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 α-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 N_O2T₃NO₂. 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.