Guest-Dependent Fluorescence Response by a Highly Emissive s-Tetrazine Pd₂L₄ Cage

Fluorescent cage molecules are highly sought after as they offer new avenues in selective chemosensing. s-Tetrazines are renowned for their fluorescent character and redox properties, yet integrating fluorescent tetrazine functionality into coordination cages remains largely unexplored. Here, we report the self-assembly of a Pd₂L₄ coordination cage from 3,6-bis(pyridin-3-yloxy)-1,2,4,5-tetrazine. NMR spectroscopy, ESI-MS, x-ray crystallography, and density functional theory (DFT) calculations reveal a rigid helical cage structure that encapsulates a tetrafluoroborate anion within its highly electron-deficient cavity. Cyclic voltammetry experiments show that both the ligand and cage can be reversibly reduced to the radical anion, while fluorescence measurements indicate that the cage completely retains the emissive properties of the free tetrazine ligand (quantum yield = 0.33, fluorescence lifetime = 120 ns). Anion exchange studies reveal pronounced binding preference for NO₃ ^‒ (K = 2300 M^‒1 in MeCN) over other monoanions. As a result, emission quenching for the bound NO₃ ^‒ is non-linear, while a linear trend is observed for the non-encapsulated PF₆ ^‒ anion-distinct fluorescence responses that reflect host-guest chemistry.