Effect of the Molecular Weight and the Ionic Strength on the Photoluminescence Quenching of Water-Soluble Conjugated Polymer Sodium Poly[2-(3-thienyl)ethyloxy-4-butylsulfonate]

The photoluminescence (PL) quenching of sodium poly[2-(3-thienyl)ethyloxy-4-butylsulfonate)] (PTE-BS) by the cationic electron acceptor methyl viologen (MV²⁺) was investigated in samples with high (HPTE-BS) and low (LPTE-BS) molecular weight. The Stern−Volmer constant decreases with decreasing the molecular weight from 8.4 × 10⁵ M^-1 down to 2.4 × 10⁴ M^-1. This decrease is attributed to the lower concentration of the polyanion in the solution of LPTE-BS due to the lower solubility of this polymer when compared with HPTE-BS. Furthermore, we demonstrate that lowering HPTE-BS solubility by increasing the solution ionic strength affects the PL and the quenching mechanism. For the four salts investigatedNaCl, KCl, MgCl₂, and CaCl₂the photoluminescence decreases with increasing the ionic strength, and the effect is conspicuous for the divalent cations investigated. The linear behavior characteristic of the quenching of the HPTE-BS luminescence by MV²⁺ disappears when the quenching is performed in the solution of CaCl₂ with ionic strength 0.3 M, which points to the formation of polymer aggregates due to the electrostatic bridging between monomers helped by the divalent cations introduced in the solution.