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 [PdCl2(PPh3)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 Eg 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.