"Through Space" Triplet Energy Transfer: Movement of Electrons through the Hemicarcerand Skeleton

Abstract

A great deal of interest has developed concerning the influence of the nature of the intervening media on electron transfer reactions in which electrons move from donor to acceptor sites separated by “insulating” material.’ Details of such processes can be probed by the study of triplet energy transfer based on the proposal of Dexter that transfer involves exchange of electrons between energy donor and acceptor.* Recently this approach was verified when Closs showed that energy transfer can occur between chromophoric groups separated by long distances, but connected by networks of single bonds.3 The rates of energy transfer within a number of related molecules, with varying interchromophoric distances and orientations, were correlated quantitatively with rates of charge transfer involving both electron and hole transfer in the same molecular ~ y s t e m . ~ The results lent increased credibility to the Dexter mechanism but did not resolve the question as to whether or not the intervening saturated structures played a special role in the process (through-bond mechanism). Here we describe the unambiguous observation of triplet energy transfer between unconnected systems separated by a molecular barrier (throughspace mechanism). An encapsulating host molecule (“hemicarcerand”) was used to separate the donor and acceptor in solution so that the through-bond mechanism as it is commonly defined would not be operative, but the path of closest approach could still be estimated with good certainty. Hemicarcerands are closedsurface hosts with rigid, physically isolated interiors that form kinetically stable complexes when large energetic barriers prevent guest egresse5 Work by Cram et al. in which the normally highly reactive cyclobutadiene was created inside a hemicarcerand confirms that species deeply buried within hemicarcerand interiors have no “bond-forming’’ contact with the extemal milieu.6 Therefore, a triplet energy donor placed inside a hemicarcerand would not come into direct contact with the energy acceptor. Both CPK molecular model examination and force field calculations7 predict that acetophenone would fit tightly within