Synthesis of Coumarin- and Spiropyran-Functionalized Quadruple-Responsive Fluorescent Polymers: Stimuli-Responsive Behaviors, Fluorescence Properties, and Multilevel Anti-Counterfeiting Applications

Abstract In this study, coumarin-functionalized homopolymer P(Cou-N-AM), spiropyran-functionalized homopolymer P(SP-N-AM), and dual-functional block copolymer P(Cou-N-AM)-b-P(SP-N-AM) were synthesized via reversible addition–fragmentation chain-transfer (RAFT) polymerization. These polymers exhibited quadruple-responsive characteristics. The experimental results demonstrated that they displayed typical lower critical solution temperature (LCST) behavior in a buffer solution at pH = 4, which can be precisely tuned by adjusting the polymerization degree and solution concentration. Moreover, these polymers possessed a definite critical pH in water. Alternate bubbling of CO2 and N2 achieved reversible cyclic variations in solution pH and transmittance, indicating excellent gas-responsive cycling stability. Upon UV irradiation, P(Cou-N-AM) showed gradual decay in characteristic UV absorption, while P(SP-N-AM) and the block copolymer exhibited reversible photoisomerization with good cycling stability. Fluorescence measurements confirmed that all the polymers exhibited fluorescence properties and could be regulated by temperature, pH, and CO2. With the increase of temperature, fluorescence underwent thermal quenching. P(Cou-N-AM) produced stable blue fluorescence under alkaline conditions, while P(SP-N-AM) realized reversible red fluorescence emission under acidic environments. The block copolymer achieved mutually exclusive reversible switching between blue and red fluorescence through acid–base regulation or CO2/N2 gas alternation. Based on the superior multistimuli responsiveness and tunable dual-color luminescence performance of the block copolymer, a multilevel and reversible intelligent anti-counterfeiting system was successfully constructed. This work provides insight for the rational design and fabrication of multistimuli-responsive fluorescent polymers, showing promising applications in multilevel anti-counterfeiting and intelligent gas sensing.