Michał Slawik

Theoretical interpretation of the electroabsorption spectrum of fullerene films

A three-dimensional analogue of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal is used to calculate the electroabsorption (EA) spectrum of buckminsterfullerene films. The approach is essentially nonempirical, with most of the input parameters estimated from theoretical calculations or from independent experiments, but in doubtful cases a comparison of the calculated and experimental EA spectra guides the choice between alternative sets of parameters. Semiquantitative agreement between the calculated and experimental EA spectra lends credence to the model, which can therefore be used to draw conclusions concerning the underlying physical mechanisms. The EA signal of the CT states is shown to be substantially affected by the off-diagonal CT interactions, and by interactions with the optically inactive Frenkel excitons, whose intramolecular parentage is tentatively assigned. The results confirm the validity of previous calculations of the ...

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.

Theoretical calculations of the electroabsorption spectra of perylenetetracarboxylic dianhydride

A three-dimensional analog of the Merrifield model is proposed to study excitons in solid perylenetetracarboxylic dianhydride (PTCDA). Its relevant parameters are estimated by independent calculations, and finally determined by fitting the experimental absorption and electroabsorption spectra of PTCDA films; the optimum values obtained from the fit correspond well with the calculated values. The results justify description of PTCDA excitons within a one-dimensional model, as proposed in the literature. The calculated spectra are generally in good agreement with the experimental ones, with the exception of the directional properties of the electroabsorption signal. A possible reason for this discrepancy is suggested. Generally, the results highlight the prominent role of charge-transfer excitons in the electroabsorption spectrum of PTCDA by showing the experimental features that are difficult to explain without invoking these states.

Electro-absorption spectrum of buckminsterfullerene : evidence for the existence of charge transfer states

Theoretical considerations suggest that the buckminsterfullerene crystal may have a distinct manifold of excited charge transfer (CT) states. In the present paper, the energies of these states are estimated and their probable properties are discussed. The recently published electro-absorption spectrum of C60, originally interpreted in terms of a set of bands originating from Frenkel (intramolecular) excitations, is demonstrated to contain considerable contributions from the CT states. The CT states are located at about 2.7 and 4 eV, in excellent agreement with theoretical estimates. Some other consequences of the existence of excited CT states are predicted that are open to experimental verification.

THEORETICAL INTERPRETATION OF THE ELECTROABSORPTION SPECTRA OF POLYACENE CRYSTALS. II. CHARGE-TRANSFER STATES

In recent articles, a two-dimensional analog of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal was successfully applied for the calculation of the electroabsorption (EA) spectra of polyacene crystals. Presently, the results of those calculations are rationalized in terms of a simplified two-state model, designed specifically for interpretational purposes. Particular attention is given to the CT manifold where symmetry considerations rule out the existence of eigenstates endowed with nonzero dipole moment, seemingly contradicting the existing experimental evidence of second-derivative EA signals. The apparent contradiction is resolved by analyzing EA contributions from pairs of zero-field eigenstates coupled by the modulating field. The new approach provides a bridge to the conventional interpretation rooted in the localized model of CT excitations, and allows to determine the limits of validity of the traditional analysis of EA spectra.

Theoretical interpretation of the electroabsorption spectra of polyacene crystals. I. Role of Frenkel states

The recently proposed two-dimensional model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal, successfully applied for the calculation of the electroabsorption (EA) spectra of polyacenes [Petelenz et al., J. Chem. Phys. 105, 4427 (1996)], is applied to probe the influence of different intramolecular excited states on the EA signal in the CT energy range. The study highlights the preeminent role of Frenkel excitons that are energetically close to the CT manifold, even if they are inactive in conventional absorption spectroscopy. The high-energy intense excitons are found to have a lesser effect. The study suggests EA spectroscopy as a potential tool to detect and locate some of the absorption-inactive intramolecular excitations.

Two-dimensional model of charge transfer excitons in polyacene crystals

Abstract A two-dimensional analog of the Merrifield model of the coupling between Frenkel and charge transfer (CT) excitations of a molecular crystal is proposed and applied for the calculation of the band structure of the CT excitons in the crystals of linear polyacenes. The solutions are compared with those obtained within the previously used linear model and the limitations of the latter are discussed. Some possible applications of the new approach for the interpretation of experimental data are suggested.

Intermediate vibronic coupling in charge transfer states: Comprehensive calculation of electronic excitations in sexithiophene crystal

A comprehensive theory of linear vibronic coupling in a coupled manifold of Frenkel and charge-transfer states in an infinite molecular crystal is presented and applied for sexithiophene. The approach, valid in the intermediate-coupling regime, includes up to three-particle terms of the Philpott expansion, with the vibronic wavefunctions represented in the Lang-Firsov basis. As a stringent test, the scheme is used to reproduce the complete set of available sexithiophene absorption and electroabsorption spectra within a unified theoretical framework. The input is based primarily on independent calculations and to some extent on independent experiments, with explicit fitting contained within the limits set by the estimated inherent errors of a priori parameter estimates. Reasonably good quantitative agreement with experimental spectra is achieved. The results resolve some existing interpretational ambiguities and expose some peculiarities of electric field effect on vibronic eigenstates of Frenkel parentage, highlighting the role of charge-transfer interactions.

Microscopic calculation of the electroabsorption spectra of polyacene crystals

A two-dimensional analogue of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal is applied for the calculation of the electroabsorption (EA) spectra of polyacene crystals. The approach is essentially non-empirical, with most of the necessary input data estimated either from theoretical calculations or from independent experiments. Good agreement between the calculated and experimental EA spectra lends credence to the model and allows us to draw conclusions concerning the accuracy of band gap estimates based on polarisation energy calculations. The conclusions may be potentially useful for other molecular crystals.

Effect of the Coupling with Frenkel States on the Electroaborption Signal of Charge Transfer Excitons

Abstract The recently proposed nearest-neighbour model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal, successfully applied for the calculation of the electroabsorption (EA) spectra of polyacenes and fullerene, is applied to probe the influence of different intramolecular excited states on the EA signal in the CT energy range. The study highlights the pre-eminent role of Frenkel excitons that are energetically close to the CT manifold, even if they are inactive in conventional absorption spectroscopy. The high-energy intense excitons are found to have a lesser effect. The study suggests EA spectroscopy as a potential tool to detect and locate some of the absorption-inactive intramolecular excitations.