Understanding fluorescence quenching in polymers obtained by RAFT

Reversible addition-fragmentation chain transfer (RAFT) polymerization is a versatile process to obtain polymers with controlled architecture and molecular weight from a large variety of monomers. This appears to be a very attractive method for the synthesis of fluorescent polymers for different applications. However, fluorescent polymers obtained by RAFT are known to suffer strong fluorescence quenching. Here, we report the quenching of coumarin 343 (C343) fluorescence by four different chain transfer agents (CTA) used in RAFT polymerization: carboxymethyl dithiobenzoate (CMDB), tert-butyl dithiobenzoate (tBDB), menthonyl dithiobenzoate (MDB), and bis(3-methylbutyl)-2-(thiobenzoylthio)succinate (MBTS). The quenching ability of these CTAs was compared with that of a macroCTA (poly(DcA)-MBTS) obtained from the RAFT polymerization of N-decylacrylamide (DcA) using MBTS as chain transfer agent. The results of stationary and time-resolved fluorescence measurements of C343 with the different CTA and macroCTA can be interpreted with a single kinetic model, which considers the reversible formation of a C343-CTA exciplex and both static and dynamic quenching of C343. The photophysical properties of tert-butyl mercaptan (tBM), resulting from the aminolysis of the tBDB dithioester, were also studied using the same experimental conditions, confirming that the thiol group does not quench the C343 fluorescence. This was further tested by converting poly(DcA)-MBTS to the corresponding thiol-ended chain, eliminating the fluorescence quenching of coumarin previously observed.