Growth and characterization of l-histidinium - 4-nitrobenzoate (1:1) multi-component molecular complex

A novel phase by reaction of l-histidine with 4-nitrobenzoic acid (4NBA) was obtained by solvent assisted grinding (SAG). The complex was crystallized from aqueous solution and characterized by X-ray powder diffraction (XRPD), single-crystal X-ray diffraction (SCXRD), infrared spectroscopy (FTIR) and thermal analysis (DSC/TGA). Additionally, the non-linear optical (NLO) properties were assessed for second harmonic generation (SHG). XRPD confirmed that a new phase was obtained, and this can be obtained pure from solution. However, when the complex appeared in SAG experiments, it was always in equilibrium with the precursors. The crystal structure of the complex revealed that a proton transfer took place from the carboxylic acid group of 4NBA to the unprotonated nitrogen atom in the imidazole ring of l-histidine to yield l-histidinium - 4-nitrobenzoate (LH4NBE) molecular complex. The FTIR spectroscopy depicted accurately the molecular characteristics of LH4NBE and the thermal analysis showed that it is stable up to 190 °C. Unfortunately, the complex displayed poor NLO properties and the sample suffered damage upon laser excitation.