Design of Emission Ratiometric Metal-Ion Sensors with Enhanced Two-Photon Cross Section and Brightness

To develop metal-responsive fluorescence probes with optimized nonlinear optical properties for two-photon excitation fluorescence microscopy (TPEM), we investigated the utility of donor−acceptor architectures where the metal-ion coordinates to the acceptor rather than donor site of the fluorophore. Zn(II) binding to such a probe consisting of a tris(picolyl)amine-substituted acceptor site connected with an anisole donor moiety and an oxazole central core resulted in a significant increase of the two-photon absorption cross section (δ) and fluorescence brightness (ηδ). The fluorophore showed also a strong shift of the emission peak upon saturation with Zn(II), rendering the proposed approach suitable for two-photon excited emission ratiometric sensing.