DFT study of conjugational electronic structures of aminoalkyl end-capped oligothiophenes up to octamers

The molecular geometries and electronic properties of a series of bis(aminoalkyl) end-capped oligothiophenes (BRnTs) were investigated by means of the density functional theory (DFT). The calculations were performed on dimers up to octamers in the neutral and ionic species using the B3LYP/6-31G(d,p) level of theory. The results obtained show that the conjugated systems in the p- and n-doped oligomers had more aromaticity, with expanded and planar chains. The calculated energy gap values between the frontier molecular orbitals for the end-capped oligomers were larger than those for the unsubstituted oligomers, in which with increase in the oligomer chain length, the conduction band gap decreased. The calculated first excitation energies of BRnTs at the TD-B3LYP/6-31G(d,p) level indicated that both doped oligomers (p- and n-type) had lower excitation energies than the neutral states, and that they displayed red shifts in their absorption spectra. Moreover, the results obtained for the natural bond orbital (NBO) analysis showed that closing the end-side oligothiophene chains with the aminoalkyl groups eased the hole or electron transfer, owning to better charge delocalization through the backbone structures of BRnTs.     

Theoretical investigation of structures and electronic states of a series of phenyl-capped oligothiophenes

The structures and electronic states of a series of phenyl-capped oligothiophenes (PnTs) and their ionic species were investigated by means of the density functional theory (DFT). The calculations were performed on the oligomers formed by n repeating units, where n ranges from 2 to 6, using the B3LYP/6-31G** level of theory. The results obtained show that the end-substitution plays a fine-tuning effect on the geometries, electronics, and excitation states. It was found that the oligomers in the doped state have more satisfactory structural and electronic characteristics for the conducting polymers. The conjugated system in the doped oligomers has more aromaticity, with expanded and planar chains. The calculated energy gap values between the frontal molecular orbitals for the PnTs indicate that with increasing the oligomer chain length, the conductive band gap decreases. The calculated ?rst excitation energies of the PnTs at the TD-B3LYP/6-31G** level reveal that the doped PnTs have lower excitation energies than the neutral states. The oligomer chains with a phenyl ring as the end-capped group display red shifts in their absorption spectra. The end-caped substituted oligothiophenes display better characteristics than the unsubstituted ones. It could be anticipated that the phenyl-caped substitution would be helpful to charge-carrier hopings between chains, and thereby, enhance the conductivity.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

Through versus cross electron delocalization in polytriacetylene oligomers: a computational analysis.

Herein, we report a systematic theoretical investigation of the molecular and electronic properties of unsubstituted polytriacetylene (PTA) and iso-polytriacetylene (iso-PTA) oligomers, which are characterized by through and cross pi-conjugation pathways, respectively. The goal of the study is to compare through versus cross conjugation on the basis of the computed molecular geometries of the neutral, anionic, and cationic species, the electron affinities, ionization potentials, excitation energies, and nonlinear optical properties for oligomers up to the nonamer. Differences in the effective conjugation length are directly related to electron delocalization in cross- and through-conjugated pathways. As in the through-conjugated oligomers, that is, the PTAs, the frontier orbitals of the iso-PTA oligomers are delocalized along the entire carbon backbone, suggesting that pi-delocalization can extend through cross-linked carbon atoms. However, in contrast to the PTA oligomers, the bond lengths remain strictly constant and the reduction of the energy gap beyond the trimer is completely due to the correlation contribution. On the other hand, in the anions and cations, the bond lengths do change significantly with increasing chain length. Therefore, oxidation or reduction of the iso-PTA oligomer appears to switch on delocalization through cross-linked carbon atoms. Obviously, the effective conjugation length is specific and depends on the observable considered.     

Dft Study of the Structural and Electronic Properties of Conducting Oligo(<Emphasis Type="Italic">p</Emphasis>-Fluorophenylthiophene)

A comprehensive theoretical study on the conducting oligomeric systems is carried out in view of their potential application in electrochemical charge storage. Density functional theory (DFT) calculations are carried out on a series of oligomers made up of 3-(p-fluorophenyl)-thiophene (FPT) to estimate the geometric and electronic structures, conjugated lengths, bandwidths, and energetic properties of polymeric systems. The calculations are performed on the dimer up to octamer chains in the ground state and both pand n-doped phases. The results obtained show that the conjugated system in p- and n-doped oligo(FPT)s has a higher distance with more planar chains with respect to their neutral forms. The band gap energy between the frontier molecular orbitals decreases dramatically for both ionic states, and approaches a low limiting value with increasing oligomer length. The charge delocalization through the monomer rings along the backbone oligo(FPT)s reveals that the p- and n-doped states had more suitable properties, reflecting the electron and hole transport characteristics for conductivity, respectively. The calculated first excitation energies for oligo(FPT)s at the time-dependent B3LYP/6-31G(d,p) level of theory indicate that both doped oligomers have lower excitation energies, which display a red shift in their absorption spectra. For polymeric systems, the evolution of ionization potential, electron affinity, electron chemical potential, molecular hardness, and thermodynamic stability is made through the extrapolated oligomer ones.     

Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers

Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaron. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory.     

Electronic structure and properties of some oligomers based on fluorinated 1H-phospholes: n- versus p-type materials

A series of oligothiophenes and novel oligophospholes, consisting of fluorinated and perfluoroarene-substituted structures, were investigated using density functional theory (DFT) methods. The study focused on the geometrical structures and electronic properties. The degree of π-conjugation in the neutral oligomers was probed by different approaches including analysis of predicted Raman spectra. The character of the charge carrier of the new substituted oligomers, either electron (n-type doping) or hole (p-type doping) transport, was predicted by comparing their properties, including the frontier orbital HOMO and LUMO, excitation, and reorganization energies, with those of their non-substituted parent oligomers. The quantum chemical DFT results are consistent with available experimental data on the oligothiophenes for both geometries and conductivity properties. The results strongly suggest that an effective way of designing new materials with n-type conductivity is to introduce electron-withdrawing groups into the oligomer backbone. Calculated results were subsequently obtained for oligomers based on 1H-phospholes, which are predicted to have potentially useful properties as novel semiconductor materials.     

Blue/red-shift of the p-Phenylene-ethynylene Oligomers Absorption Spectra

The absorption spectra of the p-phenylene-ethynylene(p-PPE) oligomers(up to n = 12) were estimated by DFT and TD-DFT methods. The effective conjugation length(ECL) of the corresponding polymer was obtained by extrapolating the first excitation energies of the oligmers to infinite chain length with an alternative exponential function. The absorption spectral red-shift mainly depends on the ?-conjugation segment of oligomers resulting from the planarization of the backbone. The excitation mechanism of the rotamer has been investigated sufficiently by analyzing the orbital density variation upon the conformational rotations around the cylindrical triple-bonded carbon which is believed to impact significantly on the optical spectrum. The calculated results further indicate that rotation about the cylindrical triple bond interrupts the conjugation of rod-like oligomers to lead an angle-dependence of the corresponding excitation energy. The results are helpful to interpret the conformational-dependent spectroscopic phenomena of p-phenyleneethynylene and derivatives oligomers and polymers observed in ensemble and single molecule spectroscopy.     

On low‐lying excited states of extended nanographenes

Low‐lying excited states of planarly extended nanographenes are investigated using the long‐range corrected (LC) density functional theory (DFT) and the spin‐flip (SF) time‐dependent density functional theory (TDDFT) by exploring the long‐range exchange and double‐excitation correlation effects on the excitation energies, band gaps, and exciton binding energies. Optimizing the geometries of the nanographenes indicates that the long‐range exchange interaction significantly improves the C C bond lengths and amplify their bond length alternations with overall shortening the bond lengths. The calculated TDDFT excitation energies show that long‐range exchange interaction is crucial to provide accurate excitation energies of small nanographenes and dominate the exciton binding energies in the excited states of nanographenes. It is, however, also found that the present long‐range correction may cause the overestimation of the excitation energy for the infinitely wide graphene due to the discrepancy between the calculated band gaps and vertical ionization potential (IP) minus electron affinity (EA) values. Contrasting to the long‐range exchange effects, the SF‐TDDFT calculations show that the double‐excitation correlation effects are negligible in the low‐lying excitations of nanographenes, although this effect is large in the lowest excitation of benzene molecule. It is, therefore, concluded that long‐range exchange interactions should be incorporated in TDDFT calculations to quantitatively investigate the excited states of graphenes, although TDDFT using a present LC functional may provide a considerable excitation energy for the infinitely wide graphene mainly due to the discrepancy between the calculated band gaps and IP–EA values. © 2017 Wiley Periodicals, Inc.     

Cyclic oligothiophenes: novel organic materials and models for polythiophene. A theoretical study

Cyclic oligothiophenes (CnT, n = 6-30, even only) in syn- and anti-conformations are studied theoretically at the B3LYP/6-31G(d) level of theory. Strain energies, ionization potentials, HOMO-LUMO gaps, bond length alternations, NICS values, and IR and Raman spectra have been studied. The properties of anti-conformers change systematically with increasing ring size and were studied in detail in neutral, radical cation, and dication forms. In syn-conformation, the oligomers lose their nearly planar ring shape and bend significantly for n > 14, and thus properties cannot be related to ring size. The HOMO-LUMO gap in C14T-syn is even lower than polythiopehene. IR and Raman spectra calculated at the B3LYP/6-31G(d) level are used to differentiate syn- from anti-conformations. The properties of cyclic oligomers are compared to those of the linear system, and cyclic oligothiophenes are revealed as good models for polythiophene. To assist the experimental study of known cyclic oligomers having dibutyl substituents on alternate thiophene rings, the corresponding dimethyl-substituted oligomers are also studied. The experimentally evaluated HOMO-LUMO gaps for alternately dibutyl-substituted cyclic oligomers match the calculated values; however, they are significantly higher than those of the unsubstituted analogues.     

Probing electron transfer dynamics at MgO surfaces by Mg-atom desorption

Ultraviolet excitation of high surface area MgO films using 4.7 eV femtosecond pulses results in neutral Mg-atom desorption with hyperthermal kinetic energies in the range 0.1-0.4 eV. The Mg-atom hyperthermal energies and power dependences are similar to those previously observed using nanosecond pulsed excitation. Femtosecond two-pulse correlation measurements reveal the existence of different dynamical paths for Mg-atom desorption. One mechanism displays a sub-150 fs time scale and involves the simultaneous or near-simultaneous transition of two electrons to a 3-coordinated Mg(2+) site. Other paths display picosecond time scales that we associate with dynamics following electron trapping at 3-coordinated Mg(2+) surface sites.     

Structural and electronic properties of polyacetylene and polyyne from hybrid and Coulomb-attenuated density functionals

The bond length alternation (BLA), the highest-occupied-lowest-unoccupied (HO-LU) orbital energy gap, and the corresponding excitation energy are determined for trans-polyacetylene (PA) and polyyne (PY) using density functional theory. Results from the Coulomb-attenuated CAM-B3LYP functional are compared with those from the conventional BHHLYP and B3LYP hybrid functionals. BLA values and HO-LU gaps are determined using both finite oligomer and infinite chain calculations, subject to periodic boundary conditions. TDDFT excitation energies are determined for the oligomers. The oligomer excitation energies and HO-LU gaps are then used, in conjunction with the infinite chain HO-LU gap, to estimate the infinite chain excitation energy. Overall, BHHLYP and CAM-B3LYP give BLA values and excitation energies that are larger and more accurate than those obtained using B3LYP. The results highlight the degree to which excitation energies can be approximated using the HO-LU gaps-at the infinite limit, this approximation works well for B3LYP, but not for the other functionals, where the HO-LU gap is significantly larger. The study provides further evidence for the high-quality theoretical predictions that can be obtained from the CAM-B3LYP functional.     

An application of RPA theory to conjugated systems in the excited states

In order to obtain a relationship between the molecular dimension and the correlation effect, RPA method has been applied to the calculation of electronic transition energies of linear polyenes. It has been found that the effect of electron correlation on the excitation energy decreases with increasing the size of molecule. The calculated oscillator strengths are remarkably improved by RPA calculation.     

Relationship between orbital energy gaps and excitation energies for long‐chain systems

The difference between the excitation energies and corresponding orbital energy gaps, the exciton binding energy, is investigated based on time‐dependent (TD) density functional theory (DFT) for long‐chain systems: all‐trans polyacetylenes and linear oligoacenes. The optimized geometries of these systems indicate that bond length alternations significantly depend on long‐range exchange interactions. In TDDFT formalism, the exciton binding energy comes from the two‐electron interactions between occupied and unoccupied orbitals through the Coulomb‐exchange‐correlation integral kernels. TDDFT calculations show that the exciton binding energy is significant when long‐range exchange interactions are involved. Spin‐flip (SF) TDDFT calculations are then carried out to clarify double‐excitation effects in these excitation energies. The calculated SF‐TDDFT results indicate that double‐excitation effects significantly contribute to the excitations of long‐chain systems. The discrepancies between the vertical ionization potential minus electron affinity (IP–EA) values and the HOMO–LUMO excitation energies are also evaluated for the infinitely long polyacetylene and oligoacene using the least‐square fits to estimate the exciton binding energy of infinitely long systems. It is found that long‐range exchange interactions are required to give the exciton binding energy of the infinitely long systems. Consequently, it is concluded that long‐range exchange interactions neglected in many DFT calculations play a crucial role in the exciton binding energies of long‐chain systems, while double‐excitation correlation effects are also significant to hold the energy balance of the excitations. © 2016 Wiley Periodicals, Inc.     

Effect of microhydration on the electronic structure of the chromophores of the photoactive yellow and green fluorescent proteins

Electronic structure calculations of microhydrated model chromophores (in their deprotonated anionic forms) of the photoactive yellow and green fluorescent proteins (PYP and GFP) are reported. Electron-detachment and excitation energies as well as binding energies of mono- and dihydrated isomers are computed and analyzed. Microhydration has different effects on the excited and ionized states. In lower-energy planar isomers, the interaction with one water molecule blueshifts the excitation energies by 0.1-0.2 eV, whereas the detachment energies increase by 0.4-0.8 eV. The important consequence is that microhydration by just one water molecule converts the resonance (autoionizing) excited states of the bare chromophores into bound states. In the lower-energy microhydrated clusters, interactions with water have negligible effect on the chromophore geometry; however, we also identified higher-energy dihydrated clusters of PYP in which two water molecules form hydrogen-bonding network connecting the carboxylate and phenolate moieties and the chromophore is strongly distorted resulting in a significant shift of excitation energies (up to 0.6 eV).     

Mechanisms of hydrolysis-oligomerization of aluminum alkoxide Al(OC3H7)3

As one of the representative superinsulating materials, the aluminum trioxypropyl Al(OC(3)H(7))(3) aerogel may be applied in launch vehicles and manned spacecrafts. In this study, the structures and hydrolysis mechanisms of the monomer, dimers, and trimers of Al(OC(3)H(7))(3) in neutral and alkaline environments were studied at the B3LYP/6-31G(d,p) level by using the CPCM solvation model to understand the fundamental chemistry of Al(OC(3)H(7))(3) hydrolysis and oligomerization. Our calculation shows that the first-order hydrolyses of the monomer and oligomers are energetically favorable in both alkaline and neutral solutions. In alkaline solutions, they are more apt to oligomerize than to hydrolyze due to high energy barriers and large binding energies in the formation of anionic species. For the oligomers under neutral condition (1) Al(OC(3)H(7))(3) is linked by four-membered Al-O rings with pentacoordinated bridging and tetracoordinated Al atoms, (2) the hydrolyzed propoxy groups will be expelled by solvent molecules, and (3) partly hydrolyzed species can condense to oligomers with bridging OH groups or O atoms.     

Radical ions and radicals in electron-irradiated acrylates and N-isopropylacrylamide: a quantum chemical study

The semiempirical quantum chemical methods MNDO, AM1 and PM3 were used to investigate the performance of the single excited configuration interaction (SCI) approximation for calculating low energy excitation energies of open-shell systems. Systematic calculations were done for eight radicals formed by reactions of H√, OH√ and e⁻_aq with various acrylates and N-isopropylacrylamide. The calculated electronic spectra show a reasonable correlation with experimental data for both neutral radicals and radical ions. The AM1 as well as the PM3 formalism can be successfully applied to calculate the low energy excited states of these types of open shell systems. The best correlation between experimental and calculated excitation energies was obtained using the PM3 method (correlation coefficient 0.96, overall average error 0.16 eV).     

Energy-consistent relativistic pseudopotentials for the 4d elements: atomic and molecular applications

Recently reported energy-consistent relativistic pseudopotentials have been used with series of matching correlation consistent basis sets in benchmark calculations of various atomic and molecular properties. The basis set convergence of the 4d metal electron affinities and 5s2-->5s0 excitation energies are reported at the CCSD(T) level of theory, and the effects of valence and 4s4p correlation are investigated. In addition the impact of correlating the low-lying 3d electrons was also studied in all-electron Douglas-Kroll-Hess (DKH) calculations, which also included the ionization potentials and 5s2-->5s1 excitation energies. For all four atomic properties, higher order coupled cluster calculations through CCSDTQ are reported. The final calculated values are generally all within 1 kcal/mol of experiment. A notable exception is the ionization potential of Tc, the currently accepted experimental value of which is suggested to be too high by about 3 kcal/mol. Molecular calculations are also reported for the low-lying electronic states of ZrO and RuF, as well as the ground electronic state of Pd2. The effects of spin-orbit coupling are investigated for these cases in pseudopotential calculations. Wherever possible, the pseudopotential results have been calibrated against DKH calculations with correlation consistent basis sets of triple-zeta quality. In all cases the calculated data for these species are in very good agreement with experiment. In particular, the correct electronic ground state for the RuF molecule (4Phi92) was obtained, which was made possible by utilizing systematic sequences of correlation consistent basis sets.