Color in Poly(3,4-ethylenedioxythiophene) with Profound Implications for Electronic, Electrochemical, and Optical Functions

Abstract

Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been synthesized by a facile electropolymerization route by using poly(diallyldimethylammonium) chloride (PDDA) as the counter-ion source. To enhance their efficacy, the fullerene derivative N-methyl fulleropyrrolidine (N-FP) was embedded in the PEDOT/PDDA films; the electron-conducting ability of the N-FP came to the fore, as the conductivity, optical contrast, and ion-storage capacity were greater in the PEDOT/PDDA/N-FP film than in the PEDOT/PDDA film. Both the PEDOT/PDDA and PEDOT/PDDA/N-FP films showed an unprecedented dramatic digression from the expected optical response of conventional PEDOT, exhibiting distinct π–π* absorptions in the visible region in air, corresponding to a bandgap of 1.1–1.3 eV, which is outside the established range (1.6–1.7 eV). The neutral state of these films showed split components, which was simultaneously accompanied by a reversible color change from bright blue (in oxidized form) to deep brown (in reduced form). This is a most unusual color transition for PEDOT, as it opposes the well-established colors that vacillate between dark blue (reduced) and sky blue (oxidized) tones. Atomic force microscopy and Kelvin probe force microscopy provided evidence for the higher nanoscale current-carrying capacity and lower localized work function for PEDOT/PDDA/N-FP than for PEDOT/PDDA; both the energetics and conductivity are conducive for fast redox switching. The serendipitous but easily reproducible synthesis method and results for PEDOT/PDDA and PEDOT/PDDA/N-FP pave the way for the utilization of this material for electronic, electrochemical, and optical functions. This is different from what is currently known about the molecular-level feature control of macroscopic properties.