Remarkable Photophysics and Amplified Quenching of Conjugated Polyelectrolyte Oligomers

We report the photophysics and fluorescence quenching of a series of monodisperse, anionic π-conjugated oligomers that are molecularly dissolved in aqueous solution. These structurally well-defined oligomers feature oligo­(phenylene ethynylene) backbones with two −CH₂COO^– units on each repeat unit, with overall lengths of 5, 7, and 9 repeats. The ionic oligomers display a structured fluorescence band with high quantum efficiency in water, in contrast to the low fluorescence quantum efficiency and pronounced aggregation displayed by structurally similar oligomeric and polymeric (phenylene ethynylene) conjugated polyelectrolytes studied previously. Stern–Volmer (SV) fluorescence quenching studies using cationic charge- and energy-transfer quenchers reveal that all of the oligomers give rise to SV quenching constants (K_SV) in excess of 10⁶ M^–1, with values increasing with oligomer length, consistent with the amplified quenching effect. The amplified quenching effect is proposed to occur due to the formation of comparatively small oligomer aggregates.