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.

High-efficiency RTP of TX-Cl crystal was induced by one-dimensional π–π stacking, which significantly enhances both intersystem crossing (S₁ → T_n) and radiative rate (T₁ → S₀). SMWLE was harvested by precisely controlling aggregating degree.

Herein, we report a new pyrene (PY)-based derivative, 1-(4-(phenylsulfonyl)phenyl)pyrene (1-DPS-PY), whose two crystals (crystal-B and crystal-G) exhibit sky-blue and green excimer fluorescence, respectively. Crystal-B and crystal-G have a similar dimeric π–π PY stacking in the crystals, but with a different single-molecule conformation. The experimental and theoretical investigations demonstrate that the substituent orientation of 4-(phenylsulfonyl)phenyl (DPS) plays a crucial role in modulating the excited-state properties of the PY excimer. The small twist angle (Φ) between DPS and PY facilitates the larger overlap between two PY planes in the excited state, resulting in more red-shifted excimer emission, and vice versa. Moreover, crystal-B shows a photoluminescence (PL) efficiency (η_PL) as high as 77% due to the better discreteness of PY dimer stacking, in contrast to η_PL = 60% of crystal-G with the non-discrete PY dimer stacking. Organic light-emitting diodes (OLEDs) were fabricated with doped and non-doped device structures. The devices showed a gradual emission change from deep blue to sky blue with increasing doping concentration, corresponding to the emission species transformation from a monomer to an excimer. A non-doped device exhibited the best performance with an external quantum efficiency (EQE) of nearly 4.5% and a maximum luminance of 13 733 cd m⁻². These results provide us with a new understanding of the mechanism of the PY excimer, and a new strategy to design the polymorphic PY dimers for high-efficiency excimer fluorescence with tunable emission color.

Selenanthrene with a folded geometry was used as a model to study the purely organic room-temperature phosphorescence (RTP) behavior under high pressure, and an unprecedented phenomenon of pressure-induced RTP enhancement was deeply studied.

Herein, an unusual blue shift in excimer fluorescence with increasing temperature was observed from a crystal with a discrete π–π anthracene dimer, originating from the more temperature-sensitive intermolecular geometry of the π–π dimer (e.g., π–π distance) compared to the intramolecular geometry of the monomer, which is confirmed by temperature-dependent single-crystal X-ray diffraction (XRD) measurements.