p-Type Semiconducting Ladder Poly(pyrrolobenzothiazine)s: Effects of N-Alkyl Side Chains on the Chain Conformation, Electronic Structure, and Charge Transport Properties

In this work, we show how N-alkyl substitution affects the chain conformation, electronic structure, and optical and charge transport properties of π-conjugated ladder poly(pyrrolobenzothiazines)s (LPBTs). We found that the π-conjugated backbones of the LPBTs have a donor–acceptor motif, which enabled a small bandgap of 1.5 eV that is unchanged by N-alkyl substitution. We found that partial protonation of the LPBTs in acid solutions resulted in increased backbone flexibility evidenced by thermochromism in solution and planar/nonplanar chain conformational variation with degree of protonation that we saw in density functional theory (DFT) calculations. The average field-effect hole mobility increased from 1.3 × 10–3 cm2/(V s) in LPBT-Me to 3.1 × 10–3 cm2/(V s) in LPBT, which can be explained by the increased crystallinity and decreased lattice disorder in LPBT. The results of our investigations of the solution and solid-state properties of the two ladder poly(pyrrolobenzothiazine)s provide new insights into the structure–property relationships of π-conjugated ladder polymers.