Structural and optoelectronic properties of L-Citrulline based systems: An overview through Density Functional theory

Abstract

L-citrulline, a non-proteinogenic amino acid, forms a variety of molecular crystal structures, including anhydrous, hydrated states, and different salts. These structures have garnered significant interest owing to their wide-ranging applications and their utility in studying how long-range interactions affect the structural, electronic, and optical characteristics of molecular crystals. A deeper theoretical comprehension of these features could enhance the application of citrulline polymorphs as key components in pharmaceuticals and contribute to the optimization of synthesis and stability processes for therapeutic purposes. Additionally, these insights may unlock new uses in fields such optoelectronics or sensing, based on their electronic structure characteristics. In this study, we utilized Time-Dependent Density Functional Theory (TD-DFT) to probe the electronic structure of L-citrulline at the molecular level, testing various hybrid DFT functionals to pinpoint those that best capture the electronic properties. The PBE0 and HSE06 functionals emerge as particularly effective, providing accurate depictions of the UV/Vis spectrum with errors of 1.25 % and 0.26 % respectively, in predicting absorption peak maxima. For solid-state analysis, we adopted the DFT GGA + TS dispersion corrected approach to obtain detailed structural geometries. Our research reveals that citrulline crystals have band gaps between 4.5 and 5.0 eV, with direct band gaps observed in the δ-, mono-, and dihydrated forms, while the α-form exhibits an indirect band gap. These crystals also demonstrate substantial charge carrier effective masses and notable optical anisotropy, offering valuable insights into the complex electronic and optical behaviors of citrulline systems and paving the way for their potential use in sensor technologies and optoelectronic devices. Finally, to assess crystal stability, phonon calculations were performed. Analyses of phonon dispersion relations and density of states were analyzed, providing further insight into the dynamic stability and lattice vibrational properties of the various citrulline polymorphs.