electron coupling between donor and acceptor applied to the whole donor-bridge-acceptor system (inside the two state or multistate model), and HSE is the superexchange term. The HSE depends on the coupling between donor-bridge (HDB) and bridge-acceptor (HBA) and the energy barriers between orbitals (EDA and EDB), following the formula: (6) where :…”

“…For this reason, EC values have been computed to find the optimal distance and geometry to pack the layers in the material 6,49 .…”

“…Electron transfer (ET) reactions are fundamental processes involved in most biological phenomena 1 including oxidation 2 , photosynthesis 3 or DNA processes 4 and also in industrial applications as enzymatic reactions 5 , organic semiconductors 6 , solar cells 7 , molecular wires 8 , or biosensors 9,10 . For these, many efforts have been done experimentally and computationally to characterize ET processes 11 .…”

Efficient Constrained Density Functional Theory Implementation for Simulation of Condensed Phase Electron Transfer Reactions

“…In comprehensive studies of charge transfer in anionic and cationic organic dimers, Blumberger and coworkers 89,90 Table 2. According to Table 2 …”