AIE-Active Cyano Substituted Dimethoxy–Phenyl Derivatives for Nonlinear Optics: Spectral, Structural, and DFT Studies

This study presents donor-π-acceptor type compounds of 3,5-bis(OCH₃)-C₆H₃-C₆H₄-CN (1) and 3,5-bis(OCH₃)-C₆H₃-C₆H₄-CH₂-CN (2), synthesized and characterized through analytical and spectroscopic techniques. Single-crystal X-ray diffraction of compound 2 revealed a triclinic system with a P-1 space group. Despite its centrosymmetric nature, noncovalent interactions, such as CH…π interactions in the crystal packing, prevent antiparallel alignment in bulk, enabling nonlinear optical (NLO) activity. Solvatochromic studies indicated negative solvatochromism with absorption shifts of 72 nm for (1) and 63 nm for (2), attributed to high-ground state dipole moments. The cyano moiety suppresses fluorescence in solution due to twisted confirmation but is enhanced in aggregated states via aggregation-induced emission (AIE) at 90% THF/H₂O for (1) and 70% for (2). Thin film studies with polymethyl methacrylate (PMMA) showed increased emission intensity. Second-order NLO properties, evaluated using the Kurtz–Perry method, revealed compound 2 exhibiting 2.7 times the second harmonic generation (SHG) efficiency of compound 1 due to restricted antiparallel molecular packing. Computational studies of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at the B3LYP/6–31+G∗∗level of theory confirmed experimental results, validating optical and NLO properties of both compounds. These findings highlight the potential of these compounds for advanced optoelectronic applications.