Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins as Donors, Viologen as Acceptor, and Octa Acid Capsule as the Mediator

Results
of our study on ultrafast electron transfer (eT) dynamics
from coumarins (coumarin-1, coumarin-480, and coumarin-153) incarcerated
within octa acid (OA) capsules as electron donors to methyl viologen
dissolved in water as acceptor are presented. Upon photoexcitation,
coumarin inside the OA capsule transfers an electron to the acceptor
electrostatically attached to the capsule leading to a long-lived
radical–ion pair separated by the OA capsular wall. This charge-separated
state returns to the neutral ground state via back electron transfer
on the nanosecond time scale. This system allows for ultrafast electron
transfer processes through a molecular wall from the apolar capsular
interior to the highly polar (aqueous) environment on the femtosecond
time scale. Employing femtosecond transient absorption spectroscopy,
distinct rates of both forward (1–25 ps) and backward eT (700–1200
ps) processes were measured. Further understanding of the energetics
is provided using Rehm–Weller analysis for the investigated
photoinduced eT reactions. The results provide the rates of the eT
across a molecular wall, akin to an isotropic solution, depending
on the standard free energy of the reaction. The insights from this
work could be utilized in the future design of efficient electron
transfer processes across interfaces separating apolar and polar environments.