Through-Space Cation–π Quadrupole–Monopole Design Enables Selective Suppression of Third-Order Nonlinear Optical Kerr Effects

Abstract

Organic materials capable of exhibiting high-order nonlinear optical responses are central to the advancement of photonic technologies. However, the molecular-level mechanisms that govern selective control over nonlinear optical response-order remain limited. Here, we employ a quadrupole–monopole design strategy based on intramolecular through-space cation–π interaction to uncover a new principle for tuning nonlinear optical behavior in purely organic systems. A series of π+–π conjugates were synthesized and systematically analyzed using Z-scan measurements. We demonstrate that intramolecular quadrupole–monopole coupling selectively suppresses the third-order Kerr effect while enabling a pure fifth-order nonlinear optical response. The magnitude of the fifth-order response correlates with the electrostatic interaction strength between π and π+ centers, following a 1/R4 dependence characteristic of monopole-induced dipole polarization. These findings establish electrostatic through-space coupling as a general design principle for inverting the optical nonlinearity hierarchy in organic materials.