Synthesis and properties of Multi-Stimuli responsive Water-Soluble copolymers with high porphyrin content

Abstract

Water-soluble porphyrins have played an important role and showed potential application value in photodynamic therapy, sensor and aqueous photocatalysis. Most of the reported water-soluble porphyrin polymers have low porphyrin content, which is difficult to give full play to the role of porphyrin. To improve the content of porphyrin in polymers, we introduced the hydrophilic chain into the porphyrin molecule to obtain the hydrophilic porphyrin monomer, and then copolymerized with the hydrophilic monomer N-isopropylacrylamide to obtain the water-soluble copolymers with higher porphyrin content. The prepared porphyrin copolymers aqueous solution also showed responsive behavior to temperature, ionic strength and pH. Firstly, temperature affects the solvability and photophysical properties of copolymers. When heating the temperature above their cloud point, the copolymers chain aggregated, leading to the decreased light transmittance and enhanced emission intensity. Secondly, the copolymer aqueous solution showed responsive behavior to ionic strength. After adding different sodium salts into the copolymer aqueous solution, the changes order of cloud point was completely consistent with the Hofmeister series, and their emission intensity also varied. Finally, the copolymer aqueous solution showed responsive behavior to pH. When the pH of copolymer aqueous solution decreased, the inner nitrogen atoms in porphyrin ring were protonated, resulting in an increase in the cloud point of their aqueous solution. At the same time, the maximum absorption peak gradually red-shifted, and the emission intensity also decreased. Therefore, based on the decrease in fluorescence, the protonated copolymer could be used as photo-thermal conversion materials. The measured photo-thermal conversion efficiency (PCE) of the copolymer in the pH = 1.0 aqueous solution was 25.34%.