Solvent-dependent reactivity of azo-BF₂ switches

Azo-BF₂ complexes are visible and near-infrared (NIR) light-activated photoswitches showcasing versatility in applications ranging from energy storage to three-dimensional displays. While highly appealing as molecular switches, factors that affect their photophysical properties and solution-state reactivity still need to be teased out. In this paper, we present the synthesis and characterization of two azo-BF₂ analogues (1 and 2), each modified with structurally distinct dimethylamine-substituted naphthalene moieties. In addition to investigating the effect of the π-system expansion on the photophysical and photoswitching properties of the system, we also elaborate on the stability (<i>i.e.</i>, whether they undergo solvolysis or 1,2-BF₂ shift) of the switches in different solvents. Specifically, switch 1 absorbs in the NIR region, enabling its activation with 700 nm light, while switch 2 exhibits enhanced separation between the <i>trans</i> and <i>cis</i> isomer absorption bands, resulting in an improved photostationary state. As for the solvent effect, we discovered that polar aprotic solvents induce an intramolecular 1,2-BF₂ shift in both switches, transforming the azo-BF₂ photoswitches into boron difluoride hydrazone fluorophores, whereas polar protic solvents facilitate the solvolysis of the azo-BF₂ into the starting hydrazone derivative. In the former, the donor number of the solvent is a major factor in determining the obtained outcome, while in the latter, it is the solvent's hydrogen-bond donation capability. These insights into the design strategy and solvent-mediated reactivity of azo-BF₂s will contribute to their further development into efficient NIR-responsive photoswitches, paving the way for innovative applications in smart materials and molecular devices.