Poly[3,3′-dialkyl-2,2′-(ethyne-1,2-diyl)bis(thiophene)]: Electrically conducting and fluorescent polymers incorporating a rigid acetylenic spacer

Abstract

A series of poly[3,3′-dialkyl-2,2′-(ethyne-1,2-diyl)bis(thiophene)]s, comprising a rigid carbon-carbon triple bond between two bithiophene repeating units, were synthesized. The improved rigidity of the polymer backbone led to an increased fluorescent quantum yield in comparison to poly(3-alkylthiophene)s. A generic trend depicting decreasing Stokes shift in the fluorescence spectra with increasing pendant alkyl chain length was observed. The incorporation of the acetylenic spacer also resulted in a significant red shift in the absorption spectra in comparison to poly(3-alkylthiophene)s, corresponding to an increase in effective conjugation over the entire series of polymers. These polymers, upon doping with iodine or ferric chloride, gave electrical conductivity in the range of 10° to 10-4 S cm-1. Thermochromism studies showed a blue shift in absorption peak as the temperature changes from 25 to 180°C. The influence of alkyl chain length and the acetylenic spacer on the conductivity and UV-VIS absorption is also discussed. In situ electrochemical doping studies were monitored using UV-VIS-near infrared absorption spectroscopy and showed the evolution of polaron bands at around 1.4 eV.