Cationic conjugated polyelectrolytes with molecular spacers for efficient fluorescence energy transfer to dye-labeled DNA

Two water-soluble conjugated polyelectrolytes, poly(9,9′-bis(6-N,N,N- trimethylammoniumhexyl)fluorene-alt,-1,4-(2,5-bis(6-N,N,N- trimethylammoniumhexyloxy))phenylene) tetrabromide (Pli) and poly((10,10′-bis(6-N,N,N-trimethylammoniumhexyl)-10H-spiro(anthracene-9, 9′-fluorene))-alt-1,4-(2,5-bis(6-N,N,N-trimethylammoniumhexyloxy)) phenylene) tetrabromide (P2i) are synthesized, characterized, and used in fluorescence resonance energy transfer (FRET) experiments with fluoresceinlabeled single-stranded DNA (ssDNA-Fl). Pli and P2i have nearly identical π-conjugated backbones, as determined by cyclic voitammetry and UV-vis spectroscopy. The main structural difference is the presence of an anthracenyl substituent, orthogonal to the main chain in each of the P2i repeat units, which increases the average interchain separation in aggregated phases. It is possible to observe emission from ssDNA-Fl via FRET upon excitation of P2i. Fluorescein is not emissive within the ssDNA-Fl/Pli electrostatic complex, suggesting Fl emission quenching through photoinduced charge transfer (PCT). We propose that the presence of the anthracenyl "molecular bumper" in P2i increases the distance between optical partners, which decreases PCT more acutely relative to FRET.