Near-Infrared Cocrystal Nanofluorophore with Enhanced Two-Photon Absorption Cross Sections

The growing demand for organic nanoprobes that combine broadband two-photon absorption (TPA) with near-infrared (NIR) optical excitation continues to drive advancements in biological imaging and advanced photonics. However, the design and preparation of such nanoprobe materials remain a significant challenge. Here, we employ co-crystallization strategy to fabricate a TPA crystal material, BP4TC (donor BP4VA, acceptor TCNB), exhibiting deep red fluorescence. Nanoprecipitation with an amphiphilic surfactant yields water-dispersible BP4TC nanoparticles (BP4TC-NPs, ≈50 nm), which are subsequently used for open-aperture Z-scan measurements. This marks the first experimental determination of the TPA cross-section for a molecular multicomponent solid. Prominent reverse-saturable and saturable absorption characteristics span 700-1000 nm, with corresponding TPA cross-sections decreasing monotonically with increasing wavelength. First-principles calculations demonstrate that BP4TC exhibits a stronger intermolecular charge transfer capacity, verifying its enhanced TPA ability. The TPA cross-section of BP4TC was further computed using the TDDFT approach, and the result indicates that BP4TC is a potent two-photon absorber. Under 900 nm excitation, BP4TC-NPs produce bright, low-background fluorescence in A549 cells (human lung cancer cells), exhibiting negligible cytotoxicity. Notably, by combining broadband NIR-I TPA with favorable biological properties, these cocrystal materials establish a multifunctional platform for biological multiphoton imaging and NIR photonics, while providing an experimentally validated blueprint for advanced nonlinear optical nanomaterials.