Conjugated Polyelectrolyte (CPE) Poly{3-[6-(N-methylimidazolium)hexyl]-2,5-thiophene} Complexed with DNA: Relation between Colloidal Level Solution Structure and Chromic Effects

We report on the solution structure and colorimetric changes for aqueous cationic conjugated polyelectrolyte, poly[3- [6-(N-methylimidazolium)hexyl]-2,5-thiophene] bromide (P3ImiHT), mixed with double stranded (anionic) DNA-P3ImiHT-(DNA)x, where x is the molar ratio of DNA base pair to polymer repeat unit. Small-angle X-ray scattering (SAXS) indicates that P3ImiHT forms loose arrays of rodlike chains involving electrostatic repulsion. SAXS and transmission electron microscopy confirm that the DNA addition reduces this repulsion and leads to networks of ~10 nm sized aggregates (x < 0.5 or 1.3 < x ≤ 2.4); or compact submicrometer sized particles with rough surface and ~2 nm thick rodlike chains (0.5 ≤ x ≤ 1.3). The absorption spectra red-shift from 440 nm (x < 0.5) to 510 nm (x ≥ 0.5) on increasing x, indicating an increased chain planarity and conjugation length concomitant with the rodlike segments. The photoluminescence maxima blue-shift from 680 nm (x = 0) to 661 nm (x = 0.5) and 628 nm (x = 2.4), indicating a decrease in interchain interactions. 16-fold dilution does not change the solution structure but reduces interchain energy transfer.