Structures, Phase Behavior, and Fluorescent Properties of 3-Phenyl-1-(pyridin-2-yl)-1 H -pyrazol-5-amine and Its ZnCl 2 Complex

The synthesis of the asymmetric ligand 3-phenyl-1-(pyridin-2-yl)-1 H -pyrazol-5-amine ( L1 ) and its single-crystal X-ray structure are reported. L1 displays crystallographic symmetry (orthorhombic, Pccn ) higher than its molecular symmetry (point group C 1 ) and also displays supercooling, with a difference in the melting and solidification points of over 100 °C. Upon complexation with ZnCl 2, L1 engages in both primary cation and secondary anion coordination via hydrogen bonding, and the complex exhibits a room-to-low-temperature single crystal-to-crystal phase transition. The ZnCl 2 complex becomes a birefringent fluid mixed with crystalline domains at high temperatures, as detected by polarized optical microscopy. Examination of the photoluminescence properties showed that the emission intensity increased and a pronounced bathochromic shift was observed in the emission maximum upon going from solution to the solid state, for both the ligand and complex, consistent with aggregation-induced emission behavior.