Modeling Photoelectron Spectra of Conjugated Oligomers with Time-Dependent Density Functional Theory

Abstract

With the aim of producing accurate band structures of conjugated systems by employing the states of cations, TDDFT calculations on conjugated oligomer radical cations of thiophene, furan, and pyrrole with one to eight rings were carried out. Benchmarking of density functional theory and ab initio methods on the thiophene monomer shows that the ∆SCF ionization potential (IP) is most accurate at the B3LYP/6-311G* level. Improvement of the basis set beyond 6-311G* leads to no further changes. The IP is closer to experiment at B3LYP/6-311G* than at CCSD(T)/CCPVQZ. For longer oligomers the ∆SCF IPs decrease too fast with increasing chain length with all density functionals. CCSD/6-311G* performs well if the geometries are optimized at the CCSD level. With MP2 geometries IPs decrease too fast. Peak positions in photoelectron spectra were determined by adding appropriate TDDFT excitation energies of radical cations to the ∆SCF IPs. The agreement with experiment and with Green function calculations shows that TDDFT excited states of radical cations at the B3LYP/6-311G* level are very accurate and that absorption energies can be employed to predict photoelectron spectra.

Electronic structure of conducting organic polymers: insights from time‐dependent density functional theory

“…An additional issue with DFT is that all properties depend too strongly on system size. Thus, the decrease in IPs with increasing oligomer length is overestimated which means that IPs of long oligomers and polymers are too low, even with functionals that predict IPs of small molecules correctly . This problem can also be addressed with certain range‐separated hybrid functionals …”

“…As mentioned previously, the first IP can be calculated with the ΔSCF method as the energy difference between energies of neutral molecule and cation. The higher IPs can be obtained with TDDFT by calculating excited states of the cations, selecting the states that correspond to the ions that have the hole in the appropriate orbitals (i.e., the states that transfer β‐electrons from lower‐lying orbitals to the semi‐occupied molecular orbital (SOMO)), and adding the excitation energies to the first IP . Comparison of excited state energies of cations with orbital energies has revealed that DFT with certain range‐separated hybrid functionals produces accurate orbital energies for valence and innervalence electrons down to around −30 eV …”

“…Comparison of bond length in the thiophene monomer at CCSD/6‐311G* and MP2/6‐311G* reveals that the main reason for the large differences in theoretical results is that the associated energy differences are very small. For instance, CCSD/6‐311G* and MP2/6‐311G* predict quite different BLA values, 0.068 Å and 0.040 Å, respectively but recalculation of the energy with CCSD at the MP2 geometry led to an increase by only 0.04 kcal/mol …”

“…Thus, the decrease in IPs with increasing oligomer length is overestimated which means that IPs of long oligomers and polymers are too low, even with functionals that predict IPs of small molecules correctly. 53 This problem can also be addressed with certain range-separated hybrid functionals. 54 A special kind of range-separated hybrid functionals are the so-called ' -tuned' range-separated hybrids, [55][56][57] where is a flexible parameter that determines at which distance from the nucleus the amount of HF exchange starts to increase.…”

Ferrocene Orientation Determined Intramolecular Interactions Using Energy Decomposition Analysis

“…Although many recent quantum mechanical studies using high level post-HF methods, such as Møller-Plesset perturbation theory (MP2), coupled-cluster singles/doubles (CCSD) and coupled-cluster triples CCSD(T) in combination with a number of large basis sets such as TZV2P+f [17] and using various Density Functional Theory (DFT) models including BHLP, B3LYP, BLYP, BP86, local spin-density approximation (LSDA) [26] and BPW91 [13] models, the puzzle of E-Fc or S-Fc remains. Nevertheless, all consistently suggest that the eclipsed Fc exists in equilibrium as the minimum structure of Fc [2,17,18,20,27]. Studies using the B3LYP/m6-31G(d) model indicated that in addition to the quantum mechanical method, the basis set needs to including more d-functions of Fe [2,3].…”