Nanofibers of Hydrogen-Bonded Two-Component Gel with Closely Connected p- and n‑Channels and Photoinduced Electron Transfer

An D–A–D gelator (DTCQ) was designed and synthesized using 2,3-dimethyl-5,8-di­(thiophen-2-yl)­quinoxaline and <i>N</i>-alkyl 3-aminocarbazole units as acceptor and donor, respectively, which were linked by a single bond. The compound could gelate several solvents, such as benzyl alcohol, aniline, acetophenone, and <i>o</i>-dichlorobenzene, as well as self-assemble into one-dimensional (1D) nanofibers in gel phase. The absorption and infrared spectra of the gels indicated that π–π interactions between aromatic moieties, intermolecular hydrogen bonds between amide units, and van der Waals forces were the driving forces for the formation of 1D self-assemblies and gel. DTCQ gel was red and emits red fluorescence because it has a strong absorption band at 487 nm and an emissive band at 620 nm. Moreover, DTCQ and a fullerene carboxylic acid formed two-component gel, in which the two compounds developed a hydrogen bond complex and self-assembled into 1D nanofibers with closely connected p- and n-channels. The nanofibrous xerogel film can rapidly generate a photocurrent under visible-light radiation through electron transfer from the gelator to fullerene, and then, the excellent exciton separation and charge transfer to two electrodes

Amplifying Emission Enhancement and Proton Response in a Two-Component Gel

A glutamide gelator, <b>1</b>, was synthesized, and a weak emission enhancement was observed during its gelation. In addition, <b>1</b> could be an excellent scaffold for successfully embedding an energy acceptor, <b>2</b>, into its aggregate to obtain highly efficient energy transfer. An amplification of the emission enhancement was observed in the two-component gels compared to that of the neat gel of <b>1</b> during gel formation. For example, <b>1</b> induced only a 2.5-fold increase in emission intensity, whereas a 23-fold enhanced emission could be observed in the two-component gel with only 1.6 mol % <b>2</b>. Furthermore, two-component gels had an excited proton response. In systems with low acceptor concentrations, the hot solution red-shifted the fluorescence from blue to yellow upon the addition of a proton, which continuously blue-shifted with decreasing temperature to form the gel given that the binding of the gelator to the proton is weakened during coassembly. Moreover, the casting film formed by the two-component wet gel had an excellent response to volatile acids such as hydrochloric acid, trifluoroacetic acid, and so on and could be reversibly recovered by exposure to NH₃

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

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 <b>1</b>–<b>6</b>) that exhibit prominent aggregation-induced emission (AIE) properties with high solid-state fluorescence quantum yields (29–65%). Among them, 2,7-bis­(4-methoxyphenyl)-9<i>H</i>-fluoren-9-one (<b>2</b>) and 2,7-bis­(4-ethylphenyl)-9<i>H</i>-fluoren-9-one (<b>6</b>) display reversible stimuli-responsive solid-state luminescence switching. Compound <b>2</b> 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 <b>6</b> 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 <b>2</b> and <b>6</b> 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