Nitro-functionalized oligothiophenes as a novel type of electroactive molecular material:: Spectroscopic, electrochemical, and computational study

A novel series of terthiophenes bearing electron-donor and electron-acceptor groups at the end a-positions has been prepared. The analysis of the UV-vis, infrared, and Raman spectra, performed with the aid of density functional theory calculations, shows that the asymmetrically substituted nitro compounds PhT3NO2 and BrT3NO2 behave as push-pull systems and present an intense photoinduced charge transfer in the visible spectrum. The symmetrically substituted dinitro compound NO2T3NO2 displays a highly delocalized structure with a low single-double bond length alternation and also displays a low-energy absorption band in the visible region. The novel nitroterthiophenes possess attractive electrochemical properties since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the oligothiophene backbone and leads to the formation of stable cations even for NO2T3NO2. Reduction is mainly nitro-centered but also affects the conjugated structure. Radical anions and dianions are formed for PhT3NO2 and BrT3NO2. Dianions, not radical anions, and trianions are obtained for NO2T3NO2. Nitro-functionalized terthiophenes are shown to be very promising as electroactive molecular materials since they behave as push-pull systems, present a very intense photoinduced charge transfer in the visible region, and could act as both n- and p-channel conductors in organic electronic transistors.