Origin of the Failure of Density Functional Theories in Predicting Inverted Singlet–Triplet Gaps

Abstract

Recent experimental and theoretical studies have shown several new organic molecules that violate Hund’s rule and have the first singlet excited state lower in energy than the first triplet excited state. While many correlated single reference wave function methods have successfully predicted excited-state energetics of these low-lying states, conventional linear-response time-dependent density functional theory (TDDFT) fails to predict the correct excited-state energy ordering. In this article, we have explored the performance of combined DFT and wave function methods like doubles-corrected TDDFT and multiconfiguration pair-density functional theory for the calculation of inverted singlet–triplet gaps. We have also tested the performance of the excited-state DFT (eDFT) method for this problem. Our results have shown that it is possible to obtain inverted singlet–triplet gaps both by using doubles-corrected TDDFT with a proper choice of double-hybrid functionals or by using eDFT.

On the origin of the inverted singlet–triplet gap of the 5th generation light-emitting molecules

“…The excitation energies calculated at the ADC(2) and CC2 levels agree well with values obtained using computationally more expensive ab initio levels of theory. [2][3][4][5][6][7][8]25 Correlated ab initio methods (SCS-CC2, SCS-ADC(2), SA-CASSCF, and SC-NEVPT2, RASSCF, ADC(2) and EOM-CCSD) employed in previous studies 2-6,25 and here yielded consistently inverted singlettriplet gaps in the range of [À0.66, À0.03] eV for molecule 1 and 2. Literature values are collected in Table S4 of the ESI.…”

“…[2][3][4][5]8,25 The reason for this is proposed to be the inability of conventional TDDFT methods to include double excitation contributions, 26,27 which seems to be the prerequisite for the description of an inverted singlet-triplet gap. [2][3][4]25 In this work, we computationally study electronic excited states of the INVEST molecules shown in Fig. 1 with the aim to understand the underlying reasons for the inverted singlettriplet gap, and to determine which computational methods are suitable for correct recognition of the INVEST property.…”

“…[4][5][6]10 Recent computational studies have shown that timedependent density functional theory (TDDFT) methods fail to predict an inverted S 1 -T 1 energy separation. [2][3][4][5]8,25 The reason for this is proposed to be the inability of conventional TDDFT methods to include double excitation contributions, 26,27 which seems to be the prerequisite for the description of an inverted singlet-triplet gap. [2][3][4]25 In this work, we computationally study electronic excited states of the INVEST molecules shown in Fig.…”

“…2–5,8,25 The reason for this is proposed to be the inability of conventional TDDFT methods to include double excitation contributions, 26,27 which seems to be the prerequisite for the description of an inverted singlet–triplet gap. 2–4,25…”

Organic molecules with inverted singlet-triplet gaps

“…Interestingly, negative Δ E ST with high accuracy could be well realized by employing methods merging wavefunction and correlation functionals. Moreover, Ghosh and Bhattacharyya calculated seven azaphenalene derivatives (M-18 to M-21, Azine, Azine-4N and Azine-7N, Figure 2 ) by combining DFT and wave function methods, unveiling that inherent INVEST gaps could be obtained by using doubles-corrected TD-DFT with suitable double-hybrid functionals or excited-state DFT ( Ghosh and Bhattacharyya, 2022 ). Overall, the molecular design of inherent INVEST emitters should consider both minimal exchange integrals leading to small singlet-triplet gaps, and significant double excitation character in electronic transitions in extended π-conjugated heteroatom-containing molecular systems.…”