Synthesis and properties of a water-soluble aggregation-induced emission enhancement conjugated polymer

A conjugated polymer (P-0) containing tetraphenylethene (TPE) groups has been designed and synthesized by Suzuki coupling reaction between 1, 2-di[4-(6-bromohexyloxy) phenyl]-1, 2-di(4-bromophenyl) ethene (monomer 2) and benzen-1, 4-bis (boronic acid ) propane-1, 3-diol diester (monomer 3). A water-soluble cationic conjugated polyelectrolyte (P-l) was obtained by quaternization of P-0 with trimethylamine at the end of side-chains as a novel method for investigation into the aggregation-induced emission enhancement (AIEE) phenomena and mechanism. The structures of P-0 and P-l were characterized by ¹H-NMR, MALDI-TOF-MS and GPC. P-l is completely soluble in water and insoluble in common organic solvents such as THF and acetone. When a poor solvent such as THF was added into P-1 aqueous solutions, a slight decrease of the fluorescent intensity of P-l is observed before the volume rate of THF:H₂O arrived at 60%. However, a dramatic enhancement of luminescence is observed and concomitant with an obvious blue-shift of PL maxima when volume rate of THF:H₂O arrived at 70%. Addition of heparin into P-l aqueous solution showed a gradual increase in fluorescence intensity at 519 nm without PL maxima shift. Compared with its unit analogue, 1, 2-di [4-(6-(N, N, N-trimethyl ammonium) hexyloxy) phenyl]-1, 2-diphenylethene dibromide (MC-1), the emission enhancement of P-l showed higher sensitivity and better linearity than that of MC-1. Since heparin has four negative charges per repeat unit, and each polymer repeat unit has two positive charges, the concentrations for the positive and negative charges are the same (2.0μmol/L or 20. 0 μmol/L) at the saturation point. This indicates that the aggregation-induced emission enhancement stems from the restriction of their intramolecular rotations due to electrostatic attraction as the main driving force between the oppositely charged polymer and heparin, which can eliminate some disturbance factors such as aggregation-caused quenching of light emission. In summary, the present results indicate that cationic conjugated polyelectrolyte with AIEE property is expected to be generalized for studying AIE (E) mechanism and the detection of biological analytes of interest.