Azobenzene-mediated reversible conformational switching in 9,9'-bifluorenylidene-based molecular machines.

Abstract

Intrinsically dynamic molecules offer attractive building blocks for molecular machines, yet controlling their conformational motion with external stimuli remains a fundamental challenge. Here, we report Azo-BF-Azo, a visible-light-driven molecular machine in which an intrinsically dynamic 9,9'-bifluorenylidene (BF) core is coupled to two photoswitchable tetra-ortho-fluorinated azobenzene (Azo) units that act as molecular actuators. By using variable-temperature NMR spectroscopy and UV-vis spectroscopy, together with computational studies, we demonstrate that irradiation at 525 nm induces Azo trans-cis photoisomerization and shifts the BF conformational equilibrium from the closed toward the open state, whereas 456 nm irradiation reverses Azo isomerization and promotes recovery of the BF-closed state. DFT calculations reveal that the extended trans-Azo units favor intramolecular π-π interactions that stabilize the closed BF conformation, while their conversion to the folded cis state disrupts these interactions and alters the steric environment surrounding the BF core. Variable-temperature NMR spectroscopy provides experimental support for the role of these intramolecular interactions in regulating the BF conformational equilibrium. The photoinduced metastable state further undergoes spontaneous thermal relaxation in the dark, recovering approximately 65% of the initial state. Together, these results demonstrate that local photoisomerization can reshape the conformational energy landscape of a second, intrinsically dynamic molecular degree of freedom through switchable noncovalent interactions. This work provides a strategy for converting intrinsic molecular dynamics into externally programmable motion and establishes a design principle for constructing integrated molecular machines in which one type of molecular motion regulates another.

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