Crown-annelated oligothiophenes as model compounds for molecular actuation

Crown-annelated quater- (4T) and sexithiophenes (6T) with oligooxyethylene chains of various lengths attached at the 3-positions of the terminal thiophene rings have been synthesized. Analysis of the cation-binding properties of the macrocycles by H-1 NMR and UV-vis spectroscopy reveals the formation of a 1:1 complex with Ba2+, Sr2+, or Pb2+ and shows that cation complexation results in a conformational transition in the pi-conjugated system. Theoretical analysis of this process by density functional methods predicts that this conformational transition results in a narrowing of the highest occupied-lowest unoccupied molecular orbital gap of the conjugated system with a decrease of the redox potentials (E-1(0) and E-2(0)) associated with the formation of the 4T cation radical and dication. Cyclic voltarmmetry shows that, depending on the binding constant, the presence of metal cation produces a negative or a positive shift of E-1(0) while E-2(0) always shifts negatively. This unusual behavior is discussed in terms of interplay between electrostatic interactions and conformational changes associated with cation binding.