Qualitatively Incorrect Features in the TDDFT Spectrum of Thiophene-Based Compounds

Abstract

Ab initio molecular electronic structure computations of thiophene-based compounds constitute an active field of research prompted by the growing interest in low-cost materials for organic electronic devices. In particular, the modeling of electronically excited states and other time-dependent phenomena has moved toward the description of more realistic albeit challenging systems. We demonstrate that due to its underlying approximations, time-dependent density functional theory predicts results that are qualitatively incorrect for thiophene and thienoacenes, although not for oligothiophene chains. The failure includes spurious state inversion and excitation characters, wrong distribution of oscillator strengths and erroneous potential energy surfaces. We briefly analyze possible origins of this behavior and identify alternative methods that alleviate these problems.

Excited state dynamics of thiophene and bithiophene: new insights into theoretically challenging systems

“…The spectra were decomposed into contributions from different states, S 1 (blue) and S 2 (red) for thiophene, S 1 (red) and S 3 + S 4 (blue) for bithiophene. The color code is consistent with the one used in our previous study, 23 and reflects the character of the pp* states. The energy windows used for the sampling of the initial conditions of the molecular dynamics simulations are also indicated.…”

“…Achieving a balanced description of these states using electronic structure methods is not an easy task. In a recent letter 23 we showed that CIS (configuration interaction singles) and TDDFT invert ordering of the two pp* states. This is somewhat surprising for TDDFT, which is usually considered reliable for pp* states.…”

“…Our ADC(2) computations support this view as no B 2 minimum was located. The geometries resulting from the TDDFT 23 and CASSCF 66 optimizations are most likely spurious. The vertical excitation energies of bithiophene are also reported in Table 1.…”

“…ADC (2) has been successfully applied to an important number of molecular systems [28][29][30] and, more specifically, for thiophene-based molecules. 23,31,32 In the case of thiophene, 23 ADC(2) reproduces the electronic state ordering given by CASPT2 at the ground-state geometry, while TDDFT suffers from its approximation and inverts the character of the first two electronic states. When it comes to excited-state properties and dynamics, ADC(2) is considered to be more robust than CC2 (approximate coupled cluster singles and doubles) as its eigenvalue problem is Hermitian.…”

Low-Lying ππ* States of Heteroaromatic Molecules: A Challenge for Excited State Methods

“…The inversion of the L a and L b states was also spotted for 9H-adenine 35 by comparing TD-DFT estimates to high level reference values obtained with wave function-based approaches, such as EOM-CCSD(T) 36 (equation of motion-coupled cluster singles and doubles with perturbative triples) or CASPT2 37 (complete active space second-order perturbation theory), even though the ambiguity still remains in this case. 35 More recently, two of us have unraveled similar discrepancies for the low-lying ππ* states of thiophene and thienoacenes, 38 which constitute popular building blocks in organic electronics. Regardless of the exchange-correlation functional used, we found not only a spurious state inversion but also a wrong distribution of oscillator strengths and erroneous potential energy surfaces.…”