Spectroscopic properties of oligothiophenes

Oligomer molecules exhibit a benzoid structure with a single bond between the rings. This structure allows various conformers in solution at room temperature which show deviations from planarity but only small energy differences [9]. 

These properties result in the observed wide structureless absorption spectra. In contrast to the absorption the fluorescence band is well structured. This is explained by the planar quinoid structure of the fluorescence state. In quantum chemical calculations [6] the bonds between thiophene rings exhibit electron densities like double bonds. 

absorption and fluorescence bands, shift to longer wavelengths with increasing oligomer size as shown in Fig. 17.3. 

It can be seen from the absorption spectra that all oligothiophenes ( T) except 2T can be excited well with the third harmonic of a Nd YLF laser (349 nm). 2T was excited with the second harmonic of a sub-picosecond dye laser system at 308 nm [7]. 

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