Unsaturated linkages in dioxythiophene-benzothiadiazole donor-acceptor electrochromic polymers: The key role of conformational freedom

The perspective of generating new colors commonly difficult to attain such as cyan blue and greens of various tones and saturations has motivated the design of conjugated polymer finely tuned in their molecular structure to reassemble the optical features desired in the context of non-emissive electrochromic device (ECD) applications. Herein, we report on a series of soluble donor-acceptor (DA) conjugated polymers involving 3,4-dioxythiophenes (DOTs) and 2,1,3-benzothiadiazole (BTD) constructed in combination with unsaturated linkages, namely ethynylene and trans-ethylene, and compare these to their fully polyheterocyclic DA control analogues with careful emphasis on optical, electrochemical, and electrochromic (EC) properties. As confirmed by spectroelectrochemical analysis, ethynylene linkers hindered the formation of a defined bipolaronic transition in the near-IR and were thus found disruptive with respect to the EC potential of their subsequent alternating copolymers. On the other hand, the presence of trans-ethylene spacers incorporated in the DA polymeric backbones allowed further narrowing of the energy gap so that new colors distinct from that exhibited by the control polymers were accessed including saturated green, a complementary color in the realization of polymeric EC displays. Systematic spectroelectrochemical analysis of each novel DA polymer is provided that offers clear evidence of the requirement for conformational freedom and stable quinoidal geometries upon electrochemical oxidation as cathodically coloring electrochromic polymers (ECPs) are switched reversibly to their transmissive doped state.