Electron transfer and multi-electron accumulation in ExBox⁴⁺

Abstract

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. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.