Assessment of TD‐DFT‐ and TD‐HF‐based approaches for the prediction of exciton coupling parameters, potential energy curves, and electronic characters of electronically excited aggregates

Abstract

The reliability of linear response approaches such as time-dependent Hartree-Fock (TD-HF) and time-dependent density functional theory (TD-DFT) for the prediction of the excited state properties of 3,4;9,10-tetracarboxylic-perylene-bisimide (PBI) aggregates is investigated. A dimer model of PBI is investigated as a function of a torsional motion of the monomers, which was shown before to be an important intermolecular coordinate in these aggregates. The potential energy curves of the ground state and the two energetically lowest neutral excited and charge-transfer (CT) states were obtained with the spin-component scaling modification of the approximate coupled-cluster singles-and-doubles (SCS-CC2) method as a benchmark for dispersion corrected TD-HF and a range of TD-DFT approaches. The highly accurate SCS-CC2 results are used to assess the other, computationally less demanding methods. TD-HF predicts similar potential energy curves and transition dipole moments as SCS-CC2, as well as the correct order of neutral and CT states. This supports an exciton trapping mechanism, which was found on the basis of TD-HF data. However, the investigated TD-DFT methods provide generally the opposite character for the excited states. As a consequence, these TD-DFT results have unacceptably large errors for optical properties of these dye aggregates.

The dimer-approach to characterize opto-electronic properties of and exciton trapping and diffusion in organic semiconductor aggregates and crystals

“…To keep the computations manageable, the intra-monomer degrees of freedom were not varied but frozen to the values obtained from the monomer ground-state optimization. 28,29,[33][34][35]51,52 The ground state PESs were computed by Møller-Plesset perturbation theory to second-order in combination with spincomponent-scaling (SCS-MP2) using the resolution of identity approximation. [76][77][78] For all dimer computations, we employed the SVP (split valence plus polarization) basis sets.…”

“…In particular, for perylene-based aggregates, they are so small that the mixing and consequently the electronic structures of the states, vary strongly already as a function of the relative orientation of the monomers. 29,33,35 Increasing the size of the aggregate to a trimer (tetramer) the number of states increase to 3 (4) Frenkel and 6 (12) CT states, and the energy separation between the upper and lower Frenkel state becomes larger. The limit for a large number of states (or large aggregates) is 4| J|.…”

“…1, right hand side). Despite the high symmetry of the homo-dimer, Frenkel and CT states can mix 28,33,34 which gives rise to the four adiabatic states which are sketched in the middle of the figure. The amount of mixing depends on the energy difference of Frenkel-and CT states which is very small for many dyes.…”

“…50 This failure persuaded us that, for the description of the emission spectroscopy of many organic aggregates, a more comprehensive approach has to be developed. 29,33,34,[51][52][53][54][55] This approach, which we coined as 'dimer approach', is the main topic of this perspective. However, to clarify the differences to standard approaches that are based on monomers as central units, we briefly summarize their model Hamiltonians in Section 2.…”

QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

“…[37,38] Furthermore, intermolecular interactions are taken into account on a fully quantum-mechanical level. [39,40] Ground-state geometries and energies, excitation energies, and ionization potentials were calculated with the ωB97X-D functional [41] that was shown to provide very accurate ground-state geometries, excitation energies, and ionization potentials. [42][43][44] Furthermore, it yields reliable intermolecular potential energies for dimers composed of molecular semiconductors.…”