Selective Growth of Polymorphs: An Investigation of the Organic Nonlinear Optical Crystal 5-Nitro-2-thiophenecarboxaldehyde-4-methylphenylhydrazone

An organic nonlinear optical crystal, 5-nitro-2-thiophenecarboxaldehyde-4-methylphenylhydrazone (NTMPH), has been prepared and investigated with regard to polymorphism, growth, and characterization of the structural and physical properties. Recrystallization of NTMPH in various solvents under different conditions showed the existence of three crystalline phases, red greenish plates (NTMPH-I) in space group P2₁/n, red orange prisms (NTMPH-II) in space group Pna2₁, and black needle (NTMPH-III) in space group P2₁/n. The crystal structures of NTMPH showed that three different intermolecular forces dominate the stabilization of one- or two-dimensional structures (called secondary structures in this work) of the three phases of the NTMPH crystals, namely the hydrogen bonding, dipole-dipole interactions, and the van der Waals forces for NTMPH-I, -II, and -III crystals, respectively. The thermal analysis showed that within the trimorphs the NTMPH-I crystal has the lowest thermodynamic stability with the lowest melting point and enthalpy, but NTMPH-II and -III crystals possess a higher thermodynamic stability and are called thermodynamic phases. The NTMPH-I crystal is easily crystallized in most conditions and is called a kinetic phase, whereas NTMPH-II and -III are crystallized with a higher supercooling (equal to a higher chemical potential), which also means that higher energy barriers for the nucleation of NTMPH-II and -III must be overcome. Since crystal structures cannot be designed precisely based on the molecular properties, especially for flexible molecules, the finding and selective growth of interesting polymorphic crystals is an essential step for the development of highly active nonlinear optical materials.