Intramolecular Energy and Electron Transfer within a Diazaperopyrenium-Based Cyclophane

Abstract

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 DAPP^3+•-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.