B-N Lewis pair functionalized mechanochromic and photoresponsive materials

Functional polymeric materials are an area of extensive research as their unique features open up new applications in various industries. The integration of dynamic functionality into polymers endows them with reversibility and responsiveness to external stimuli. Tunable and dative B-N Lewi pairs allow for stimuli-responsiveness and this interaction is particularly important in the development of responsive polymeric materials. Our goal is to construct responsive polymer networks that utilize dynamic B-N Lewis pairs as cross-links, enabling tunable bulk mechanical properties. In addition, by incorporating photoswitches into supramolecular B-N Lewis pair systems, selective light irradiation can induce changes in assembly behaviors, providing another pathway of stimuli responsiveness. In the first project, polystyrene-based organoborane polymeric Lewis acids were prepared by introducing bis(bithiophenyl)borane and diarylaminesubstituted bithiophenes as pendent groups in polymers. As the dynamically crosslinked polymer network formed upon addition of pyridine-functionalized PDMS, the fluorescence of the network can be varied by reversible supramolecular B-N bond formation. Mechanical-induced cleavage of the dynamic B-N bonds upon stretching the bulk material restores the original fluorescence of boron-containing chromophores, leading to a mechano-induced chromic response. The polymeric Lewis pairs can recombine instantly upon stress relief, regenerating the optical properties of the bound state. In the second project, we designed and synthesized pyridine-functionalized dithienylethene (DTE) photoswitches with boron-functianlized fluorenes for the construction of dynamic supramolecular assembly of polymeric materials through B-N Lewis pair formation. By incorporating DTE photoswitches that show ‘ring-open’ to ‘ring-closed’ switching behaviors, triggered by UV or ambient light respectively; preliminary results from DOSY NMR confirmed the formation of high-molecular-weight assemblies that upon UV irradiation can undergo conformational changes. UV-vis absorption spectra, photoswitching behavior, and fatigue resistance were analyzed for the molecular complexes and the polymeric system. Computational results provided insights into differences in absorption between the ‘ring-open’ and ‘ring-closed’ isomers of various boron complexes. Overall, the results demonstrated the construction of photoresponsive supramolecular assemblies from B-N Lewis pairs by incorporating DTE photoswitches as building blocks.