Enhanced Second Harmonic Generation from an Organic Self-Assembled Eutectic Binary Mixture: A Case Study with 3‑Nitrobenzoic and 3,5-Dinitrobenzoic Acids

This work illustrates the use of powder second harmonic generation (powder SHG), temperature-resolved second harmonic generation (TR-SHG), and second harmonic generation microscopy (SHGM) in monophasic and multiphasic sample studies. The commercial powder of 3,5-dinitrobenzoic acid was found to exhibit a significant second harmonic generation signal, whereas only two centrosymmetric polymorphic forms have been reported for this compound. Second harmonic generation techniques were used in combination with chromatography, differential scanning calorimetry, and powder X-ray diffraction to show that the SHG activity of 3,5-dinitrobenzoic acid powder originates from a chemical impurity (3-nitrobenzoic acid) present in the commercial powder under the form of a new metastable noncentrosymmetric polymorph. The metastable equilibria between 3,5-dinitrobenzoic acid and 3-nitrobenzoic acid were studied, and SHG analyses performed on crystallized binary mixtures showed significant enhancements of the SHG signal compared to that of the pure noncentrosymmetric phase. This is due to the formation of a suitable eutectic microstructure that enables quasi phase matching (QPM). In particular, powders from near-eutectic compositions exhibit SHG signals up to 20 times higher than that of the powder containing pure 3-nitrobenzoic acid noncentrosymmetric phase. This observation could provide the basis for a new route to achieve new, efficient materials for second-order frequency conversion.