Endowing nitro-compounds with bright and stimuli-responsive luminescence based on propeller-like AIEgens

An electron-rich nitro group, having higher sensitivity to various environments, is attractive for constructing stimuli-responsive luminescent materials. However, it also causes the fluorescence quenching of luminogens, making a formidable challenge to achieving intense luminescence in nitro-compounds. Herein, we design a group of bright nitro-compounds with high fluorescence efficiency, long-wavelength emission and multi-stimuli-responsive behaviors by combining through-space conjugated AIEgens and nitro groups. Two bright nitro-compounds were constructed by introducing a propeller-like skeleton with the π-conjugated rings, showing a strong intramolecular spatial delocalization of π-electrons. This character not only stabilizes the (π, π*) states to enable their energy below the n → π* transitions caused by the nitro groups, enormously hindering the intersystem crossing process and promoting radiative transition, but also provides a strong steric hindrance of aromatic rings to vastly restrict the nonradiative decay of the singlet excitons, thereby enabling a high fluorescence efficiency, up to 95%. Moreover, the propeller-like chromophore and the nitro group with strong electron-withdrawing capacity work in synergy to induce an environmentally responsive conversion of local excitation and charge transfer states, endowing these nitro-compounds with stimuli-responsive luminescence, including solvatochromism, thermochromism, and mechanochromism in a reversible way, which varies from green to red. These findings outline a fundamental principle for the construction of bright nitro-compounds with stimuli-responsive behaviors, endowing traditional nitro-compounds with new features for potential applications.