Shape-Adaptable Water-Soluble Conjugated Polymers

Abstract

Cationic water-soluble conjugated polymers with a non “rigid-rod” aspect ratio can be prepared by Suzuki coupling condensation polymerization of 2,7-dibromo-9,9-bis(6'-bromohexyl)fluorene and varying ratios of p- and m-phenyl diboronic acids, followed by reaction with trimethylamine. The resulting polymers are designated M_nP_m⁺, where the subscripts in M_n and P_m correspond to the ratio of meta and para linkages, respectively. Absorption and fluorescence spectra indicate facile energy transfer via interachain or intrachain mechanisms. The emission of M_nP_m⁺ with m > 50 matches that of M₀P₁₀₀⁺. Excitations ultimately reside in the polymer segments with largest number of para linkages. FRET experiments show that the polymers with a more flexible structure are better donors to fluorescein-labeled double-stranded DNA. Similar results are obtained when intercalated ethidium bromide is used as the acceptor. These results indicate that there is better spatial registry between the flexible conjugated polymers and the secondary structure of DNA.