Sensitive Thermal-Undoping Characteristics of the Self-Acid-Doped Conjugated Conducting Polymer Poly[2-(3′-thienyl)ethanesulfonic acid]

The structure and properties of the poly[2-(3′-thienyl)ethanesulfonic acid] (P3TESH) treated at temperatures from 20 to 70°C were investigated by thermal analysis, ultraviolet-visible-near-infrared and infrared spectroscopies, electron spin resonance, solid-state ¹³C nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and conductivity measurements. P3TESH is susceptible to thermal undoping at low temperatures (40°C for thin solid film). Upon undoping, the color of P3TESH changes from the original brownish green to brownish orange. The thermal undoping is irreversible due to a permanent structure change resulting from the nucleophilic attack of the -CH₂CH₂SO₃^- side chain at carbocations to yield C-O bonding and a coupling of the unpaired electrons in polarons on the main chains. As the solid film is subjected to undoping, its conductivity changes drastically from 10^-2 to 10^-7 S/cm. The sensitivity of P3TESH's color and conductivity changes are high compared to those of other conjugated polymers. Such characteristics allow P3TESH to be useful for the permanent recording of a temperature increase in the low-temperature range.