Intramolecular charge ordering in the multi molecular orbital system (TTM-TTP)I3

Abstract

Starting from the structure of the (TTM-TTP)I(3) molecular-based material, we examine the characteristics of frontier molecular orbitals using ab initio (CASSCF/CASPT2) configurations interaction calculations. It is shown that the singly occupied and second-highest-occupied molecular orbitals are close to each other, i.e., this compound should be regarded as a two-orbital system. By dividing virtually the [TTM-TTP] molecule into three fragments, an effective model is constructed to rationalize the origin of this picture. In order to investigate the low-temperature, symmetry breaking experimentally observed in the crystal, the electronic distribution in a pair of [TTM-TTP] molecules is analyzed from CASPT2 calculations. Our inspection supports and explains the speculated intramolecular charge ordering which is likely to give rise to low-energy magnetic properties.

Ab initio derivation of multi-orbital extended Hubbard model for molecular crystals

“…In order to estimate the energies of the expected levels, we set the occupation numbers for both MOs as 1.5. 29 Under these conditions, for (TTM-TTP) + , we obtain that the SOMO is u and HOMO−1 is g, in agreement with extended Hückel calculations, 4 and the resulting energy difference obtained from the ROHF calculation is ∼ 0.4 eV. 29 For [Au(tmdt) 2 ], we obtain that the SOMO is g and HOMO−1 is u, in agreement with DFT-based ab initio calculations.…”

“…29 Under these conditions, for (TTM-TTP) + , we obtain that the SOMO is u and HOMO−1 is g, in agreement with extended Hückel calculations, 4 and the resulting energy difference obtained from the ROHF calculation is ∼ 0.4 eV. 29 For [Au(tmdt) 2 ], we obtain that the SOMO is g and HOMO−1 is u, in agreement with DFT-based ab initio calculations. 22 The notations for label of MOs are adopted to those in Ref.…”

“…3 and 26-28) is now considered as another candidate of multi-molecular-orbital system. 29,30 In this compound, the non-magnetic insulating behavior at low temperature has been confirmed. [31][32][33][34] A charge ordering reflected by the alternation of valence of TTM-TTP molecule along the stacking direction has been proposed initially.…”

“…The relevant fragment MOs are simply the left and right part of MOs, ϕ L and ϕ R , where the center fragment is omitted due to its small weight. 29 We denote the corresponding annihilation operators by c L, j, σ and c R, j, σ . The operator correspondence between the original MOs and the fragment MOs basis is given by…”

“…33,34 The experimental analysis based on Raman-scattering [35][36][37] and x-ray 38 measurements suggested a new type of charge-ordered state, "intra-molecular charge ordering (ICO)," which cannot be described by the conventional single-orbital approximation. By performing wavefunction-based ab initio calculations for the isolated ionic TTM-TTP molecule, 29 we previously revealed that this system has a multiconfigurational character.…”