SYNTHESIS AND CHARACTERIZATION OF MIXED OLIGOHETEROCYCLES BASED ON END-CAPPED OLIGOTHIOPHENES

A homologous series of mixed oligoheterocycles 2-6, based on the end-c apped quinquethiophene EC5T 1, was synthesized by the introduction of electronegative heteroatoms, such as oxygen and nitrogen, into the con jugated Jc-system. This led to novel structures in which the central t hiophene unit of the parent compound 1 is substituted by other heteroc ycles (thiazole, 1,3,4-thiadiazole, furan, oxazole, Id,4-oxadiazole) t hat have a more pronounced acceptor character. The characterization of the optical and electrochemical properties clearly reveal the influen ce of the heteroatoms on the electronic properties. The electron-withd rawing character of the central heterocycles renders the oxidation of the oligomer more difficult while reduction is facilitated. Moreover, in some cases a hypsochromic shift of the longest-wavelength absorptio n and emission is observed along with a significant enhancement of the fluorescence quantum yield in solution and in the solid state. The HO MO/LUMO energy difference, determined from optical and electrochemical measurements, corresponds qualitatively well with the values obtained from semiempirical calculations. An X-ray structural determination wa s performed on oligoheterocycle 6 and the experimental and calculated data are compared.