Heterobetaines based on quinolizinium cation as donor–acceptor building blocks for organic NLO materials

D–A + chromophores containing a quinolizinium-based azonia cation as the electron acceptor and various π-excessive N-heterocycles as electron donors were synthesized under standard Stille coupling conditions. To investigate the influence of molecular architecture on nonlinear optical (NLO) behavior, a series of D - –A + heterobetaine chromophores was also prepared. The electronic and optical properties of these donor–acceptor systems were examined experimentally and complemented by theoretical studies of their structural, electronic, and NLO characteristics. The calculations revealed that the absence of a conjugated π-bridge between the donor and acceptor units leads to a pronounced enhancement of the NLO response, as reflected by significantly increased first hyperpolarizabilities (β). The heterobetaine structures exhibit enhanced intramolecular charge separation and stronger donor–acceptor electronic interactions, resulting in an improved nonlinear optical response. Comparative analysis highlights the important role of direct donor–acceptor interactions in governing the electronic structure and second-order NLO properties. The observed structure–property relationships were rationalized using Density Functional Theory (DFT) and correlated Hartree–Fock (HF)-based computational methods, providing insight into the molecular geometries, frontier orbital distributions, charge-transfer characteristics, and NLO parameters of the quinolizinium-based heterobetaines.