Folding fluorescent probes for self-reporting transesterification in dynamic polymer networks

Dynamic exchange reactions in covalent adaptable networks (CANs) are difficult to probe directly via various macroscopic mechanical methods. Herein, we report a fluorescent strategy for directly reporting the dynamic bond exchange in transesterification-based CANs by using folding molecular probes. The folding probes (PDI-dimers) consist of two perylene diimide (PDI) cores, a spacer of dynamic esters between the two PDI cores, and reactive terminal groups. During transesterification in CANs, the PDI-dimers unfold their PDI excimers to show a sharp fluorescent color change from orange to bright yellow. This visual strategy is demonstrated by a crosslinked thiol-Michael network (TMN) and poly(4-hydroxybutyl acrylate) network (PHBA). The dynamic behaviors like stress relaxation and self-stiffening in these CANs can be directly read out via the change of fluorescent color. This method can provide quantitative information and show spatiotemporal resolution and therefore, can be applied to probe various dynamic chain exchange mechanisms in crosslinked materials.