Solvent-dependent reactivity of azo-BF2 switches

Abstract

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.e., 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 trans and cis 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.