Broadband nonlinear optical enhanced by expanding charge transfer dimensionality in pyrene-based chalcone derivatives

Designing functionalized molecular structures to modulate intramolecular charge transfer (ICT) characteristics plays a pivotal role in significantly enhancing the optical nonlinearity of pyrene-based chalcone derivatives. Building upon this foundation, we designed three molecular systems (X4, X2, and X1) featuring diethylaminophenyl groups via one-dimensional and two-dimensional (1D and 2D) extension of the chalcone backbone, simultaneously regulating pyrenyl charge density and π-π* transition intensity. Experimental assessment of 1D and 2D extended structures revealed distinct variations in optical nonlinearity. Broadband femtosecond Z-scan measurements (532-900 nm) and non-degenerate phase-object pump-probe (POPP) experiments demonstrated significantly enhanced nonlinear optical responses in X4 and X2 relative to X1, while key parameters including two-photon absorption cross-sections, excited-state absorption coefficients, and nonlinear refractive indices exhibited remarkable proximity between X4 and X2. From a charge transfer perspective, these observations can be rationalized by the saturation behavior of charge density on the central pyrene core with increasing numbers of electron-donating arms. This phenomenon is analogous to the finite volume of a container, wherein the saturated pyrene unit suppresses charge transfer, thereby limiting further enhancement of optical nonlinearity. These findings establish that while the 2D extended molecule system (X4) achieves greater absolute nonlinear enhancement, the 1D extended molecule system (X2) delivers a superior enhancement ratio per structural unit. This validates 1D extension as a highly efficient design strategy, providing valuable principles for optimizing pyrene-chalcone derivatives and related nonlinear optical materials.