Quantifying through-space charge transfer dynamics in π-coupled molecular systems

Understanding the role of intermolecular interaction on through-space charge transfer characteristics in pi-stacked molecular systems is central to the rational design of electronic materials. However, a quantitative study of charge transfer in such systems is often difficult because of poor control over molecular morphology. Here we use the core-hole clock implementation of resonant photoemission spectroscopy to study the femtosecond charge-transfer dynamics in cyclophanes, which consist of two precisely stacked p pi-systems held together by aliphatic chains. We study two systems, [2,2] paracyclophane (22PCP) and [4,4] paracyclophane (44PCP), with inter-ring separations of 3.0 and 4.0 angstrom, respectively. We find that charge transfer across the pi-coupled system of 44PCP is 20 times slower than in 22PCP. We attribute this difference to the decreased inter-ring electronic coupling in 44PCP. These measurements illustrate the use of core-hole clock spectroscopy as a general tool for quantifying through-space coupling in pi-stacked systems.