One-Electron Modulation of Photophysics in a Through-Space Donor–Acceptor System

Through-space donor–acceptor interactions provide an attractive strategy for controlling molecular electronic structures and excited-state properties. However, the influence of one-electron transformations on the photophysical behavior of such systems remains relatively unexplored. Herein, we report an isolable carbazole–xanthylium cation (5+) and its corresponding neutral radical (5•), in which the donor and acceptor units are held in close spatial proximity. Single-crystal X-ray diffraction reveals short donor–acceptor contacts of approximately 3.37 and 3.45 Å for the cation and radical, respectively. The cation exhibits a broad nearinfrared absorption extending to approximately 930 nm, whereas one-electron reduction affords a neutral radical displaying visible absorption and red photoluminescence with a quantum yield of 3.7 %. Density functional theory calculations and natural transition orbital analyses reveal pronounced through-space charge-transfer (TSCT) character in both species, despite their substantially different excitation energies. Excited-state geometry optimizations further support the distinct energetic landscapes associated with their charge-transfer excited states. These findings demonstrate that one-electron reduction can substantially modulate the photophysical properties of a spatially confined donor–acceptor system while preserving its underlying TSCT character.