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)flourene-alt-1,4-(2,5-bis(6-N,N,N-trimethylammoniumhexyloxy))phenylene) tetrabromide (P1i) and poly((10,10'-bis(6-N,N,N-trimethylammoniumhexyl)-10H-spiro(anthracene-9.9'-fluorene))-alt-1,4 (2,5-bis-N,N,N-trimethylammoniumhexyloxy))phenylene) tetrabromide (P2i) are synthesized, characterized, and used in fluorescene resonance energy transfer (FRET) experiments with fluorescein-labeled single-stranded DNA (ssDNA-Fl). P1i and P2i have nearly identical pi-conjugated backbones, as determined by cyclic voltammetry and UV vis spectroscopy. The main structural difference is the presence of an anthracenyl substituent, orthogonal voltammetry 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 seperation 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/P1i electostatic complex, suggesting Fl emission quenching through photoinduced charge transfer (PCT). We propose that the purpose of the anthracenyl "molecular bumper" in P2i increases the distance between optical partners, which decreases PCT more acutely relative to FRET.