Solvent-modulated second harmonic generation in <i>N</i> -alkylated thiohydantoin derivatives: synthesis, characterization, and first-principle insights

This study explores the synthesis (five new thiohydantoin derivatives from MAS1 to MAS5, while an REF molecule was used as a standard for comparative DFT studies), spectroscopic characterization, and optoelectronic properties of six newly designed thiohydantoin derivatives using a combined experimental and computational approach. The molecular structures of all synthesized compounds were confirmed through FTIR and NMR spectroscopy, while DFT calculations at four functionals provided insights into vibrational modes, chemical shifts, and electronic behavior. Among the utilized functionals D3-B3LYP demonstrated the best agreement with experimental data, establishing its reliability for predicting electric field induced second harmonic generation (EFISHG) and electro-optical Pockel's effect (EOPE). Frontier molecular orbital (FMO) analysis revealed a reduced energy gap in nitro-substituted derivatives, particularly MAS3 (Δ<i>E</i> = 3.32 eV), which exhibited enhanced intramolecular charge transfer (ICT) and large hyperpolarizabilities, responsible for EFISHG applications. Molecular electrostatic potential (MEP) maps highlighted significant charge separation, while ELF and LOL analyses confirmed strong π-electron delocalization, further supporting their electro-optic potential. Notably, MAS3 displayed the highest first hyperpolarizability (<i>β</i> _tot = 3217 a.u.), surpassing the reference <i>p</i>-nitroaniline (<i>p</i>NA), indicating strong potential for second-harmonic generation (SHG) and EOPE-based devices. Dynamic hyperpolarizability studies at 1460 nm and 1907 nm revealed frequency-dependent behavior, with MAS3 showing exceptional <i>γ</i>(-2<i>ω</i>;<i>ω</i>,<i>ω</i>,0) values (124 212 a.u.), suggesting utility in EFISHG measurements and electro-optic modulation. Thermodynamic stability, light-harvesting efficiency (LHE), and radiative lifetime (<i>τ</i> _r = 8.8 ns for MAS3) further underscore the promise of these derivatives for optoelectronic applications. This work not only validates the solvent modulated EFISHG and EOPE capabilities of thiohydantoin-based systems but also provides a robust framework for designing advanced organic materials for photonics and telecommunications.