Conjugated polyelectrolyte (CPE) poly[3-[6-(N-methylimidazolium)hexyl]-2,5-thiophene] complexed with aqueous sodium dodecylsulfateamphiphile: synthesis, solution structure and “surfactochromic” properties

Abstract

We report on the synthesis, solution structure and photophysical properties of poly[3-[6-(N-methylimidazolium)hexyl]-2,5-thiophene] bromide (P3ImiHT) when complexed with sodium dodecylsulfate (SDS). Synthesis of P3ImiHT used a Grignard metathesis (GRIM)-type route developed by McCullough, followed by quaternisation of the bromohexyl side groups of poly[3-(6-bromohexyl)thiophene] with N-methylimidazole. P3ImiHT was mixed with either SDS or deuterated SDS to form the P3ImiHT(SDS)_x complex, where x is the molar ratio of surfactant to polyelectrolyte repeat unit, in D₂O and studied using small-angle neutron scattering (SANS) and optical spectroscopy. Marked differences in behaviour are observed upon interaction of P3ImiHT with SDS compared with the related poly[3-(6-trimethylammoniumhexyl)thiophene] bromide (P3TMAHT). At room temperature, P3ImiHT forms charged aggregates with electrostatic repulsion which are eliminated by the SDS addition. For x ≤ 1 P3ImiHT and SDS are molecularly mixed and form ellipsoidal (x = 1/5) or sheet-like (x = 1/2–1) P3ImiHT(SDS)_x aggregates. No visible precipitation is observed around the nominal charge compensation point (x = 1). For x > 1, P3ImiHT(SDS)_x aggregates coexist with SDS rich micelles which turn from thick rod-like (x = 3/2) to non-charged (x = 2) and charged ellipsoidal micelles (x = 5). This transition is driven by decreasing free ion fraction. For x = 5, P3ImiHT(SDS)_x forms a lamellar phase with a periodicity of ∼270 Å. The structural transitions are accompanied by an initial red-shift from 422 nm (x = 0) to 459 nm (x = 1), followed by a reverse blue-shift to 400 nm (x = 5) of the UV/vis absorption maxima. The photoexcitation spectra follow this trend but are ∼50 nm red-shifted, thus indicating energy transfer within the density of states after photoexcitation. The photoluminescence maximum is gradually blue-shifted from 643 nm to 597 nm on increasing x from 0 to 5, indicating a decrease in polymer–polymer interactions.