Crystal growth, Spectroscopic Characterisation, and Third-order Nonlinear Optical response of a novel...

Single crystals of the N-methylurea-gallic acid (NMU-GA, 1:1) cocrystal were grown by the slow evaporation solution technique. Solubility increased from 25.65 to 41.58 g/100 mL over 30-50 °C (62.1% rise). Single-crystal X-ray diffraction confirmed monoclinic P2₁/c symmetry; PXRD confirmed phase purity. Fourier Transform Infrared Spectroscopy (FTIR) confirmed the distinct hydrogen-bonding framework of R²₂(8). UV-Vis spectroscopy showed a cut-off at 247 nm, 78% optical transparency, and a direct bandgap of 4.46 eV. Photoluminescence showed violet-blue emission at 418 nm with a 68 nm Stokes shift; time-resolved PL confirmed bi-exponential nanosecond decay. TG/DTA confirmed thermal stability to 152 °C, decomposition onset at 224 °C, and an exothermic DTA peak at 263 °C. Vickers microhardness showed a Reverse Indentation Size Effect (RISE) with Meyer coefficient n = 2.586. Dielectric measurements (303-403 K, 10 Hz-30 MHz) showed ε' converging to ∼4.2 above 1 MHz with negligible high-frequency loss. Z-scan at 632.8 nm gave n₂ = 2.634 × 10^-16 m²/W (self-focusing), β = 6.456 × 10^-5 m/W (reverse saturable absorption), and |χ3| = 1.572 × 10^-7 esu (n₀ = 1.5), an effective third-order response that, together with the wide UV-transparent window, establishes NMU-GA as a promising multifunctional cocrystal for optical-limiting applications. DFT calculations (B3LYP/6-311++G(d,p)) corroborate the R²₂(8) synthon and give a HOMO-LUMO gap of 4.99 eV, in agreement with the experimental bandgap.