Light emission of organic luminogens: Generation, mechanism and application

It is always a challenge for planar polycyclic aromatic molecules to achieve high efficiency in solids owing to their frequent encounter with aggregation-caused quenching (ACQ). An anthracene derivative with one-side meta-substituted triphenylamine (TPA) was found to show high-efficiency excimer fluorescence (ηPL = 76.8%) in G-phase (green) crystals as well as a long lifetime, in sharp contrast with that of a monomer in a doped film (ηPL = 36.6%) and that of B-phase (blue) crystals (ηPL = 8.1%). In essence, the excimer-induced enhanced emission can be ascribed to the special intermolecular stacking in the solid state, namely, discrete antiparallel dimeric stacks between anthracene moieties in G-phase crystals, which are responsible for greatly suppressed non-radiative deactivation due to a uniform emissive state preventing the formation of an energy-trapping “dark” state. Moreover, a G-phase could be obtained through a thermally-activated phase transition from B-phase crystals, corresponding to the completely synchronized change of fluorescence properties. The present results consolidate a novel strategy of designing discrete dimeric stacking of planar polycyclic aromatic molecules to achieve high-efficiency fluorescence in the solid state by an excimer-induced enhanced emission (EIEE) mechanism.

Discrete face-to-face stacking of anthracene inducing high-efficiency excimer fluorescence in solids via a thermally activated phase transition

Here, we report the self-assembly of Amoc (9-anthracenemethoxycarbonyl)-capped dipeptides, which self-assemble to form injectable, self-healable, and shape-memory hydrogels with inherent antibacterial properties. Amoc-capped dipeptides self-assemble to form nanofibrillar networks, which are established by several spectroscopic and microscopic techniques. The inherent antibacterial properties of hydrogels are evaluated using two Gram-positive Staphylococcus aureus, Bacillus subtilis and three Gram-negative Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhi bacteria. These hydrogels exhibit potent antibacterial efficacy against Gram-positive and Gram-negative bacteria. The minimum inhibitory concentrations (MIC50) for the hydrogels on Gram-positive bacteria are in the range of 10–200 μM hydrogelator concentrations. The biocompatibility and cytotoxicity of the hydrogels are evaluated using 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT), hemolysis, and lipid peroxidation (LPO) ...

Investigations of Peptide-Based Biocompatible Injectable Shape-Memory Hydrogels: Differential Biological Effects on Bacterial and Human Blood Cells

In order to facilitate pyrene (PY) excimer formation in solids, several imidazole-containing PY derivatives were designed and synthesized. Among them, a new compound 1,4,5-triphenyl-2-(pyren-1-yl)-4,5-dihydro-1H-imidazole (IM-PY) was achieved with two crystalline polymorphs (crystal-G and crystal-B), both of which presented PY dimer stacking. One crystal showed green PY excimer emission while the other one exhibited blue monomer emission, despite their similar π–π stacking of PY dimers. Both experimental and theoretical investigations revealed the nature of this distinct emission: the dimer in crystal-B had a smaller overlap area of π–π stacking than that in crystal-G, which substantially hindered PY excimer formation in crystal-B. Through the analysis of PY-based excimer from a series of samples, the π–π overlap area of PY dimer plays a more crucial role in the formation of an excimer than interplanar π–π distance in crystals. This work not only reveals a key structural factor on PY excimer formation in solids, but also provides us with a better understanding of the structure–property relationship between the PY dimer structure and fluorescence property.

A key stacking factor for the effective formation of pyrene excimer in crystals: degree of π–π overlap

Organic light‐emitting materials are centered on their excited states. However, excited‐state geometry is hardly solved. Herein, the planar distance of an anthracene excimer is determined by integrating a monodisperse dimeric anthracene stacking in crystal and high‐pressure technique. The crystal exhibits the pure excimer fluorescence at ambient conditions, and a unique photophysical phenomenon is observed under pressure: fluorescence peak keeps constant first and then starts a gradual redshift. The theoretical potential energy curve indicates that this starting point of redshift corresponds to the excimer equilibrium geometry, which will be innovative to determine the excited‐state π–π interplanar distance (Re) in experiment. Moreover, high‐pressure synchrotron powder X‐ray diffraction experiment tactfully determines Re = 3.330 Å at the pressure point of initial redshift. This work not only suggests the Re determination by an original pathway but also reports a unique excimer photophysics, which may provide a reference benchmark for the further theoretical and experimental studies on aromatic excimer.

Monodisperse π–π Stacking Anthracene Dimer under Pressure: Unique Fluorescence Behaviors and Experimental Determination of Interplanar Distance at Excimer Equilibrium Geometry

Polyaromatic compounds are significant members of leading candidates for organic semiconductors and optical materials. However, a thorny problem of polyaromatic materials is that their good emissive abilities in solutions are seriously weakened in solids due to strong π–π interactions between aromatics. As a typical case, the intermolecular π–π interaction tends to form excimers for polyaromatic system, which were always considered to quench fluorescence and decrease luminous efficiency in the past decades. Herein, anthracene is modified by meta-substituted bromobenzene to facilitate the formation of discrete dimeric stack in solids, leading to the enhanced anthracene excimer fluorescence. Particularly, instead of excimer quenching fluorescence, the more anthracene dimers in solids, the higher fluorescence efficiency, namely, excimer-induced enhanced solid-state emission. This work not only provides a meta-substituted strategy for molecular design to form excimer in solids but also demonstrates that high-...

Discrete Dimeric Anthracene Stackings in Solids with Enhanced Excimer Fluorescence

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Discrete Dimeric Anthracene Stackings in Solids with Enhanced Excimer Fluorescence