aggregation-induced emission (AIE) properties with high solid-state fluorescence quantum yields (29–65%). Among them, 2,7-bis(4-methoxyphenyl)-9H-fluoren-9-one (2) and 2,7-bis(4-ethylphenyl)-9H-fluoren-9-one (6) display reversible stimuli-responsive solid-state luminescence switching. Compound 2 transforms between red and yellow crystals (the emission wavelength switches between 601 and 551 nm) under the