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 deposit ed onto platinum and gold surfaces, leading to a densely packed layer as confirmed by contact angle measurements. Analysis of the electroche mical behavior of this layer shows that an irreversible coupling react ion occurs during the first positive scan, which leads to a longer con jugated species, as indicated by its highly reversible redox cycle at a lower potential. This newly formed species, when desorbed from the p latinum surface, has been characterized by fluorescence spectroscopy, which also confirms that the obtained conjugation length corresponds t o that of a quinque- or a sexithiophene. Discussion of various possibl e reaction schemes leads us to propose that the electrogenerated radic al cations of terthiophene units dimerize through an intermolecular co upling within the self-assembled monolayer, occurring at the beta' pos itions of the thiophene units. Although not yet well recognized, such beta, beta'-coupling is compatible with electron delocalization. This intermolecular dimerization on beta' positions of thiophenes is confir med by the fact that the terminal alpha-CH bonds are still present, an d can undergo further electrochemical coupling with thiophene derivati ves introduced in the electrolyte. (C) 1998 Elsevier Science S.A. All rights reserved.