Amphiphilic block copolymer of 9-vinylphenanthrene and methacrylic acid: Fluorescence quenching of phenanthrene residue in aqueous medium

Amphiphilic block copolymers of 9-vinylphenanthrene (VPh) and methacrylic acid (MA) were prepared by a two-step living anionic polymerization of VPh and trimethylsilyl methacrylate followed by hydrolysis of the trimethylsilyl ester groups. Bis(2-hydroxyethyl) terephthalate, an oxidative quenching agent with amphiphilic nature, strongly quenched the fluorescence of phenanthrene groups in the block copolymer in aquecus solution. Apparent second-order rate constants k_q for the quenching ranged in the magnitude of 10¹¹?10¹²M^?1 s^?1 in the borate (pH 9) and phosphate (pH 7) buffers, whereas those in DMF solution were found to be ～10⁹M^?1 s^?1. No such difference in k_q for the aqueous and DMF solutions was observed with the related random copolymer. The results suggest that a considerable increase in the effective concentration of the quencher around the VPh sequences in the block copolymer resulted from hydrophobic association. Fumaric acid (FA), an anionic quencher, did not quench the fluorescence of the copolymer at pH 9 and 7, presumably because of the lack of accessibility of the quencher to the copolymer due to electrostatic repulsion. However, in neat water in which only a part of the carboxyl groups of MA sequences are dissociated and therefore the charge effect is minimized, FA quenched the fluorescence, with the k_q value approximating the diffusion control limit.