π‐Conjugation Engineering in New Ladder‐Type Pyrazinoquinoxaline‐Based Chromophores: A Route to High‐Performance <scp>NLO</scp> Materials With Ultradeep <scp>HOMO</scp> Levels

ABSTRACT In this study, a series of novel chromophores featuring pyrazinoquinoxaline (PQ)‐type variant conjugated bridges and powerful push‐pull effects was designed and systematically investigated. These PQ chromophores possess unique ladder‐type D‐π D ‐A, D‐π A ‐π D ‐A, and D‐π A ‐A configurations, first proposed and explored in the nonlinear optical (NLO) field. A comprehensive investigation of core properties, including aromaticity, static hyperpolarizability, dipole moments, excited‐state properties, stability, 2D second‐order nonlinear response spectra, and structure–property relationships was conducted through multiple advanced analytical methodologies. The structure of PQ conjugated bridges, acceptor strength, and dielectric surrounding were found to exert significant impacts on the hyperpolarizabilities and dipole moments of new PQ chromophores. Intriguingly, in contrast to traditional polyene‐bridged chromophores, PQ‐type systems exhibit a V‐shaped hyperpolarizability curve with increasing pyrazine rings on the conjugated bridge, accompanied by enhanced transition dipole moment squared, elevated polarity, a sharp decline in HOMO energy levels, and diminished aromaticity. The crucial states governing hyperpolarizability in PQ‐type chromophores are S 3 or S 4 state. The number of pyrazine rings on the PQ‐type bridge determines the key factors influencing hyperpolarizability. DN‐PQ0‐AS1 and DN‐PQ6‐AS1 exhibit remarkable hyperpolarizabilities and dipole moments in chloroform and acetonitrile, resulting in outstanding molecular hyperpolarizabilities. Significantly, DN‐PQ6‐AS1 and DN‐PQ5‐AS1 exhibit ultra‐deep HOMO energy levels of −8.26 eV and −8.16 eV respectively, indicating that these chromophores possess exceptional oxidation resistance, chemical stability, and air stability. This directly solves the urgent problem of insufficient stability in the processing and polarization of NLO materials. The unique D‐π A ‐π D ‐A, D‐π D ‐A, and D‐π A ‐A configurations in this system contain donor‐acceptor pairs with various lengths and strengths, resulting in multilevel and multiscale characteristics of electronic transitions and charge transfers. These structural features critically influence hyperpolarizability density and 2D NLO responses. Several PQ chromophores exhibit excellent EOPE/OR performance (exceeding 1 × 10 −22 esu). Notably, DN‐PQ4‐AS1 shows exceptional SHG response values (1.902 × 10 −23 esu). This mechanistic understanding of novel PQ conjugate systems provides innovative design strategies for developing multifunctional high‐performance NLO materials.