Polymorphic crystal growth for NLO responses by band gap engineering, and charge transfer analysis: experimental vs theoretical exploration

We focus on a new polymorphic crystal of 4H-1,2,4-triazole-3,5-diamine for to evaluate its nonlinear optical (NLO) responses. The grown crystals by slow evaporation method are analyzed experimentally by single crystal X ray diffraction (SC-XRD), vibrational and electronic analysis. The crystal exhibits monoclinic system with space group P21/c and has been assigned a deposition number of 2348100 after depositing its structure at Cambridge Crystallographic Data Centre (CCDC). Theoretical calculations further support the experimental results in respect of their structural, optical and geometrical parameters. The results also showcase an energy gap (Egap) of 2.88 eV between its highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). It also displays a stable chemical property in different solvent phases and has a Vickers hardness scale range of 70–90 Vickers Pyramid Number (HV), showing its adaptability to various chemical environments. In the context of its nonlinear optical (NLO) responses, the value ranges for the linear polarizability (<α0>) fall between 2.80 and 3.30 electrostatic units (esu). Similarly, the first polarizability (β0) ranges from 2.39 to 6.45 esu, while the second hyperpolarizability (γ0) spans from 1.21 to 8.50 esu. This comprehensive study illuminates the interplay between crystal polymorphism, band gap tuning, and electronic structure manipulation, offering valuable insights for developing advanced materials with tailored NLO properties.