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

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.