Regioregular poly(thiophene-3-alkanoic acid)s: water soluble conducting polymers suitable for chromatic chemosensing in solution and solid state

Regioregular polythiophenes with pendant carboxylic acid functionality, poly(thiophene-3-propionic acid) (PTPA, ) and poly(thiophene-3-octanoic acid) (PTOA, ), were prepared as water soluble conducting polymers and their chemosensory response was studied. Treatment with aqueous base generated intensely colored water soluble conducting polymer salts, with the color varying both as a function of counter ion size and length of the carboxyalkyl substituent. PTPA showed a different colorimetric response to each of the alkali earth metals whereas the longer chain PTOA was only sensitive to ions larger than Et4N+. Distinct color changes were also noted in studies of divalent cations known to selectively bind carboxylates, such as Zn2+, Mn2+, and Cd2+. Cast films of PTPA have been found to act as sensors to acid vapor (Δλmax of up to 132 nm). IR shows that the polymer self-assembles upon HCl vapor exposure, and that the process is completely reversible, leading to a good solid-state sensor for acids. This work demonstrates that regioregular polythiophenes containing acid side-chains have tremendous potential in the development of new sensors.