Multistate second-order nonlinear optical switches based on dimethyldihydropyrene-azobenzene hybrid system

This study pioneers the application of quantum chemical methods to systematically elucidate the evolution of second-order nonlinear optical (NLO) responses in multistate molecular switches derived from a dual photochromic dimethyldihydropyrene (DHP)-azobenzene (AB) hybrid system during isomerization processes. Through time-dependent density functional theory (TDDFT) simulations, we unravel the intricate structure-property correlations and establish a theoretical framework for molecular-level control of NLO performance. The results reveal that, in this hybrid system, a cascade isomerization pathway (E-C ↔ Z-C ↔ Z-O → E-O ↔ E-C) can be induced by wavelength-selective photoexcitation and thermal treatment, exhibiting efficient “ON–OFF–OFF–ON–ON” NLO switching behavior. The derivatives featuring the E-C structure exhibit the largest dynamic β_HRS value (8.4 × 10⁴ a.u.) in acetonitrile, surpassing that of the conventional DO3 system by a factor of 5.7, attributed to enhanced intramolecular charge transfer (ICT) and extended π-conjugation. We find that the donor-DHP/acceptor-AB linkage (Series 1 compounds) yields optimal NLO performance, with β_HRS values 32–243 % higher than those of the reversed configuration, offering a viable design strategy for high-performance NLO materials. The DHP-AB hybrid system emerges as a promising NLO switch candidate for multilevel optical memory devices with non-destructive readout capabilities. Our findings provide a molecular design blueprint for tunable NLO switches, enabling development of advanced optoelectronic technologies.