Intramolecular Energy and Electron Transfer within a Diazaperopyrenium-Based Cyclophane

Molecules
capable of performing highly efficient energy transfer
and ultrafast photoinduced electron transfer in well-defined multichromophoric
structures are indispensable to the development of artificial photofunctional
systems. Herein, we report on the synthesis, characterization, and
photophysical properties of a rationally designed multichromophoric
tetracationic cyclophane, DAPP­Box4+, containing a
diaza­pero­pyrenium (DAPP2+) unit and an extended
viologen (Ex­BIPY2+) unit, which are linked together
by two p-xylylene bridges. Both 1H NMR
spectroscopy and single-crystal X-ray diffraction analysis confirm
the formation of an asymmetric, rigid, box-like cyclophane, DAPP­Box4+. The solid-state superstructure of this cyclophane reveals
a herringbone-type packing motif, leading to two types of π···π
interactions: (i) between the Ex­BIPY2+ unit and the
DAPP2+ unit (π···π distance
of 3.7 Å) in the adjacent parallel cyclophane, as well as (ii)
between the Ex­BIPY2+ unit (π···π
distance of 3.2 Å) and phenylene ring in the closest orthogonal
cyclophane. Moreover, the solution-phase photophysical properties
of this cyclophane have been investigated by both steady-state and
time-resolved absorption and emission spectroscopies. Upon photoexcitation
of DAPP­Box4+ at 330 nm, rapid and quantitative intramolecular
energy transfer occurs from the 1*Ex­BIPY2+ unit to the DAPP2+ unit in 0.5 ps to yield 1*DAPP2+. The same excitation wavelength simultaneously
populates a higher excited state of 1*DAPP2+ which then undergoes ultrafast intramolecular electron transfer
from 1*DAPP2+ to Ex­BIPY2+ to
yield the DAPP3+•–Ex­BIPY+• radical ion pair in τ = 1.5 ps. Selective excitation of DAPP2+ at 505 nm populates a lower excited state where electron
transfer is kinetically unfavorable.