Conjugated oligomers based on various combinations of thiophene and 3,4-ethylenedioxythiophene (EDOT) moieties have been synthesised. Comparison of the optical and electrochemical properties shows that the introduction of a bis-EDOT core in the middle of the system produces a decrease of the HOMO-LUMO gap which is attributed to an enhancement of the planarity and rigidity of the pi-conjugated system. (C) 2000 Elsevier Science Ltd. All rights reserved.