Single-crystal structural, spectroscopic, quantum chemical computational, and nonlinear optical studies on L-thioproline and L-thioprolinium picrate complex

Single crystals of the compounds L-thioproline (LTP) and L-thioprolinium picrate (LTPP) were grown by slow evaporation method. The compound LTP crystallized in the orthorhombic space group P2₁2121 and LTPP in the monoclinic space group P2₁. The crystal structures of both compounds were precisely determined by single-crystal X-ray diffraction technique. The molecules of LTP exhibit zigzag (Z1) and straight (S2) head-to-tail hydrogen bonds that are characteristic feature observed in parent amino acids. The packing of LTPP molecules is significantly different from LTP as there are no direct hydrogen-bonded interactions between L-thioprolinium cations. The title compounds were characterized by FTIR to confirm the presence of functional groups. The UV–Vis spectra for both the compounds was also recorded. The optical energy band gap (Eg) of LTP and LTPP was found as 5.64 and 2.67 eV respectively, from Tauc’s plot. The fluorescence emission spectra were recorded and the compounds show significant emission in the visible region. Also, thermal stability of grown crystals was investigated by thermogravimetric and differential thermal analytical studies. The optimized molecular structure and vibrational modes were carried out by DFT/B3LYP method for LTPP compound. Second harmonic generation efficiency of LTPP was 1.32 times of KDP.