Electron transfer and multi-electron accumulation in ExBox⁴⁺

Molecules capable of accepting and storing multiple electrons are crucial components of artificial photosynthetic systems designed to drive catalysts, such as those used to reduce protons to hydrogen. ExBox⁴⁺, a boxlike cyclophane comprising two π-electron-poor extended viologen units tethered at both ends by two p-xylylene linkers, has been shown previously to accept an electron through space from a photoexcited guest. Herein is an investigation of an alternate, through-bond intramolecular electron-transfer pathway involving ExBox⁴⁺ using a combination of transient absorption and femtosecond stimulated Raman spectroscopy (FSRS). Upon photoexcitation of ExBox ⁴⁺, an electron is transferred from one of the p-xylylene linkers to one of the extended viologen units in ca. 240 ps and recombines in ca. 4 ns. A crystal structure of the doubly reduced species ExBox²⁺ was obtained. From outside the box: A viologen-based cyclophane ExBox⁴⁺ is photoexcited revealing a new through-bond pathway for electron transfer to ExBox⁴⁺. The solid-state structure with both redox states ExBox ⁴⁺ and ExBox²⁺ present, confirms that transferred electrons cause ExBox²⁺ to become flattened and form semiquinoidal structures capable of stabilizing unpaired electrons.