A Single Crystal of Uniform Photoredox-Active Nanochannels for Monodisperse Photo-Oxidative Oligomerization

A single crystal of aligned identical nanotubes with uniform channel lengths and diameters might serve as a porous crystal template with a monodisperse channel structure. However, the polydisperse nature of nanotubes reported to date makes them difficult to fabricate a porous single crystal with monodisperse channels. This work reported the fabrication of a porous single crystal comprising uniform photoredox-active nanotubes ([Eu₂₄L₃₆]_n, where L is a fluoroalkyl-rich bis(β-diketonate) ligand). [Eu₂₄L₃₆]_n was fabricated by length-restricted heterochiral columnar crystallization of P- and M-Eu₆L₉ unit tubes to form Eu₂₄L₃₆ nanotubes. X-ray diffraction analysis of [Eu₂₄L₃₆]_n revealed that Eu₂₄L₃₆ nanotubes were one-dimensionally aligned and spatially isolated in the crystal lattice, thereby forming monodisperse Eu₂₄L₃₆ channels. [Eu₂₄L₃₆]_n absorbed pyrrole (Py) monomers upon soaking in Py liquid through a crystal-to-crystal process ([Eu₂₄L₃₆]_n + mPy → [mPy⊂Eu₂₄L₃₆]_n), which was directly observed by X-ray diffraction. Photoirradiation of [mPy⊂Eu₂₄L₃₆]_n drove photo-oxidative oligomerization of the confined Py, during which the nanotube crystal template preserved its crystallinity because of the soft photo-oxidation with the Eu³⁺ cores. Thus, the confined photo-oxidative oligomerization process ([mPy⊂Eu₂₄L₃₆]_n + hν → [(Py)_m⊂Eu₂₄L₃₆]_n) can be observed directly by X-ray diffraction, which indicated the formation of ∼(Py)₂₄ inside the monodisperse Eu₂₄L₃₆ nanochannels. Likewise, monodisperse photo-oxidative oligomerization of Py to give octa- and nonapyrrole was achieved using the Eu₆L₉ unit tube as a template. [Eu₂₄L₃₆]_n also absorbed ferrocene (Fc) through a crystal-to-crystal process ([Eu₂₄L₃₆]_n + mFc → [mFc⊂Eu₂₄L₃₆]_n), where the crystalline-state photoinduced electron transfer from the incorporated Fc to the Eu³⁺ cores can be monitored by quenching of the Eu³⁺-red luminescence.