The Green Box: An Electronically Versatile Perylene Diimide Macrocyclic Host for Fullerenes

The powerful electron accepting ability of fullerenes
makes them
ubiquitous components in biomimetic donor–acceptor systems
that model the intermolecular electron transfer processes of Nature’s
photosynthetic center. Exploiting perylene diimides (PDIs) as components
in cyclic host systems for the noncovalent recognition of fullerenes
is unprecedented, in part because archetypal PDIs are also electron
deficient, making dyad assembly formation electronically unfavorable.
To address this, we report the strategic design and synthesis of a
novel large, macrocyclic receptor composed of two covalently strapped
electron-rich bis-pyrrolidine PDI panels, nicknamed the “Green
Box” due to its color. Through the principle of electronic
complementarity, the Green Box exhibits strong recognition of pristine
fullerenes (C60/70), with the noncovalent ground and excited
state interactions that occur upon fullerene guest encapsulation characterized
by a range of techniques including electronic absorption, fluorescence
emission, NMR and time-resolved EPR spectroscopies, cyclic voltammetry,
mass spectrometry, and DFT calculations. While relatively low polarity
solvents result in partial charge transfer in the host donor–guest
acceptor complex, increasing the polarity of the solvent medium facilitates
rare, thermally allowed full electron transfer from the Green Box
to fullerene in the ground state. The ensuing charge separated radical
ion paired complex is spectroscopically characterized, with thermodynamic
reversibility and kinetic stability also demonstrated. Importantly,
the Green Box represents a seminal type of C60/70 host
where electron-rich PDI motifs are utilized as recognition motifs
for fullerenes, facilitating novel intermolecular, solvent tunable
ground state electronic communication with these guests. The ability
to switch between extremes of the charge transfer energy continuum
is without precedent in synthetic fullerene-based dyads.