Quantitatively Correct UV-vis Spectrum of Ferrocene with TDB3LYP

Cataloged from PDF version of article.The ultraviolet-visible light (UV-vis) absorption spectrum of ferrocene is modeled with time-dependent density functional theory employing LSDA, BLYP, B3LYP, and CAM-B3LYP functionals in combination with 6-31G*, 6-31+G*, CC-PVTZ, and aug-CC-PVTZ basis sets. With the exception of LSDA, all functionals predict a reasonable Fe-CP distance of similar to 4.67 angstrom. Diffuse functions are essential for the strongly allowed states at high energy but of lesser consequence for the visible range of the spectrum. Dipole forbidden states are examined with vibrationally excited structures, obtained from the normal modes of the infrared (IR) spectrum. Despite earlier claims, TDB3LYP predicts the UV-vis spectrum of ferrocene quantitatively correct. TDBLYP predicts a large number of spurious charge-transfer states, TDCAM-B3LYP and TDwB97)CD are correct in the low-energy region but overestimate the energy of strongest peak of the spectrum by 0.8 eV. The amount of charge transfer involved in "d-d transitions" is equal to that in "charge-transfer states"

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

Cataloged from PDF version of article.Conducting organic polymers (COPs) became an active field of research after it was discovered how thin films rather than insoluble infusible powders can be produced. The combination of the properties of plastics with those of semiconductors opened the research field of organic electronics. COPs share many electronic properties with inorganic semiconductors, but there are also major differences, e. g., the nature of the charge carriers and the amount of the exciton binding energy. Theoretical analysis has been used to interpret experimental observations early on. The polaron model that was developed from one-electron theories is still the most widely used concept. In the 1990s, time-dependent density functional theory (TDDFT) became available for routine calculations. Using TDDFT, electronic states of long oligomers can be calculated. Now UV spectra of neutral and oxidized or reduced species can be compared with in situ UV spectra recorded during doping. Likewise states of cations can be used to model photoelectron spectra. Analysis of states has resolved several puzzles which cannot be understood with the polaron model, e. g., the origin of the dual absorption band of green polymers and the origin of a 'vestigial neutral band' upon doping of long oligomers. DFT calculations also established that defect localization is not crucial for spectral changes observed during doping and that there are no bound bipolarons in COPs. (C) 2014 John Wiley & Sons, Ltd

mproved prediction of properties of π-conjugated oligomers with range-separated hybrid density functionals

Cataloged from PDF version of article.Range-separated hybrid functionals along with global hybrids and pure density functionals have been employed to calculate geometries, ionization energies (IP)s, electron affinities (EA)s, and excitation energies of neutral and oxidized polyenes, thiophene, and furan oligomers. Long-range correction with 100% HF exchange solves the problem of density functional theory with incorrect chain length dependence of IPs and energy gaps. There is a possibility of overcorrection, if the short-range part of the functional with no or low HF exchange is too small. The wB97XD functional with 22% of HF exchange in the short-range and a range-separation parameter of 0.2 seems to be just right for conjugated systems at all chain lengths. The wB97XD functional additionally produces negative orbital energies in very good agreement with IPs and EAs. With correct orbital energies, band gaps correspond to transport gaps (E t) and not to optical gaps (Eg). Et is much larger than Eg in the gas phase, but the difference is significantly smaller in the solid state. The accuracy of the negative orbital energies is good down to about 30 eV so that valence and innervalence PE spectra can be modeled. wB97XD is therefore suitable for calculating band structures of conjugated polymers employing orbital energies. © 2011 American Chemical Society

Koopmans' springs to life

Cataloged from PDF version of article.The meaning of orbital energies (OOEs) in Kohn-Sham (KS) density functional theory (DFT) is subject to a longstanding controversy. In local, semilocal, and hybrid density functionals (DFs) a Koopmans' approach, where OOEs approximate negative ionization potentials (IPs), is unreliable. We discuss a methodology based on the Baer-Neuhauser-Livshits range-separated hybrid DFs for which Koopmans' approach "springs to life." The OOEs are remarkably close to the negative IPs with typical deviances of +/- 0.3 eV down to IPs of 30 eV, as demonstrated on several molecules. An essential component is the ab initio motivated range-parameter tuning procedure, forcing the highest OOE to be exactly equal to the negative first IP. We develop a theory for the curvature of the energy as a function of fractional occupation numbers to explain some of the results

Modeling photoelectron spectra of conjugated oligomers with time-dependent density functional theory

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 ΔSCF 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 ΔSCF 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 ΔSCF 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. © 2010 American Chemical Society

Effects of perfluorination on thiophene and pyrrole oligomers

The effect of perfluorination on thiophene and pyrrole oligomers in neutral, cationic, and anionic states was investigated with density functional theory at the (TD)B3P86-30%/6-31G* level. For the title compounds fluorination leads to planarization. For pyrroles a band gap reduction of 0.58 eV results, as unsubstituted pyrroles are nonplanar and disordered in the solid state. For thiophene the band gap is slightly increased as long thiophene oligomers are almost planar. Ionization energies and electron affinities increase upon fluorination by 0.65 and 0.60 eV for polythiophene and by 0.45 and 0.90 eV for polypyrrole. Conduction band widths increase by 0.5 for polythiophene and by 0.7 eV for polypyrrole. Spectra of charged (doped) forms are almost identical to those of the parent systems. Like parent systems, fluorinated oligomers with chain lengths of more than six rings develop a third UV absorption that increases in strength and decreases in energy upon chain length increase. © 2010 American Chemical Society

Electronic structure analysis of a new quinoid conjugated polymer

The low lying unoccupied orbitals of oligomers of 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b'] dithiophene (CDM) are not delocalized over the whole molecule. Is such electron localization in the conduction band of poly-CDM responsible for its low n-type conductivity? Are polymers of the tricyclic thioketone (TCT) with more delocalized unoccupied orbitals a better alternative for stable n-dopable conducting polymers? Monomer through tetramer of TCT have been optimized with density functional theory. IP, EA, energy gap, and band width of the corresponding polymer were obtained by extrapolation. Comparison with data for oligomers of 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b'] dithiophene and of thiophene indicates that the novel polymer would have a small band gap and would fulfil the conditions for n-dopability and high mobility of n-type carriers. © Springer-Verlag 2000

Theoretical investigation of excited states of large polyene cations as model systems for lightly doped polyacetylene

Electronic excitations of polyene cations with chain lengths of up to 101 CH units were investigated as model systems for lightly doped polyacetylene (PA). Since high level ab initio calculations such as complete active space perturbation theory (CASPT2) are limited to systems with about 14 CH units, the performances of time-dependent Hartree-Fock (TDHF) and time-dependent density functional theory (TDDFT) were evaluated. It turned out that TDDFT excitations energies are much more accurate for polyene cations than for neutral polyenes. The difference between TDHF and TDDFT excitation energies for the first allowed excited state of C49H51 + is only 0.30 eV with pure DFT and 0.21 eV with a hybrid functional. For open-shell systems, pure DFT is found to be superior to DFT-hybrid functionals because it does not suffer from spin-contamination. Pure TDDFT excitation energies and oscillator strengths for small open-shell polyene cations compare well with high level ab initio results. Excitation energies are found to be almost independent of the geometry, i.e., the size of the defect. Localization of the defect, however, shifts oscillator strengths from the HOMO-LUMO transition to higher lying excited states of the same symmetry. Lightly doped PA is predicted to exhibit several strong absorptions below 1 eV. © 2007 American Chemical Society

Density functional theory investigation of substituent effects on building blocks of conducting polymers

Substituted heterocyclic dimers were calculated employing density functional theory (DFT) and analyzed with the natural bond orbits method (NBO). Substitution in 3- and 4-positions leads to parallel shifting of HOMO and LUMO but does not reduce energy gaps. For bridge dimers, HOMO-LUMO gaps correlate with π-electron densities in the carbon backbone and energy gap reduction correlate with the strength of π-π* interactions from the backbone to the bridging group. Alternating donor-acceptor groups do not reduce energy gaps and lead to systems with average HOMO and LUMO levels compared to the parent molecules

Theoretical investigation of excited states of oligothiophenes and of their monocations

Excitation energies of neutral thiophene oligomers with chain lengths of up to 25 rings and charged thiophene oligomers with chain lengths of up to 20 rings were calculated with time-dependent Hartree-Fock and time-dependent density functional theory (TDDFT). As recently for polyene cations, very good agreement is found between TDDFT and high-level ab initio calculations and with experimental results wherever data are available. For short thiophene oligomer cations, two sub-band transitions are predicted; for long chains, a third transition develops. Defects are found to be delocalized in bare cations; the inclusion of counterions induces localization. Despite the strong influence of counterions on the geometry, the influence of counterions on the spectra is small for the first two sub-band peaks. Since counterions are directly involved in the electron transitions contributing to the third sub-band peak of longer oligomers, the inclusion of counterions lowers the energy of this absorption peak. The agreement between theoretical spectra based on delocalized geometries and experimental spectra shows that defect localization (electron phonon coupling) is not the underlying cause of the two sub-band transitions. Investigation of the electronic configurations that contribute to the excited states does not confirm the nature of the transitions predicted with the polaron model. © 2007 American Chemical Society