Crystal Engineering of a Tetraphenol with Bispyridines toward Hydrogen-Bonded Emissive Cocrystals

Herein, we report the cocrystals of an AIEgenic tetraphenol, namely tetrakis(4-hydroxybiphenyl)ethylene (THBPE), with a few bispyridyl aza donors, built with phenol···pyridine hydrogen bonding. We employed two different crystallization conditions: diffusion of hexane into THF and slow evaporation from ethyl acetate. The former condition readily afforded cocrystals in all instances except one, whereas the latter afforded only one. Attempted cocrystallization with 4,4′-bipyridine (BP) did not result in any cocrystals but instead afforded THF solvates of the tetraphenol. The rigid coformers 4,4′-azopyridine (AP) and 1,2-bis(4-pyridyl)ethylene (BPE) resulted in nonemissive close-packed cocrystal solvates. In both cases, the THF solvent competes with the pyridyl moiety for hydrogen bonding at two of the four possible sites in the tetraphenol, which is uncommon for a phenol···pyridine supramolecular synthon. However, the synthon wins out in the crystals from ethyl acetate solution with one coformer (AP) to form a fully hydrogen bonded square lattice (sql) network. The moderately flexible 1,2-bis(4-pyridyl)ethane (BPA) led to a close-packed hydrated cocrystal solvate. The highly flexible 1,2-bis(4-pyridyl)propane (BPP) afforded emissive cocrystals, with nanotubular pores of 10 Å diameter. The latter cocrystal adopts aqtz topology with 8-fold interpenetration. The last two cocrystals are emissive in the solid state, which is unusual for phenol···pyridine hydrogen-bonded complexes. The quantum yields of the solid-state emission exhibited by the lastlatter two cocrystals are 16% and 25%, which are higher than that of the tetraphenol at 9%. The observed emission property can be correlated with how uniquely and efficiently the AIEgens are packed in the solid with the assistance of coformers. The cocrystallization experiments with rigid tetraphenol and diverse bispyridyl coformers also provide insight into the formation of cocrystals along with solvates and hydrated solvates. Notably, four among the five cocrystals reported herein have Z′ = 2.