Electronic-Symmetry-Tuned Emission Beyond 1500 nm in Erbium(III)-Phthalocyanine Complexes for High-Resolution In Vivo Biosensing

High-resolution in vivo biosensing is essential for advancing biology and medicine yet remains challenging. The second near-infrared region (NIR-II, 1000-2000 nm), particularly beyond 1500 nm, offers superior imaging resolution, owing to reduced scattering and minimal autofluorescence; however, no molecular probes have allowed biosensing in this range. Here, we introduce an electronic-symmetry-tuned emission strategy in monofunctionalized Er(III)-phthalocyanine complexes, establishing a molecular sensing platform operating beyond 1500 nm. In these complexes, efficient triplet energy transfer from the phthalocyanine ligands generates bright 1530 nm Er(III) emission, while modulation of electronic symmetry via peripheral substituents tunes the excitation profiles. Exploiting this mechanism, we develop responsive probes for pH and Cu(II) sensing that deliver excitation-ratiometric 1530 nm signals, enabling dynamic, quantitative, and anatomically resolved chemical sensing in vivo. This platform opens new avenues to study biological processes and improve disease diagnostics.