Water-soluble conjugated polyelectrolytes with molecular bumper for efficient FRET biosensor

We report the design, synthesis and photophysical properties of two new water-soluble conjugated polyelectrolytes, poly(9,9' ?bis(6-NNN- trimethylammoniumhexyl)fluorene-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))-alt1, 4-(2,5-bis(6-N,N,N-trimethylammoniumhexyloxy))phenylene) tetrabromide (P2i). They have same electronic conjugation but the main structural difference is the presence of the anthracenyl substituent orthogonal to the polymer main backbone in P2i. Fluorescence resonance energy transfer (FRET) from the two polymers as FRET donors to fluorescein-labeled single stranded DNA (ssDNA·Fl) was studied. It is observed the emission from ssDNA-Fl via FRET by excitation of P2i with a FRET efficiency of -60%. Fluorescein is not emissive within the ssDNA-Fl/Pli complex. We also observed clear PL quenching (Φ = 0.8 → Φ = 0.27) of fluorescein for P2i after electrostatic complexation with ssDNA-Fl. It suggests a quenching pathway of E emission through photo-induced charge transfer (PCT) in the electrostatic complex. Both P1i and P2i have same HOMO-LUMO electronic structures and a similar thermodynamic driving force for either FRET or PCT. It appears that PCT operates to a larger extent with P1i, proposing that the presence of the "molecular bumper" in P2i increases Fl emission by increasing the donor-acceptor distance, which decreases more acutely PCT quenching, relative to FRET.