Charge-Transfer Exciton Flows: Red Luminescent Zn₈D₁₄A₄ Nanotubes

Precise arrangement of multi-electron-donor (D) and acceptor (A) molecules can drive directional flows of photo-generated molecular excitons and charge-transfer (CT) excitons along with the proper D-A sequence. Here, a highly D-A-accumulated (14D + 4A) non-statistical triple-wall nanotube [(Zn²⁺)₈(L_D)₁₄(L_A)₄] driven by linkages with eight zinc ions (Zn²⁺) is fabricated (L_D and L_A are anthracene- and anthraquinone-based ditopic ligands, respectively). The absolute D and A positions are successfully determined by x-ray diffraction analysis on a single crystal of (Zn²⁺)₈(L_D)₁₄(L_A)₄, realizing their nanotubular arrangements with a non-statistical D-A sequence. The four L_A ligands are located at the inner positions of the nanotube and undergo CT interactions with the outer four L_D ligands, and the other 10 L_D ligands form dimers [(L_D)₂] through π-stacking or excitonic interactions between the anthracene rings. The intra-tube L_D/L_A CT interactions produce a CT absorption band at long wavelength, enabling the (Zn²⁺)₈(L_D)₁₄(L_A)₄ nanotube to absorb long-wavelength green light through CT excitation (L_D/L_A + hν_(CT) → L_D ^•+/L_A ^•-). Long-wavelength light excitation provides a bright red excimer emission from the anthracene dimer units [(L_D)₂ ^*]. This is most probably arising from flows of the low-energy photon-generated CT excitons.