Piotr Petelenz

Reconsideration of the electroabsorption spectra of the tetracene and pentacene crystals

Abstract The method of direct resolution of an electroabsorption spectrum into a set of second derivatives of Gaussian bands is applied to the spectra of tetracene and pentacene reported by Sebastian et al. The present fit is much better than in the original paper. The method of Hamilton R factors is used to assess further refinements of the fit. The results suggest the existence of an additional charge transfer (CT) band in the tetracene spectrum, located at 2.77 eV, and may indicate the existence of a few new bands in the pentacene spectrum. The results are interpreted in terms of a theoretical model proposed previously for anthracene and generalized here for triclinic cystals. Based on this model, it is suggested that CT interactions may strongly contribute to the Davydov splitting in tetracene and pentacene. The effect of sample inhomogeneity on electroabsorption spectra is discussed.

Theoretical interpretation of the electroabsorption spectrum of the anthracene crystal

Abstract An attempt is made to calculate the second-derivative contribution to the electroabsorption signal of the anthracene crystal. A model proposed previously for the description of charge transfer (CT) states is modified to include some effects of the coupling of excitons with intramolecular vibrations. It is argued that only the CT states from the edge of the Brillouin zone ( k = ±π) can contribute to electroabsorption, and even those are forbidden unless accompanied by phonons. The results of such an approach are in good agreement with experiment, which can be further improved in the future by ameliorating some approximations of the treatment. They seem to indicate that the “optical” band gap in anthracene is 4.25 eV, rather than 4.42 eV as supposed previously in the literature. This considerably reduces the discrepancy between the “optical” and “electrical” band gaps of this crystal.

THEORETICAL EVALUATION OF THE FREQUENCY AND INTENSITY OF LOW-ENERGY CHARGE-TRANSFER TRANSITIONS IN AROMATIC HYDROCARBON CRYSTALS. I. ANTHRACENE *

Abstract Calculations are reported for the low-energy part of the charge-transfer (CT) spectrum of anthracene. It is shown that the CT states previously obtained from an electrostatic model are split by intracell and intercell electron and hole transfer integrals. The model chosen for detailed calculations consists of a quasilinear crystal with two molecules per unit cell and only nearest-neighbour interactions. Parameter values are derived from earlier electrostatic and quantum-chemical calculations and from spectroscopic observations. The resulting energy levels and band intensities are compared directly with the experimental electromodulation spectra of anthracene. It is shown that, contrary to an earlier analysis, these spectra show evidence for four rather than three CT bands in the range 3.50–3.80 eV, located at 3.528, 3.590, 3.658 and 3.760 eV. Calculated CT energies and relative intensities compare favourably with these experimental results, leading to unambiguous assignments of the bands as components of the ( 1 2 1 2 0) and (010) CT states. These assignments imply predictions about the polarization of these bands which remain to be tested experimentally.

Reinterpretation of charge-transfer bands observed in the electroabsorption spectra of solid aromatic hydrocarbons

Abstract The second-derivative bands observed in the electroabsorption spectra of solid aromatic hydrocarbons are reinterpreted. The usual interpretation in terms of charge-transfer transitions can only hold in the presence of a perturbation that induces permanent dipole moments in charge transfer states. The possibility that imperfections induce such moments is investigated. Alternatively, the moments could possibly be induced by phonons.

Triplet exciton mobilities in weak donor-acceptor complexes

Abstract The coupling between neutral and charge-transfer triplet excitons is considered from the viewpoint of its possible influence on triplet exciton migration in alternating stack donor-acceptor complexes. The results are applied for the anthracene-TCNB crystal.

Reconsideration of the electroabsorption spectrum of the anthracene crystal

Abstract A non-linear least-squares procedure is applied directly to resolve the electroabsorption spectrum of anthracene into individual bands. Statistical tests and recent theoretical results are used to determine the number of bands. The band at around 3.68 eV is confirmed and two new bands are detected at 3.842 and 4.023 eV.

Recombination of charge carriers in anthracene

Abstract A simple model of radiationless transitions proposed previously is used to rationalize recent experimental results concerning the magnetic field dependence of delayed fluorescence of the anthracene crystal, induced by high-energy radiation. It is suggested that the charge transfer (CT) state responsible for the observed effects is the (1, 0, 1) state.

Vibronic coupling in excited charge-transfer states of solid-state charge-transfer complexes

Abstract Symmetry properties of CT excited states of some weak donor-acceptor complexes are discussed in the context of vibronic coupling with intermolecular vibrations. The results are applied to the analysis of electroabsorption spectra of anthracene-PMDA.

Vibronic coupling in charge transfer complexes. II

Abstract Vibronic coupling in excited charge transfer states of crystalline donor—acceptor complexes is reconsidered in relationship to recent electromodulation experiments for anthracene—PMDA. It is demonstrated that vibronic coupling, both linear and quadratic, may substantially affect the electric field dependence of the eigenstates, which may explain, at least semiquantitatively, the observed behaviour. Some consequences for experimental determination of dipole moments by means of electromodulation spectroscopy are pointed out.

Widths of excited states of the Wannier-exciton-ionised-donor complex in CdS

The absolute half widths of the excited states of the Wannier-exciton-ionised-donor complex in CdS due to their decay into free-exciton and free-hole continua are calculated. The computed half width of the 4S state of the complex is in very good agreement with the experimental value for the only observed excited state. The results strongly support the authors earlier interpretation which rationalised the absence of the 2S and 3S states of the complex in experimental spectra in terms of the short lifetimes of these states.