Fluorescence “turn-on” metal ion sensors based on switching of intramolecular charge transfer of donor–acceptor systems

We report the synthesis and characterization of two copolymer P1 and P2 that use the poly[p-(phenyleneethynylene)-alt-(fluorenylethynylene)] as backbones and dibutylamino pendants as receptors. In P2, strong electron-deficient 2,1,3-benzothiadiazole (BTDZ) was introduced into the backbone to enhance the dipole of the donor–acceptor system. These two copolymers exhibited strong solvatochromic effect and very weak fluorescence due to the intramolecular charge transfer. The frontier molecular orbitals of P2 were spatially separated, i.e., the HOMO was localized on the dibutylamino unit, while the LUMO was localized on BTDZ unit. Upon binding with metal ions, P1 and P2 showed changes of intensity and shift of the fluorescence emission peaks. Cu2+ ion gave rise to most noticeable changes upon coordination with P1 and P2 with which a new blue emission peak at 430 nm was detected along with enhanced emission.