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 x 10(5) M-1 down to 2.4 x 10(4) 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.