Effect of end substitution on electrochemical and optical properties of oligothiophenes

A series of oligothiophenes (from bithiophene to sexithiophene) substituted at the ends of the chain by various electron donor of acceptor groups or atoms (methoxy, nitro, or bromo) have been prepared. Their electrochemical oxidations have been studied using microelectrodes in order to determine accurately the formal redox potential of the radical-cation generation. The stability of the latter can be deduced from the required potential scan rate. Electron donor substituents (methoxy and bromo) stabilize the radical-cation forms, and the dications can also be obtained from these substituted terthiophenes. The optical properties of the compounds have also been investigated. Only the nitro-substituted group induces a significant increase of the fluorescence quantum yield and of the lifetime of the excited state. For solution study, this positive effect is only limited to the short oligomeric chains for which an intramolecular charge transfer is evidenced by solvatochromic effects. 21 refs., 5 figs., 3 tabs.