π-Conjugated polymer with Alloxazine-6,9-diyl unit in the Main chain: Synthesis, chemical properties, and sensing ability for metal ions and nucleosides

Abstract A π-conjugated polymer consisting of an alloxazine-6,9-diyl unit and 9,9-dihexylfluorene-2,7-diyl units was prepared from a newly synthesized monomer by a Pd-complex-catalyzed condensation reaction. A novel model compound, 6,9-bis(9,9-dihexylfluorene-2-yl)alloxazine, was also synthesized by the same reaction using the monomer. The Mn and Mw of the polymer are 13,500 and 21,600, respectively. The onset position of the UV-vis spectrum of the polymer is observed at a longer wavelength (λonset = 540 nm) than the model compound (λonset = 520 nm), thus revealing the presence of a π-conjugated system extending along the polymer chain. The polymer and model compound are both photoluminescent in solution (λem = 581 nm for the polymer and λem = 557 nm for the model compound), and their photoluminescence (PL) gradually decreases with the addition of nucleosides (adenosine (A) and guanosine (G)) as well as metal ions (Cu(I), Cu(II), and Zn(II)). This decrease in the PL intensity upon the addition of nucleosides and metal salts is most likely caused by electron transfer from the initially excited polymer to the complexes generated by hydrogen bonding between the polymer alloxazine unit and the nucleoside. Stern-Volmer constants of the polymer are KSV(A) = 0.8 × 103 M−1, KSV(G) = 7.4 × 103 M−1, KSV(Cu(I)) = 98 × 103 M−1, KSV(Cu(II)) = 560 × 103 M−1, and KSV(Zn(II)) = 4.1 × 103 M−1 and those of the model compound are KSV(A) = 0.3 × 103 M−1, KSV(G) = 1.4 × 103 M−1, KSV(Cu(I)) = 4.2 × 103 M−1, and KSV(Zn(II)) = 4.1 × 103 M−1. Cyclic voltammetry analysis of the polymer and model compound revealed their electrochemically active nature.