Increase of conjugation length within SAMs of terthiophene-derivatives, through electrochemical intermolecular coupling

Self-assembled monolayers of 6-(2,2′:5′,2″-terthien-5yl)-hexanethiol, containing the electroactive terthiophene subunits, have been deposited onto platinum and gold surfaces, leading to a densely packed layer as confirmed by contact angle measurements. Analysis of the electrochemical behavior of this layer shows that an irreversible coupling reaction occurs during the first positive scan, which leads to a longer conjugated species, as indicated by its highly reversible redox cycle at a lower potential. This newly formed species, when desorbed from the platinum surface, has been characterized by fluorescence spectroscopy, which also confirms that the obtained conjugation length corresponds to that of a quinque- or a sexithiophene. Discussion of various possible reaction schemes leads us to propose that the electrogenerated radical cations of terthiophene units dimerize through an intermolecular coupling within the self-assembled monolayer, occurring at the β′ positions of the thiophene units. Although not yet well recognized, such β, β′-coupling is compatible with electron delocalization. This intermolecular dimerization on β′ positions of thiophenes is confirmed by the fact that the terminal α-CH bonds are still present, and can undergo further electrochemical coupling with thiophene derivatives introduced in the electrolyte.