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, DAPPBox, containing a diazaperopyrenium (DAPP) unit and an extended viologen (ExBIPY) unit, which are linked together by two p-xylylene bridges. Both H NMR spectroscopy and single-crystal X-ray diffraction analysis confirm the formation of an asymmetric, rigid, box-like cyclophane, DAPPBox. The solid-state superstructure of this cyclophane reveals a herringbone-type packing motif, leading to two types of π···π interactions: (i) between the ExBIPY unit and the DAPP unit (π···π distance of 3.7 Å) in the adjacent parallel cyclophane, as well as (ii) between the ExBIPY 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 DAPPBox at 330 nm, rapid and quantitative intramolecular energy transfer occurs from the ExBIPY unit to the DAPP unit in 0.5 ps to yield DAPP. The same excitation wavelength simultaneously populates a higher excited state of DAPP which then undergoes ultrafast intramolecular electron transfer from DAPP to ExBIPY to yield the DAPP-ExBIPY radical ion pair in τ = 1.5 ps. Selective excitation of DAPP at 505 nm populates a lower excited state where electron transfer is kinetically unfavorable.