Measured Davydov splitting in oligothiophene crystals

Abstract

The polarized absorption spectra of single crystals of oligothiophenes in a wide spectral range are reported. The experimental procedure is discussed, underlying several details which are relevant to obtain reliable spectra particularly for samples of increasing thickness. On the basis of these considerations, it has been possible to fully detect the transition to the upper Davydov exciton originating from the first molecular state. The position and shape of the main exciton peak in these materials are compared and discussed, taking into consideration the molecular arrangement and the longitudinal contribution which depends on the transition moment orientation. The Davydov splitting values as deduced from the experimental data at room temperature are also reported either for the first vibronic replica or for the electronic transition as a whole. The difference between the purely transverse and the measured Davydov splitting is discussed.

Exciton-phonon coupling in molecular crystals: Synergy between two intramolecular vibrational modes in quaterthiophene single crystals

“…12 Quaterthiophene is a widely studied molecular material [11][12][13][14][15][16][17][18][19][20][21] which possesses interesting properties for both applications and fundamental research, 22 since it can be regarded as a model system for many molecular materials crystallizing in a herringbone packing arrangement such as oligothiophenes 15,23 and p-oligophenylene vinylenes. 24,25 The crystallographic structure of the low-temperature 4T polymorph is monoclinic with four molecules per unit cell, whose axes are a = 6.09 Å, b = 7.86 Å, and c = 30.48 Å, the angle ␤ between a and c being 91.8°.…”

In-situ study of the interface formation in organic multilayers

“…Some peculiar features are observed here, partly different from what observed during direct deposition of 6T on KAP [12]. The first characteristics related to the growth of the 6T layers is the positive peak centered at about 3.4 eV, which we attribute to the known response of crystalline 6T crystal as well as of a 6T film deposited on KAP [12,13]. In this regard, the meaning of such a feature is analogous to the peak at about 3.6 eV discussed above for 4T (that is still visible, although slightly modified in shape and spectral position).…”

“…Both interference and birefringence have been ruled out as origin of these bands by considering the experimental conditions (predictable thickness of the layer giving interference, independence of the spectra of the presence of a second polarizer). In fact, these bands, separated by 0.19 eV, can be attributed to vibronic excitons originating from Frenkel excitons coupled to one effective vibrational mode [11,13]. This attribution suggests a very interesting interpretation: these bands come from a purely transversal response of the 6T crystalline film, probably given by a part of the 6T film oriented with a different contact plane with the 4T/KAP substrate.…”

Optical Properties of Molecular Crystals: The Effect of Molecular Packing and Polymorphism

“…First we compute the microscopic mechanical exciton states and then we consider the propagation of light in an anisotropic medium, including long-range interactions through the macroscopic dielectric tensor. This theoretical approach takes full account of all the crystal symmetries and the numerical calculations provide a fairly accurate description of the dispersion of the purely excitonic bands, allowing a quantitative comparison with experiments (Raimondo et al 2006;Silvestri et al 2009;Spearman et al 2005;Tavazzi et al 2006aTavazzi et al , 2006b.…”

“…The ac polarized replicas in the spectral region between 2.7 and 3.6 eV are built on the lower b polarized excitonic band and obtain their oscillator strength through an intermolecular Herzberg-Teller coupling to the nearly free ac polarized exciton (Silvestri et al 2009;Spano 2003Spano , 2004Spano et al 2007;Tavazzi et al 2006a). On the contrary, the b polarized exciton is very weakly allowed and it is more strongly coupled to phonons, showing a vibronic progression.…”