Variable Band Gap Poly(arylene ethynylene) Conjugated Polyelectrolytes

A series of poly(arylene ethynylene) (PAE) conjugated polyelectrolytes (CPEs) have been prepared using palladium-mediated (Sonogashira) coupling chemistry. The series consists of five pairs of polymers that share the same poly(arylene ethynylene) backbone. One member of each pair contains anionic sulfonate (R−SO3-) side groups, whereas the other member contains cationic bis-alkylammonium (R-N+−R-N+−R) side groups. The repeat unit structure of the poly(arylene ethynylene) backbone consists of a bis(alkoxy)phenylene-1,4-ethynylene unit alternating with a second arylene ethynylene moiety, and five different arylenes were used, Ar = 1,4-phenyl, 2,5-pyridyl (Py), 2,5-thienyl (Th), 2,5-(3,4-ethylenedioxy)thienyl (EDOT), and 1,4-benzo[2,1,3]thiodiazole (BDT). The different arylene units induce variation in the HOMO−LUMO band gap across the series of polymers, resulting in a series of materials that display absorption maxima at wavelengths ranging from 400 to 550 nm and fluorescence maxima ranging from 440 to 600 nm. The absorption and fluorescence properties of the CPEs were investigated in methanol, water, and in methanol/water mixtures. The photophysical data suggest that the CPE chains aggregate in water, but in methanol, the polymers are well solvated such that the optical properties are characteristic of the "molecularly dissolved" chains. Stern−Volmer (SV) fluorescence quenching studies were carried out using ionic naphthalene diimides as electron acceptors. The results show that the fluorescence from the CPEs was quenched with very high efficiency (amplified quenching) when the ionic diimide was charged opposite to the charge on the CPE chain. The sensitivity of the Stern−Volmer quenching response varies strongly across the series of CPEs, with the most efficient quenching seen for polymers that display efficient fluorescence when they are aggregated. The relationship between CPE side chain structure, band gap, fluorescence quantum yield, extent of chain aggregation, and fluorescence quenching efficiency is discussed.