1,2-BF₂ Shift and Photoisomerization Induced Multichromatic Response

Adaptive materials that exhibit a multichromatic response as a function of applied stimulus are highly desirable, as they can result in applications ranging from smart surfaces to anticounterfeit devices. Here we report on such a system based on an intriguing thermal 1,2-BF₂ shift that transforms a visible-light-activated azo-BF₂ photoswitch into a BF₂-hydrazone fluorophore (BODIHY) in both solution and the solid-state. Structure–property analysis, in conjunction with DFT calculations, reveals that the shift is catalyzed by the spatial proximity of an oxygen atom next to the BF₂ group and that the activation originates from an electronic and not steric effect. Theoretical calculations also show that while the energy barrier for the trans → BODIHY transformation is accessible at room temperature (thermal half-life of 30 h), the cis → BODIHY transformation has a much higher barrier, which is why the 1,2-BF₂ shift is not observed for the cis form. The photoswitching of the azo-BF₂, in conjunction with the 1,2-BF₂ shift, was then used in the multicolor modulation of a switch-containing cross-linked polydimethylsiloxane film using light and/or heat stimuli, elaborating the usefulness of the sophisticated reaction cascade that can be accessed from this simple system.