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

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 pi-pi 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 pi-pi 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 pi-pi overlap area of PY dimer plays a more crucial role in the formation of an excimer than interplanar pi-pi 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.