Excited State Electronic Interactions in Oligothiophenes with Novel Supramolecular Structure
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TION The careful control of the solid state structure of thin films of molecular crystals and more specifically of molecular semiconductors can be realized through Organic Molecular Beam Deposition (OMBD) in ultra high vacuum [1-3]. With this technique molecular thin films are fabricated thro ugh repeated stacking of monomolecular layers onto a properly selected substrate. The resulting supramolecular organisation of the grown film, which is determined by the orientation of the first overgrown layer with respect to the substrate, originates from the balance between the molecule-molecule and moleculesubstrate interactions. In this paper we report on the structure and on the optical properties of unsubstituted quaterthiophene (4T), grown on a single crystal of potassium acid phtalate (KAP [4]). This particular substrate has been selected because it induces an extremely high optical anisotropy in the oligothiophene overgrown layer, with important implications for both the study of fundamental optical properties in oligohiophenes [5,6], as well as for the applications related to polarized photo and electro - emission. The structural and optical data reported in the present paper data form the basis for modeling the excited states intermolecular coupling using a recently developed theoretical model.
ELECTRONIC ABSORPTION SPECTRA Fig. 1 shows the spectra of the 4T sample deposited on KAP taken under normal incidence conditions. The spectrum consists of a main absorption peak at 3.67 eV strongly polarized along the b axis of the substrate and a weak vibronic, unpolarized structure in the low energy tail of the spectrum. In the inset of Fig. 1 the absorption spectrum of our oriented thin films is shown superimposed with the spectrum of 4T molecules embedded in a channel forming h o s t compound (perhydrotriphenylene, PHTP) in which the intermolecular electronic coupling is suppressed and the absorption arises from excitations on individual molecules , whose electronic states are easily identified by their well resolved vibronic components [7]. The weak low energy vibronic structure , observed in the spectrum of the thin film, therefore reveals the presence in the sample of a small amount of orientational defects, in the form of randomly oriented, weakly interacting molecules. The main absorption peak which originates from the bulk of aggregated molecules is strongly blue shifted.of about 1 eV The polarization of the main absorption peak is consistent with a very high orientational order of the supramolecular architecture with a projection of the transition moment (and hence of the long molecular axis ) onto the substrate surface directed along the b axis. The angle formed by the molecular transition moment with the ab plane of KAP (substrate surface) can be inferred from the spectra taken with polarized light at oblique incidence. The intensity ratio r between the intensity of the peaks observed in the s and p polarization at oblique incidence, r = Ip/Is, yields the tilt angle α between the 4T molecular axis and the substr
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