Intramolecular Energy and Electron Transfer within a Diazaperopyrenium-Based Cyclophane

Abstract

High Resolution Image Download MS PowerPoint Slide 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 4+, containing a diazaperopyrenium (DAPP 2+ ) unit and an extended viologen (ExBIPY 2+ ) unit, which are linked together by two p -xylylene bridges. Both 1 H NMR spectroscopy and single-crystal X-ray diffraction analysis confirm the formation of an asymmetric, rigid, box-like cyclophane, DAPPBox 4+ . 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 2+ unit and the DAPP 2+ unit (π···π distance of 3.7 Å) in the adjacent parallel cyclophane, as well as (ii) between the ExBIPY 2+ 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 4+ at 330 nm, rapid and quantitative intramolecular energy transfer occurs from the 1* ExBIPY 2+ unit to the DAPP 2+ unit in 0.5 ps to yield 1* DAPP 2+ . The same excitation wavelength simultaneously populates a higher excited state of 1* DAPP 2+ which then undergoes ultrafast intramolecular electron transfer from 1* DAPP 2+ to ExBIPY 2+ to yield the DAPP 3+• –ExBIPY +• radical ion pair in τ = 1.5 ps. Selective excitation of DAPP 2+ at 505 nm populates a lower excited state where electron transfer is kinetically unfavorable.