Synthesis and aggregation-enhanced emission of polymethacrylate with pentaphenylpyrrole side group

Methacrylate monomer M-PPP, which is aggregation-enhanced emission (AEE) active and with a conjugated pentaphenylpyrrole side group, was designed and synthesized through palladium-catalyzed cyclization of arylamine and diaryl acetylene, Suzuki cross-coupling and esterification with EDCI as dehydrant. A series of homopolymers P₁, P₂, P₃ and copolymers CP₄₁, CP₂₁, CP₁₁, CP₁₂, CP₁₄ were prepared by free radical polymerization of M-PPP and copolymerization of M-PPP with BMA. The molar ratio of M-PPP unit and BMA unit in copolymers CPs was calculated based on the chemical shift and integrations of their CH₂ groups. The content of M-PPP units in the copolymers was lower than those in raw materials, mainly due to the low polymerization activity attributed to the bulky conjugated pentaphenylpyrrole side group. The monomer M-PPP, the polymers P₁, P₂, P₃ and the copolymers CP₄₁, CP₂₁, CP₁₁ were of high thermal stability with their decomposition temperature above 300 °C, while the decomposition temperatures of CP₁₂, CP₁₄ decreased slightly because of the lower content of the conjugated pentaphenylpyrrole side group. Both the monomer M-PPP and the polymers exhibited obvious AEE feature in THF/H₂O mixture. The fluorescence intensity of the polymers increased slowly when the water fraction (f_w) was above 20%, and it increased faster when f_w is above 80%. The relative fluorescence intensity (I/I₀) reached to a maximum at f_w of 95%, mainly attributed to the aggregation of the conjugated luminogen pentaphenylpyrrole segment in the side-chain and the entanglement of the flexible polymer main chains in THF/H₂O mixture, which could restrict the motion of pentaphenylpyrrole and intramolecular rotation of the benzene rings, leading to enhanced fluorescence emission. The fluorescence intensity of M-PPP decreased a little when f_w was lower than 70%, then increased sharply, and the maximal I/I₀ was 3.9 at f_w of 95%, which was higher than those of the polymers due to the lower content of conjugated pentaphenylpyrrole side group in the polymers. The copolymers CPs exhibited similar AEE behavior as the polymers P₁, P₂, P₃ in THF/H₂O mixture, and they showed better AEE feature when increasing the content of the conjugated pentaphenylpyrrole side group. Furthermore, the copolymers CPs could “turn-on” to detect lysine selectively in THF/H₂O mixture due to the interaction of BMA unit and lysine, and more sensitive fluorescence response was observed for CP₁₄ due to the higher BMA side-chain content, which is known to have stronger interaction with lysine.