Unusual Structures, Phase Behavior, and Fluorescent Properties of 3-phenyl-1-(pyridin-2-yl)-1H-pyrazol-5-amine and Its’ ZnCl2 Complex

The synthesis and single crystal structures of 3-phenyl-1-(pyridin-2-yl)-1H-pyrazol-5-amine (L1) and its complex with ZnCl₂ are reported. L1 exhibits supercooling, with a difference in melting and solidification points of over 100 ^oC. The complex [L1ZnCl₂] has a room-to-low temperature single crystal-to-crystal phase transition in the solid state, while a birefringent fluid phase mixed with crystalline domains is observed at high temperatures. Significant fluorescence enhancement is observed upon formation of the ZnCl₂ complex.