Enhanced Sensitivity of Nonlinear Optical Signatures of Aggregation-Induced Emission.

Understanding how molecular aggregation influences nonlinear optical properties is essential for advancing organic fluorophores in imaging, sensing, and photonic applications. However, the relationship between the molecular aggregation and the magnitude of nonlinear two-photon absorption cross-section remains underexplored. Here, we systematically investigate the aggregation-dependent two-photon absorption properties of the fluorophore TPAPhCN by tuning the degree of aggregation. We observe a steady increase in the two-photon absorption cross-section as the fluorophore evolves from the single molecular state to the highly aggregated state. It is found that two-photon absorption signal emerges at an early stage of aggregation that is not observable by one-photon excitation. This divergence between the nonlinear and linear optical responses enables the construction of a dual-mode optical signature, allowing TPAPhCN to function as a self-reporting probe for each aggregation state. This work presents a sensitive and structure-responsive platform for precise aggregation-state sensing in complex environments and opens new opportunities for applications in materials diagnostics and intelligent optical sensing systems.