Oligothiophenes model molecules

At present the number of oligomeric model molecules that have been analyzed for a more comprehensive study of their dynamics and spectra is very large. In the early stages of the analysis of the spectra of oligothiophenes starting from the infrared and Raman spectra published by Akimoto et al. [195] attention was focused on the dispersion of the Raman spectra as a function of CL. 

This is in full agreement with the theoretical results presented in the work of Negri and Zgierski [198] and is substantiated by recent ab initio calculations on oligothiophenes as model molecules [197]. 

In this chapter we want to focus on the applicational visions which might be developed further in the future. Different oligothiophene derivatives have been used for the production of light emitting devices (LEDs) and turned out to be valuable model molecules for a basic understanding of LED properties, e.g. the I-V characteristics [122], the role of substituents on quantum yields [23], or the influence of interfacial layers [144]. In the following we want to summarize the application of 6T for polarized electroluminescence, which was the first oligomeric material to be used for polarized LEDs [8]. 

The crystal structure of the model compound a-4T(/0-Me)4 (Scheme 11) has been investigated by Barbarella et al. [86] in comparison with its conformation in solution [87]. Crystals of a-4T(/0-Me)4 grown in hexane are monoclinic with a P2ic space group and Z = 2. As it can be seen in Figure 8, the molecules are stacked in parallel layers along the a axis similarly to unsubstituted oligothiophenes. 

Summarizing all the optical spectroscopy on isolated molecules, 6T can well be assumed to be a model compound for longer oligothiophenes and especially for polythiophene from all the experimental data as well as theoretical approaches. The similarity between 6T and polythiophene was not only revealed by the transition energies, but also appeared from many other spectroscopic experiments, e.g. photoinduced absorption (PA) spectra [91], or photoemission spectroscopy on the valence levels [92], and have been discussed elsewhere [29, 30]. 

Oligothiophenes with well defined structures have recently received a great deal of attentions not only as a model eompoimds for conducting polymers, but also as anew class of functional r-electron systems [153], Since the initial discovery of organic compounds showing metallic conductivity, for which 2000 Nobel prize in chemistry was awarded [154-156], oligo- and polythiophenes have attracted much attention as advanced molecules with practical use in electronic devices [157-160] and their potential application in field-effect transistors [161], photovoltaic devices [162] and organic electroluminescent devices [163],... 

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