Photoinduced Electron Transfer in a Self-Assembled Bis(β-cyclodextrin)-Linked Pyrene/Bis(adamantane)-Linked Methyl Viologen Donor–Acceptor System in Aqueous Solution

Pyrene linked to two β-CD (CD = cyclodextrin; PY = pyrene) molecules (CD-PY-CD) and methylviologen (MV2+) linked to two adamantane (AD) groups (AD-MV2+-AD) self-assembled in water to give toroidal nanostructures. Photoprocesses taking place in the femtosecond and nanosecond time ranges within the assembly are reported. Fluorescence of the pyrene chromophore was quenched in the toroid, suggesting very efficient electron transfer. Fast quenching of the pyrene fluorescence with a time constant of 6.85 ns was attributed to photoinduced electron transfer from pyrene to methyl viologen within the toroid assembly. Electron transfer leads to the formation of radical ion products, PY•+ and MV•+, which were identified in the nanosecond transient absorption spectra. Because of the close packing of chromophores, the radical ions undergo fast reactions with chromophores or similar ions in adjacent stacks to give dimeric products. Since the dimeric species are not very stable, the reactions are reversed at longer time scales to generate the radical ions, which then undergo back electron transfer and regenerate the starting materials.