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

Abstract

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.

Self-Assembly and Photochemistry of a Pyrene-Methyl Viologen Supramolecular Fiber System

“…Plots of the various parameters against the concentration of Ms-(MV 2+ -AD) 3 are presented in Figure S14. These plots are very similar to those observed for the CD-PY-CD@AD-MV 2+ -AD system reported recently and confirm that static quenching corresponds to PET within the nanoassembly. Using the average value of τ 1 (11.65 ns), we obtained the rate constant of PET within the assembly as k et = 7.85 × 10 7 s –1 using the expression k et = (1/τ 1 ) – (1/τ 0 ). , …”

“…We observed a similar phenomenon for a CD-PY-CD@AD-MV 2+ -AD system reported recently, where we attributed these changes to the stacking of PY over PY and β-CD over β-CD during self-assembly of the inclusion complex . Stacking of PY over PY and β-CD over β-CD is shown schematically in previous papers. , For the present CD-PY-CD@Ms-(MV 2+ -AD) 3 system also, we attribute the dampening of PY and β-CD signals and changes in the splitting pattern of viologen protons to the formation of the vesicles and fibers, which may involve stacking of PY over PY and β-CD over β-CD as described previously …”

“…Within about 400 fs, the S 2 state decays to the S 1 state and the 380 and 480 nm absorptions seen at later times are assigned to the S 1 → S n transitions. A small residual absorption is seen around 600 nm in the spectra taken at 530 fs and this is assigned to PY excimer-type species . The slow decay of the S 1 state is attributed to PET from PY to MV 2+ .…”

“…The decays become biexponential in the presence of Ms-(MV 2+ -AD) 3 as shown in Figure b–j. The biexponential fluorescence decay was attributed to diffusion-mediated and unimolecular quenching processes as described recently for the CD-PY-CD@AD-MV 2+ -AD system . The decays were fitted to the biexponential function where τ 1 corresponds to the lifetime of the fluorophore within the nanoassembly (static quenching), τ 2 results from its diffusion-mediated quenching, and χ (complex) and χ (free) are the contributions of static and diffusion-mediated processes, respectively.…”

“…Irradiation of the aqueous toroidal solution led to the formation of a CS state, which was stable for a few hours. The work was extended by employing pyrene (PY) in place of AN. , The donor CD-PY-CD and acceptor AD-MV 2+ -AD self-assembled in water to generate ring-like structures, which then stacked together to give toroids. Irradiation of the solution containing the nanostructure led to the generation of pyrene radical cation (PY •+ ) and methyl viologen radical cation (MV •+ ).…”