Conducting Poly(3,4-alkylenedioxythiophene) Derivatives as Fast Electrochromics with High-Contrast Ratios

A series of alkyl-substituted and unsubstituted poly(3,4-alkylenedioxythiophene)s were synthesized electrochemically using 3,4-alkylenedioxythiophene derivative monomers where either the size of the alkylenedioxy ring or the nature of the pendent group was varied. The specific systems studied include 3,4-ethylenedioxythiophene (EDOT), 2-methyl-2,3-dihydrothieno[3,4-b][1,4]dioxine (EDOT-Me), 2-tetradecyl-2,3-dihydrothieno[3,4-b][1,4]dioxine (EDOT-C14H29), 2-phenyl-2,3-dihydrothieno[3,4-b][1,4]dioxine (EDOT-Ph), 3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (ProDOT), 3-methyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (ProDOT-Me), 2,3,4,5-tetrahydrothieno[3,4-b][1,4]dioxocine (BuDOT), and 5,10-dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine (BuDOT-Xyl). Optoelectrochemical experiments revealed that the nature of the substitution on the polymers had little effect on the extent of conjugation of the backbone as evidenced by electronic band gaps for all polymers of approximately 1.7 eV (730 nm). These electrochromic polymers switch from a relatively transmissive light green in the oxidized form to an opaque dark blue in the reduced form, with the highest electrochromic contrast ratios accessible for PBuDOT and PEDOT-C14H29. Multiple switching studies monitoring the electrochromic contrast showed that ca. 300 nm thick polymer films could be fully switched between their reduced and oxidized forms in 0.8−2.2 s with Δ%T of 44−63%. In situ conductivity studies carried out on relatively thick polymer films (2.7−9.5 μm) deposited between large gap (200 μm) lateral growth electrodes demonstrated the low-potential turn-on for these materials, and maximum conductivities of 0.2−12.1 S/cm were attained.