Synthesis of Imine-Bridged Planar Poly(pyridinethiophene)s. Combination of Planarization and Intramolecular Charge Transfer in Conjugated Polymers

This paper describes an approach to maximize π-conjugation in conjugated polymers by combining two complementary effects: polymer backbone planarization and intense intramolecular charge transfer (ICT). Independently, each of these methods is known to promote a significant lowering of the optical band gap in conjugated polymers. However, as described here, the combination of both strategies, planarization and ICT, only has a marginal effect on reducing the optical band gaps over that of the planarized materials that possess little ICT. The polymers studied here are planar poly(pyridinethiophene)s (PPyThs). The synthesis of the requisite polymers is accomplished by Pd-catalyzed coupling of N,N'-bis(tert-butoxycarbonyl)-3,4-diamino-2,5-bis(tri-n-butylstannyl)thiophene and 2,5-bis(1'-oxo)-3,6-dibromopyridines to afford PPyThs that are then planarized via Schiff base formation upon protic activation. The planar PPyThs exhibit enormous changes in their optical absorption properties when iodide is present in solution; this dramatic change is not observed with fluoride, chloride, or bromide salts, regardless of the cation. Therefore, these polymers have a propensity to optically distinguish iodide from the other halogens.