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 2,7diphenylfluorenone 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 structures, characterization of the photophysical properties, powder X-ray diffraction, and differential scanning calorimetry provide insight into the structure−property relationships of the solid-state fluorescence behavior. The results indicate that the variable solid-state luminescence of the fluorenone derivatives is attributed to the formation of different excimers in different solid phases. Additionally, the stimuli-responsive reversible phase transformations of compounds 2 and 6 involve a structural transition between π−π stacking-directed packing and hydrogen bond-directed packing. The results also demonstrate the feasibility of our design strategy for new solid-state luminescence switching materials: introduction of both π−π stacking and hydrogen bonding into an AIE structure to obtain a metastable solid/crystalline state luminescence system.

Mechanochromic Luminescence from Crystals Consisting of Intermolecular Hydrogen‐Bonded Sheets

“…Because AIE‐active luminophores tend to form loosely‐packed assemblies, mechanical stimuli induces closed‐packing and/or conformational changes, which alter their photoluminescence properties. Another promising approach is the design of molecular structures in which different types of intermolecular interactions can compete against each other . Adopting this strategy, molecules can form several thermodynamically ( meta )stable assembled states and mechanical stimuli can induce interconversions among these states.…”

Synthesis, characterization, aggregation-induced emission, solvatochromism and mechanochromism of fluorinated benzothiadiazole bonded to tetraphenylethenes

“…36,37 Secondary bonding interactions and D-A pairs play important roles in the design of new mechanochromic materials. [38][39][40] Recently, a number of D-A compounds have been prepared and found to possess mechanouorochromic nature.…”

Contrasting Solid-State Fluorescence of Diynes with Small and Large Aryl Substituents: Crystal Packing Dependence and Stimuli-Responsive Fluorescence Switching

“…In addition, we report reversible fluorescence switching of some of the butadiynyl derivatives in the solid state by external heating and rubbing stimuli. Although there are reports on stimuli responsive fluorophores, [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] reports pertaining to a single fluorophore are still limited. Further, the potential of conjugated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 diacetylenes as stimuli responsive fluorophores in the solid powder has not been looked at yet in the literature.…”