Marcin Andrzejak

Theoretical calculation of the electro-absorption spectrum of the α-sexithiophene single crystal

An extended two-dimensional analogue of the Merrifield model of the mixing between Frenkel and charge-transfer excitons is used to calculate the electro-absorption spectrum of the α-sexithiophene single crystal. The model reflects the symmetry of the crystal and takes into account all the major interactions between the molecules. The input parameters are estimated from independent quantum-chemical and micro-electrostatic calculations. The simulated spectrum is in very good agreement with experiment, both in shape and in absolute amplitude. The results demonstrate that the eigenstates of the crystal between 2.55 and 2.85 eV are primarily of charge-transfer parentage, so that charge-transfer contributions dominate the electro-absorption spectrum in that region. This first successful reproduction of the electro-absorption spectrum of a single crystal is a stringent test of the theoretical description that confirms its validity.

Vibronic interpretation of the low-energy absorption spectrum of the sexithiophene single crystal

The classic model of vibronic coupling in dimers and tetramers is used for calculating the vibronic fine structure of the lowest-energy electronic transition in the sexithiophene crystal. Good semiquantitative agreement with experiment is achieved, lending credence to the applied model. The observed intensity pattern is interpreted in terms of the deviations from the adiabatic approximation in closely spaced electronic states. The intramolecular Herzberg–Teller corrections are of lesser importance, but are the probable cause of the minor discrepancies in the simulated spectra. The results suggest a modified interpretation of the experimental fluorescence spectra.

Quantum chemical results as input for solid state calculations: charge transfer states in molecular crystals ☆

The difference I−A between the ionization potential and electron affinity of a molecule is one of the input parameters necessary for calculating the energies of charge transfer states in molecular crystals. In the present paper, the performance of the DFT methodology with several alternative forms of the exchange-correlation potential is tested in this context. Calculations are carried out for polyacenes to select the best potentials and to set up a scheme for correcting systematic errors of the approach, which is subsequently used to calculate the relevant quantity for sexithiophene and perylenetetracarboxylic dianhydride (PTCDA). Combined with independent calculations of the electrostatic energy of charge pairs in these crystals, the results are confronted with CT state energies observed in electro-absorption spectroscopy.

Vibronic Effects in the 11Bu(11B2) Excited Singlet States of Oligothiophenes. Fluorescence Study of the 11Ag(11A1) ← 11Bu(11B2) Transition in Terms of DFT, TDDFT, and CASSCF Methods

A combined DFT/TDDFT approach has been applied for calculating the Huang-Rhys (HR) parameters along the totally symmetric normal coordinates for the 1(1)A(g)(1(1)A(1)) <--1(1)B(u) (1(1)B(2)) electronic transition in a series of oligothiophenes containing from 2 to 6 thiophene rings. The calculations required optimized molecular geometries for both the ground state and the excited molecular state. The excited state geometry optimization was carried out by means of the time-dependent density functional theory (TDDFT) based methodology implemented in the Turbomole 5.9 (1) package of programs. The results for the three smallest oligothiophenes were verified by generating the theoretical vibronic structures and comparing them with the high-resolution fluorescence spectra measured for matrix-isolated molecules. For bithiophene a comparison was also made of the theoretical results obtained for different basis sets and the most popular exchange-correlation functionals. The best results were then confronted with the HR parameters based on the molecular geometries calculated at the CASSCF level of theory. The results obtained within the DFT/TDDFT approach are in very good agreement with the available experimental data for bithiophene, terthiophene, and quaterthiophene molecules.

Calculation of refractive indices and local electric field tensors in α-sexithiophene crystal

Polarizabilities calculated for sexithiophene using Hartree–Fock and density-functional approaches with a 6-31G∗ basis set are used to calculate refractive indices and local electric field tensors in the α-sexithiophene crystal. The calculations under-estimate the (incomplete) experimental refractive indices but polarizabilities are devised that fit them. These predict that the unknown refractive index perpendicular to the crystal bc plane is larger than those in the plane. The tilted molecular long axis produces sizeable off-diagonal local-field components in the crystal ac plane. This complicates the analysis of electro-absorption spectroscopy.

Fourier transform infrared and Raman and surface-enhanced Raman spectroscopy studies of a novel group of boron analogues of aminophosphonic acids.

Five analogues of a novel group of boron derivatives of aminophosphonic acids-N-benzylamino-(3-boronphenyl)-S-methylphosphonic acid (m-PhS), N-benzylamino-(4-boronphenyl)-S-methylphosphonic acid (p-PhS), N-benzylamino-(2-boronphenyl)-R-methylphosphonic acid (o-PhR), N-benzylamino-(3-boronphenyl)-R-methylphosphonic acid (m-PhR), and N-benzylamino-(4-boronphenyl)-R-methylphosphonic acid (p-PhR)-were studied using Fourier transform infrared (FT IR), Fourier transform Raman (FT RS), and surface-enhanced Raman (SERS) spectroscopies. Analysis of obtained FT IR and FT RS spectra show that all investigated compounds in the solid state exist as dimeric species formed by an H-bonding interaction between -B(OH)(2) moieties of each monomer. In addition, comparison of the wavenumbers, intensities, and broadness of bands from the FT Raman and SERS spectra allowed information to be obtained regarding the adsorption geometry of the investigated compounds immobilized onto an electrochemically roughened silver substrate.

Polarization energy calculations of charge transfer states in the α-sexithiophene crystal

Polarization energy calculations have been performed for the low-temperature polymorph of the α-sexithiophene crystal by means of the Fourier transform method in the submolecule approximation. The sensitivity of the results to the changes of the dielectric tensor anisotropy has been tested, since this latter quantity is not known experimentally. Good stability of the results suggests that they may be used in the interpretation of experimental electroabsorption spectra.

The role of the long-range exchange corrections in the description of electron delocalization in aromatic species

In this article, we address the role of the long‐range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange‐correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long‐standing debate in the literature. The presented results indicate that the noncorrected exchange‐correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteria.

Davydov splitting in the sexithiophene crystal

The apparent discrepancy is analysed between the literature value of the level splitting calculated by ZINDO method for sexithiophene clusters and the Davydov splitting calculated for the three-dimensional crystal of α-sexithiophene in the dipole approximation with the transition moment distributed over the six rings. Consistency is restored by identifying the relevant interactions that are not taken into account in the cluster approach. The results suggest that the band in the α-sexithiophene absorption spectrum, previously assigned to the upper Davydov component, might be due to charge-transfer (CT) states; other evidence pointing to the same conclusion is reviewed.

Mesoscopic disorder in thin film spectra: absorption spectroscopy of sexithiophene

Abstract In order to simulate disorder effects in thin films, model calculations of the absorption spectrum are performed for a statistical ensemble of differently oriented sexithiophene microcrystals. The assumed distribution of crystallite orientations is based on available experimental data. The directional dependence of the non-analytic part of dipole lattice sums is identified as the leading broadening mechanism; other contributions to broadening are also discussed.