A study on the structural, spectral and the effect of hydrogen bond on the third order nonlinear optical characteristics of cytosinium maleate

A single crystal of organic NLO material, cytosinium maleate (CM) has been grown by slow solvent evaporation technique. The cell parameters and space group have been determined by single crystal X-ray diffraction (SCXRD) analysis. The crystalline nature of the material was confirmed by powder X-ray diffraction (PXRD) study. Optical transmittance of the crystal in the visible and NIR region was measured by UV–Vis-NIR spectral analysis. The lower cut-off wavelength exists at 196 nm with excellent transmission from 262 to 1100 nm. The absence of absorption in the visible region makes CM suitable for optoelectronic applications. The obtained minimum value of Urbach energy indicates good crystallinity of CM. Fourier transform infrared (FT-IR) and FT-Raman spectral technique establishes the presence of functional groups in the CM crystal. Existence of several vibrations of COO⁻ ion indicates that a proton from the carboxylic group of maleic acid is transferred to the nitrogen atom of cytosine and is responsible for the existence of N–H…O hydrogen bond. This ensures the formation of the crystal. The proton and carbon environment of CM has been determined by nuclear magnetic resonance (NMR) spectral analysis. It reveals the formation N–H…O hydrogen bond between C₄H₆N₃O⁺ cation and C₄H₃O₄^—anion. The third order NLO parameters such as nonlinear refractive index (n₂), nonlinear absorption coefficient (β) and third order NLO susceptibility (χ³) have been determined by Z-scan method. The value of χ³ is found to be 1.1898 \(\times\) $\times$ 10^–7 esu and this large value of χ³ is due to the presence of hydrogen bond in the molecular structure of CM. The obtained open and closed aperture curve exposes that CM has saturable absorption and self-focusing nature. The observed characteristics find uses in remote sensing, laser Q-switching and laser mode locking applications. The recorded Photoluminescence (PL) spectrum reveals that the crystal was suitable for ultraviolet fluorescence emission.