Protonation and Subsequent Intramolecular Hydrogen Bonding as a Method to Control Chain Structure and Tune Luminescence in Heteroatomic Conjugated Polymers

We report the effects of protonation on the structural and spectroscopic properties of 1,4-dimethoxy-2,5-bis(2-pyridyl)benzene (9) and the related AB coploymer poly{2,5-pyridylene-co-1,4-[2,5-bis(2-ethylhexyloxy)]phenylene} (7). X-ray crystallographic analysis of 9, 1,4-dimethoxy-2,5-bis(2-pyridyl)benzene bis(formic acid) complex 10, and 1,4-dimethoxy-2,5-bis(2-pyridinium)benzene bis(tetrafluoroborate salt) (11) establishes that reaction of formic acid with 9 does not form an ionic pyridinium salt in the solid state, rather, the product 10 is a molecular complex with strong hydrogen bonds between each nitrogen atom and the hydroxyl hydrogen in formic acid. In contrast, reaction of 9 with tetrafluoroboric acid leads to the dication salt 11 with significant intramolecular hydrogen bonding (N−H···O−Me) causing planarization of the molecule. The pyridinium and benzene rings in 11 form a dihedral angle of only 3.9° (cf. pyridine−benzene dihedral angles of 35.4° and 31.4° in 9, and 43.8° in 10). Accordingly, there are large red shifts in the optical absorption and emission spectra of 11, compared to 9 and 10. Polymer 7 displays a similar red shift in its absorption and photoluminescence spectra upon treatment with strong acids in neutral solution (e.g. methanesulfonic acid, camphorsulfonic acid, and hydrochloric acid). This is also observed in films of polymer 7 doped with strong acids. Excitation profiles show that emission arises from both protonated and nonprotonated sites in the polymer backbone. The protonation of the pyridine rings in polymer 7, accompanied by intramolecular hydrogen bonding to the oxygen of the adjacent solubilizing alkoxy substituent, provides a novel mechanism for driving the polymer into a near-planar conformation, thereby extending the π-conjugation, and tuning the absorption and emission profiles. The electroluminescence of a device of configuration ITO/PEDOT/polymer 7/Ca/Al is similarly red-shifted by protonation of the polymer.