β‐Substituted Terthiophene [2]Rotaxanes

Towards polythiophene polyrotaxanes : The β‐substituted terthiophene [2]rotaxanes have been synthesized (see figure). Basic optical and electrochemical properties of the synthesized [2]rotaxanes are also reported.Two kinds of β‐substituted terthiophene [2]rotaxanes were synthesized using the host‐guest pairs of the electron‐deficient cyclophane cyclobis(paraquat‐ p ‐phenylene) ( CBPQT 4+ ) and the electron‐rich terthiophenes with diethyleneglycol chains at the β‐position. One is made from the α‐position non‐substituted terthiophene ( 3 T‐β‐Rx ) and the other is made from the α‐dibromo‐substituted terthiophene ( 3 TBr‐β‐Rx ). The binding constants of the β‐substituted terthiophene threads were confirmed to be smaller than that of the α‐substituted terthiophene analogue. By UV/Vis absorption measurements, we confirmed the charge‐transfer (CT) band in the visible region with an extinction coefficient of ≈10 2 ( M −1  cm −1 ). Strong, but not quantitative, quenching of the terthiophene fluorescence was confirmed for the [2]rotaxanes. Although the β‐substituted terthiophene thread was electrochemically polymerizable, the [2]rotaxane 3 T‐β‐Rx was not polymerizable. This result indicates that the interlocked CBPQT 4+ macrocycle effectively suppresses the electrochemical polymerization of the terthiophene unit because electrostatic repulsive and steric effects of CBPQT 4+ hinder the dimerization of the terthiophene radical cations. In the electrochemical measurement, we confirmed the shift of the first reduction peak towards less negative potential compared to free CBPQT 4+ and the splitting of the second reduction peak. These electrochemical behaviors are similar to those observed for the highly‐constrained [2]rotaxanes. The β‐substituted terthiophene [2]rotaxanes reported herein are important key compounds to prepare polythiophene polyrotaxanes.