Effect of charge density on energy-transfer properties of cationic conjugated polymers

Cationic conjugated polymers (CCPs) with different charge densities are synthesized via Suzuki polymerization. The CCPs show similar optical properties in aqueous solutions but obvious difference in fluorescence resonance energy transfer (FRET) to Texas Red-labeled single-stranded DNA (ssDNA-TR). Both CCP and TR fluorescence quenching are revealed to influence the energy-transfer process. The difference in quantum yields of CCP/ssDNA complexes highlights the importance of polymer side-chain structures and charge density. A CCP with a high charge density and ethylene oxide as the side chain provides the highest quantum yield for CCP/ssDNA complexes, which favors FRET. TR quenching within the CCP/ssDNA complexes is predominantly determined by the CCP charge density. In contrast to the other two polymers, the CCP with low charge density provides the most-intense polymer-sensitized TR emission, which is due to the collective response of more optically active polymer units around TR and the minimized TR self-quenching within the CCP/ssDNA-TR complexes. These studies provide a new guideline for improving the signal amplification of conjugated-polymer-based optical sensors.