Fluorenone Organic Crystals: Two-Color Luminescence Switching and Reversible Phase Transformations between π–π Stacking-Directed Packing and Hydrogen Bond-Directed Packing

Abstract

Organic solid-state luminescence switching (SLS) materials with the ability to reversibly switch the luminescence by altering the mode of molecular packing without changing the chemical structures of their component molecules have attracted considerable interest in recent years. In this work, we design and synthesize a new class of 2,7-diphenylfluorenone derivatives (compounds 1–6) that exhibit prominent 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 stimuli of temperature, pressure, or solvent vapor. Similarly, compound 6 exhibits SLS behavior, with luminescence switching between orange (571 nm) and yellow (557 nm). Eight X-ray single-crystal structu...