Identification of Ground-State Charge-Transfer Processes in UiO-66(Zr)-Tetrazine/Dihydrotetrazine: Enhanced Iodine Packing Density by Host-Guest Complexation.

Strategic design of intrinsic charge-transfer processes in metal-organic frameworks (MOFs) enhances host-guest chemistry. Incorporating tetrazine (TZ) and dihydrotetrazine (DHTZ) linkers into UiO-66(Zr) (Zr6O4(OH)4(BDC)6, where H2BDC is benzene dicarboxylic acid) yields UiO-66(Zr)-TZ and UiO-66(Zr)-DHTZ frameworks with ligand-to-ligand (LLCT) and ground-state ligand-to-metal (LMCT) charge transfers, respectively. In UiO-66(Zr)-TZ, LLCT is evidenced by a visible n-to-π* DRS band. In UiO-66(Zr)-DHTZ, a paramagnetically modified electronic structure gives DHTZ-to-Zr(IV) LMCT, confirmed by an absorption band at 299 nm (DRS) and an EPR signal. I2 was used as a competitive acceptor to probe these charge transfers. Iodine uptake in UiO-66(Zr)-TZ interrupts LLCT, shown by a moderate N(1s) XPS red shift and a new EPR signal. In UiO-66(Zr)-DHTZ, I2 capture directly disrupts the LMCT, quenching the EPR signal and causing a -2.25 eV N(1s) XPS red shift. Suppression of the C═N IR vibration in both frameworks confirms electronic perturbation of the linkers. This work demonstrates targeted exploitation of intrinsic charge-transfer processes as a strategy for designing adsorbents, where linker chemistry selectively activates ligand-centered or metal-involved capture.