An ambipolar behavior of novel ethynyl-bridged polythiophenes—A comprehensive study

Abstract The article contains a comprehensive study regarding the synthesis and properties of novel polythiophenes, where bithiophene units are linked with either ethynyl or bisethynylarylenyl bridges. A convenient and selective method for lithiation of 2,2′-bithiophene, with the aid of n-butyllithium in dialkyl ether/n-hexane solution under mild conditions (−5 to 5 °C), is also presented. The lithiation, followed by the addition of iodine solution at room temperature constituted a robust method towards the synthesis of 5-iodo-2,2′-bithiophene, which was then used as a key substrate in the Sonogashira-type cross-coupling reactions. The coupling of 5-ethynyl-2,2′-bithiophene with 5-iodo-2,2′-bithiophene, mediated by [PdCl 2 (PPh 3 ) 2 ]/CuI couple, resulted in the formation of 1,2-bis(2,2′-bithiophene-5-yl)ethyne. The synthesized monomers of Bt–A–Bt (where Bt = 2,2′-bithiophene-5-yl) type, namely: 1,2-bis(2,2′-bithiophene-5-yl)ethyne, 1,4-bis(2,2′-bithiophene-5-yl)buta-1,3-diyne and 1,4-bis(2,2′-bithiophene-5-ylethynyl)benzene were transformed into conducting polymers in an electrochemical oxidation process. The influence of the A bridge on the stability, luminescent and electrochemical behavior was thoroughly studied. The polymers present an extraordinary stability during multiple n- and p-doping. The experimental results were also investigated theoretically, using DFT approach. For each polymer calculations of the frontier orbital energies and the corresponding E g value were done using periodic boundary conditions. For polythiophene derived from 1,2-bis-(2,2′-bitiophene-5-yl)ethyne the value of the band gap was also estimated using an oligomeric approximation. Furthermore, the presented compounds revealed an enhanced fluorescent activity, especially in the case of a monomer with bisethynylphenylene bridge and the corresponding polymer.