Wavelength-dependent tri-state photoswitching in crystals <i>via</i> synergistic dimer confinement and charge-transfer stacking

Anthracene (An) is a well-known photoactive unit that undergoes [4+4] dimerization upon UV excitation or a hetero-Diels-Alder reaction with singlet oxygen to form epoxide under visible light. Despite this dual reactivity, there is no single material exhibiting both photoresponses simultaneously. Herein, by utilizing an electron-deficient metal-organic framework (MOF) host, we obtain an An-dimer-encapsulated MOF cocrystal enabling wavelength-dependent tri-state photoswitching. Specifically, the strontium-containing MOF with preorganized arrays of naphthalene diimide (NDI) dimers and suitable cavity size of 10.3 Å is found to incorporate an An dimer in each cavity, forming extended crystalline π stacks with an A-D-D-A motif. Detailed structure-property analysis reveals the significant impact of dimer formation and charge-transfer stacking on the photophysical properties and photoactivity of the Sr-NDI@An cocrystal. Remarkably, Sr-NDI@An exhibits three distinct wavelength-selective photoresponses: (i) UV-triggered [4+4] dimerization of guest An, (ii) red-light-induced hetero-Diels-Alder reaction between singlet oxygen and An, and (iii) near-infrared-light-driven radical generation. This conceptual work opens the door toward the facile modulation of aromatic stacking for desirable complex functions.