Ionic Strength and Solvent Control over the Physical Structure, Electronic Properties and Superquenching of Conjugated Polyelectrolytes

In this paper, we investigate the photophysical properties of the conjugated polyelectrolyte poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene), MPS-PPV, dissolved in both water and DMSO as a function of the solution ionic strength. Dynamic light scattering indicates that MPS-PPV chains exist in a highly agglomerated conformation in both solvents, and that the size of the agglomerates depends on both the ionic strength and the charge of the counterion. Even though the degree of agglomeration is similar in the two solvents, we find that the fluorescence quantum yield of MPS-PPV in DMSO is nearly 100 times greater than that in water. Moreover, intensity-dependent femtosecond pump-probe experiments show that there is a significant degree of exciton-exciton annihilation in water but not in DMSO, suggesting that the MPS-PPV chromophores interact to form interchain electronic species that quench the emission in water. Given that the emission quenching properties depend sensitively on the chain conformation and degree of chromophore contact, we also explore the superquenching properties of MPS-PPV in the two solvents as a function of ionic strength. We find that superquenching may be either enhanced or diminished in either of the solvents via addition of simple salts, and we present a molecular picture to rationalize how the conformational properties of conjugated polyelectrolytes can be tuned to enhance their emissive behavior for sensing applications. Electronic Properties of Conjugated Polyelectrolytes A. D. Smith et al. 2