Synthesis and properties of chiral helical polymers based on optically active polybinaphthyls

Abstract

The chiral polymer P-1 was synthesized by the polymerization of ( R)-6,6′-dibutyl-3,3′-diiodo-2,2′-bisoctoxy-1,1′-binaphthyl ( R-M-1 ) with 5,5′-divinyl-2,2′-bipyridine ( M-1) via Pd-catalyzed Heck reaction. P-2 and P-2′ were prepared by Wittig–Horner reaction of ( R)-6,6′-dibutyl-2,2′-bisoctoxy-1,1′-binaphthyl-3,3′-dicarbaldehyde ( R-M-2 ) with 5,5′-bis (diethylphosphonomethyl)-2,2′-bipyridine ( M-2) in the presence of EtONa or NaH, respectively. P-3 was synthesized by Wittig–Horner reaction of ( R)-6,6′-di(4-trifluoromethylphenyl)-2,2′-bisoctoxy-1,1′-binaphthyl-3,3′-dicarbaldehyde ( R-M-3 ) with M-2 using NaH as a base. The four polymers have strong blue-green fluorescence due to the extended π-electronic structure between the chiral model compounds ( R)-6,6′-dibutyl-/di(4-trifluoromethylphenyl)-2,2′-bisoctoxy-1,1′-binaphthyl ( R-1 or R-2 ) and the conjugated linker 2,2′-bipyridyl group via vinylene bridge. Both monomers and polymers were analyzed by NMR, MS, FT-IR, UV–vis spectroscopy, DSC–TGA, fluorescence spectroscopy, GPC and circular dichroism (CD) spectroscopy. Based on the great differences of specific rotation values and CD spectra, P-1 and P-2 may adopt a zigzag chain configuration, while P-2′ and P-3 may adopt a helical configuration. The responsive optical properties of the two chiral helical polymers P-2′ and P-3 on transition metal ions were investigated by fluorescence, UV–vis and CD spectra. The results show that Ag + and Ni 2+ lead to nearly complete fluorescence quenching of P-2′ and P-3, Cu 2+ and Fe 2+ can cause obvious fluorescence quenching, but Zn 2+ and Cd 2+ can only produce slight fluorescence quenching. Ag +, Ni 2+, Cu 2+ and Fe 2+ can also lead to the obvious changes of UV–vis spectra of P-2′ and P-3. On the contrary, Zn 2+ and Cd 2+ cause little changes. Most importantly, the CD intensities and wavelengths of the chiral helical polymers P-2′ and P-3 exhibit the pronounced changes upon addition of Ag + and Ni 2+.