Singlet Fission in Quinoidal Oligothiophenes

Abstract

The electronic properties of quinoidal oligithiophenes make them interesting for applications in semiconductor technology. Because of their very large singlet− triplet splitting, they are promising candidates for singlet fission (SF), a process in which an initially excited singlet state is converted into two triplet excitons. Thus, the efficiency of solar cells could be increased to overcome the ShockleyQueisser limit. Here, we investigate the ability of a quinoidal bithiophene to undergo SF in solution. We calculated the ground state and low-lying excited states using a combined density functional theory and multireference configuration interaction approach including dispersion corrections. Potential energy curves along normal mode displacements were computed to detect avoided crossings between the initially excited bright singlet state and a dark doubly excited state which can be interpreted as a triplet pair overall coupled to a singlet 1 (TT). The studied quinoidal bithiophene meets the energetic requirement for SF. A path enabling intramolecular SF could not be found. In contrast, we were able to identify two vibrational modes relevant for an intermolecular SF process in the slip-stacked dimer: A promoting coordinate that couples a bright singlet state with the 1 (TT) state and a separating coordinate that localizes the triplet states on the respective monomers. These results elucidate the mechanism underlying the formation of a triplet pair and the separation of the triplet excitons after initial photoexcitation of the bright singlet state.

The DFT/MRCI method

“…[132][133][134][135] For their modeling, the use of a multiplicity independent Hamiltonian is mandatory in a DFT/MRCI treatment. 88,[136][137][138][139] Contrary to monomeric nucleobases, strange results are obtained for a hydrogen-bonded adenine-thymine Watson-Crick base pair if the original DFT/MRCI Hamiltonian is employed. In these calculations, the first excited singlet state corresponds to a double excitation with ππ 0 !…”

“…In the case of π-stacked dimers, it may be necessary to add a semiempirical dispersion correction, such as Grimme's D3 correction, 140,141 to obtain a properly bound ground state. 138 The dispersion corrections mainly affect the molecular geometry. For a given nuclear arrangement, the D3 corrections have no impact on the MOs and the DFT/MRCI Hamiltonian and hence on the electronic spectrum.…”

“…Such systems play a significant role in photobiology, photovoltaics, and in third‐generation electroluminescent emitters that are based on thermally activated delayed fluorescence (TADF) . For their modeling, the use of a multiplicity independent Hamiltonian is mandatory in a DFT/MRCI treatment …”

“…In contrast, the redesigned DFT/MRCI Hamiltonians yield a balanced description of monomer and dimer excitations. In the case of π ‐stacked dimers, it may be necessary to add a semiempirical dispersion correction, such as Grimme's D3 correction, to obtain a properly bound ground state . The dispersion corrections mainly affect the molecular geometry.…”

Singlet fission in tetracene: an excited state analysis

“…Theoretical studies have been performed in order to find new SF molecules that fulfill the critical energetic criteria E(S 1 ) > 2E(T 1 ), [12,13] and also to understand its underlying mechanism (a detailed review can be found in Ref. [14]) but many questions of how this process actually occurs are still active areas of research; these include the role of the vibrational modes [15][16][17][18][19] and charge transfer (CT) states [20,21] in the SF process.…”