Barbara Pac

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 ...

Charge-transfer exciton band structure in the fullerene crystal-model calculations

A three-dimensional analog of the Merrifield model of the coupling between Frenkel and charge-transfer (CT) excitons of the C60 fullerene crystal is presented. The model is based on the nearest-neighbor approximation for the off-diagonal interactions and is essentially nonempirical. The parameters are estimated from free-molecule data and from the results of theoretical calculations published by other authors. The band structure of CT excitons in C60 is calculated and discussed in the context of available experimental evidence. Residual ambiguities in the parametrization are removed by calculation of the electroabsorption spectrum and comparison with experiment, as described in the accompanying paper.

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.

Charge-transfer states and the band gap in crystalline fullerene

Abstract Fourier-transform calculations of the electrostatic polarization energies are reported for isolated charges and low-energy charge transfer states of the fcc of crystalline fullerene. The results essentially confirm the earlier semi-intuitive estimates, and suggest a band gap of 3.0 ± 0.1 eV. The results are comprehensively discussed in the context of the recently published experimental evidence.

Is dipole moment a valid descriptor of excited state's charge-transfer character?

In the ongoing discussion on excited states of the pentacene crystal, dipole moment values have been recently invoked to gauge the CT admixture to excited states of Frenkel parentage in a model cluster. In the present paper, a simple dimer model is used to show that, in general, the dipole moment is not a valid measure of the CT contribution. This finding eliminates some apparent disagreement between the computational results published by different research groups. The implications of our results and other related aspects of cluster-type quantum chemistry calculations are discussed in the context of the standing literature dispute concerning the mechanism of singlet fission in the pentacene crystal, notably the role of charge transfer contributions vs the involvement of an excimer-like doubly excited intermediate (D state).

Lowest Singlet Exciton in Pentacene: Modern Calculations versus Classic Experiments

Based on simple model calculations, the expected magnitude of the field-induced shift observable in electroabsorption is estimated for three alternative assignments proposed in the literature for the lowest singlet excitation of the pentacene crystal (pure Frenkel exciton, pure charge-transfer exciton, or a mixture of both). The results are compared with the corresponding experimental value, which is also known from the literature. The latter turns out to be compatible only with the mixed parentage of the pertinent state, which contains the charge-transfer contribution in the range from 25 to 70%. The conclusion is discussed in the context of the present controversies concerning the existing experimental and theoretical evidence on this subject.

Quasi‐spherical model for the calculation of rotationally averaged charge transfer integrals in the fullerene crystal

Following an inspiration from perimeter model calculations, a quasi-spherical model is applied for the estimation of the charge transfer (CT) integrals in the rotationally disordered high-temperature FCC phase of crystalline fullerene. Based on the LCAO representation of fullerene π-orbitals, the leading terms in the exchange hybrid integrals are identified, approximated and averaged over molecular orientations. The orientational averaging is facilitated by introducing a spherical analogue of π-type Clementi orbitals. ‘Effective’ values of the integrals, implicitly accounting for orbital degeneracy, are evaluated. For the (½, ½, 0) nearest neighbours the integrals compare favourably with previous estimates; the approach provides an easy way to calculate them for other crystallographic positions.

Temperature dependence of the spectral profile and total intensity of the second-harmonic signal of the fullerene crystal.

The temperature dependence of second-harmonic generation in crystalline fullerene is simulated based on a microscopic model of orientational disorder, with special emphasis on the vicinity of the phase transition. The temperature-induced changes of the total second-harmonic intensity and of the corresponding spectral profile are treated on equal footing within a consistent theoretical framework. In addition to the previously applied approach where superexchange-type exciton transfer via the charge transfer (CT) manifold was described phenomenologically, and alternative model invoking the CT states explicitly is proposed, giving deeper insight into the underlying physics. Both models yield very good agreement with experimental results.

Theoretical interpretation of the high-energy electro-absorption spectrum of crystalline fullerene

The microscopic model used previously for interpreting the low-energy electro-absorption (EA) spectrum of the fullerene crystal is applied to reproduce theoretically the corresponding signal at 4.5 eV. It is demonstrated that the spectral feature observed at that energy, in spite of its shape resembling that located at 3.7 eV and attributed to CT excitons, is in fact due to a Frenkel state.