Crystal Growth, Physicochemical Investigation, and Raman Spectroscopy: Exploration of Intramolecular Charge Transfer in NLO Chromophore DAST

Exploring potential nonlinear optical applications, we conducted experimental and theoretical investigations on pristine 4–N,N′-dimethylamino-N′-methyl-4-stilbazolium tosylate (DAST) organic single crystals. Determining structural parameters involved powder X-ray diffraction and Rietveld refinement, revealing a monoclinic crystal structure with a noncentrosymmetric and Cc space group for the grown crystal. Additionally, our experimental analysis encompassed Fourier transform infrared (FTIR), ultraviolet–visible (UV–vis), and second harmonic (SHG) investigations. Quantum chemical theoretical studies probed the structural as well as electoral properties of DAST. Density functional theory (DFT) calculations allowed the dissection of DAST’s attributes independently of the crystal field. Moreover, determining frontier molecular orbital energies and net atomic charges sheds light on the crystal’s electronic features. Natural bond orbital (NBO) analysis unveiled heightened electron delocalization around the aromatic ring, highlighting the title molecule’s potential for nonlinear optical applications. Furthermore, stabilizing interactions (O–H···O, C–H···O, C–H···π, and π···π) within the crystal structures were examined by Hirshfeld surfaces and fingerprint plots. The obtained values for dipole moment, polarizability, and first-order hyperpolarizability were μ = 6.65 D, α = 8.17 × 10–23 e.s.u., and β = 5.95 × 10–28 e.s.u., respectively. These findings underscore the promising potential of pristine DAST organic single crystals in advancing NLO applications.