Molecular Control of Second- and Third-Order Nonlinearities in Tetrathiafulvalene–Quinone Dyads and Triads

Donor–acceptor (D–A) molecules are promising candidates for nonlinear optical (NLO) applications due to their tunable electronic properties, which can enhance second- and third-order NLO responses. In this study, we investigate the NLO properties of a series of dyads and triads combining tetrathiafulvalene (TTF) as the donor and benzoquinone (Q) as the acceptor. Second-harmonic generation (SHG) and third-harmonic generation (THG) responses were measured using the Maker fringe technique, revealing significant NLO activity. The results highlight the key role of molecular architecture and symmetry in modulating the NLO response, with enhanced hyperpolarizabilities observed for selected systems. To gain microscopic insight, electric dipole moments, dipole polarizabilities, and quadratic and cubic hyperpolarizabilities were evaluated using density functional theory (DFT), while dynamic second- and third-order susceptibilities, χ(2) and χ(3), were derived from Time-Dependent Hartree–Fock (TD-HF) approach. The UV/vis/NIR absorption properties were also analyzed, showing good agreement between experimental and theoretical spectra. Overall, the combined experimental and computational results highlight the importance of donor–acceptor interplay in tuning NLO responses and provide guidelines for the design of efficient organic NLO materials.