Dielectric tensor of tetracene single crystals: The effect of anisotropy on polarized absorption and emission spectra
ABSTRACT The full UV-visible dielectric tensor and the corresponding directions of the principal axes of triclinic tetracene crystals are reported as deduced either by polarized absorption and ellipsometry measurements or by calculations based on the molecular and crystallographic data. The results allow the attribution of the polarized bands observed in both absorption and photoluminescence emission spectra. In particular, the spectral line shape and polarization of the emission are found to depend on the sample thickness, and the effect is attributed to the modification of the state of polarization of the emitted light during its propagation inside the crystal. Indeed, the directions of polarization of the lowest optical transitions and the directions of the principal axes of the dielectric tensor are demonstrated not to coincide, in contrast to the assumptions typically made in the literature, thus causing the mixed transverse/longitudinal character of light propagation.
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ABSTRACT: The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.
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ABSTRACT: Aggregation processes that occur in a solution of 3.4 × 10−5 M tetracene in decalin at 213 K and in a 1.3 × 10−4 M solution of anthracene in methylcyclohexane at 135 K show that they are second-order kinetic processes responsible for generating the corresponding dimeric structures of these acenes. The UV–Vis absorption spectra of these dimeric structures show the presence of isosbestic points. We also show that the rate constants of these processes appear to depend on the viscosity of the solvent employed. Excitonic interactions involved in these dimers are described in the light of Kasha's model.We also show that a solution of 6.7 × 10−5 of tetracene in toluene at 173 K exhibits aggregation but following a first order kinetics and that the corresponding absorption spectra describing the process at 173 K show no isosbestic points. Consequently, it is concluded that in this solvent, the tetracene does not dimerize, but it forms a complex with toluene. The strong similarity between the first band of the tetracene dimer and the tetracene/toluene complex may give rise to a strong controversy over the excitonic model to be considered. Copyright © 2014 John Wiley & Sons, Ltd.Journal of Physical Organic Chemistry 05/2014; 27(5):456-462. DOI:10.1002/poc.3283 · 1.23 Impact Factor
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ABSTRACT: We report quadruple configuration interaction calculations within the extended Pariser$-$Parr$-$Pople Hamiltonian on the excited states of aggregates of polyenes, crystalline acenes and covalently linked dimers of acene molecules. We determine the precise energy orderings and analyze the cluster wavefunctions in order to arrive at a comprehensive physical understanding of singlet fission in these diverse families of materials. Our computational approach allows us to retain a very large number of basis states, and thereby obtain the correct relative energy orderings of one electron-one hole Frenkel and charge-transfer excitons versus intra- and intermolecular two electron-two hole triplet$-$triplet excited states. We show that from the energy orderings it is possible to understand the occurrence of singlet fission in polyene and acene crystals, as well as its near total absence in the covalently linked acene dimers. As in the acene crystals, singlet fission in the polyenes is a multichromophoric phenomenon, with the well known 2$^1$A$_g^-$ playing no direct role. Intermolecular charge-transfer is essential for singlet fission in both acenes and polyenes, but because of subtle differences in the natures and orderings of the aggregate excited states, the mechanisms of singlet fission are slightly different in the two classes. We are thus able to give qualitative physical reasoning for the slower singlet fission in the polyenes, relative to that in crystalline pentacene. Our work also gives new insight to the complex exciton dynamics in tetracene crystals, which has been difficult to understand theoretically. Our large-scale many-body calculations provide us with the ability to understand the qualitative differences in the singlet fission yields and rates between different classes of $\pi$-conjugated materials.The Journal of Physical Chemistry C 02/2015; 119(6966):150217131830008. DOI:10.1021/jp5124019 · 4.84 Impact Factor