Electron delocalization in a rigid cofacial naphthalene-1,8:4,5- bis(dicarboximide) dimer

Abstract

Investigating through-space electronic communication between discrete cofacially oriented aromatic π-systems is fundamental to understanding assemblies as diverse as double-stranded DNA, organic photovoltaics and thin-film transistors. A detailed understanding of the electronic interactions involved rests on making the appropriate molecular compounds with rigid covalent scaffolds and π-π distances in the range of ca. 3.5 Å. Reported herein is an enantiomeric pair of doubly-bridged naphthalene-1,8:4,5- bis(dicarboximide) (NDI) cyclophanes and the characterization of four of their electronic states, namely 1) the ground state, 2) the exciton coupled singlet excited state, 3) the radical anion with strong through-space interactions between the redox-active NDI molecules, and 4) the diamagnetic diradical dianion using UV/Vis/NIR, EPR and ENDOR spectroscopies in addition to X-ray crystallography. Despite the unfavorable Coulombic repulsion, the singlet diradical dianion dimer of NDI shows a more pronounced intramolecular π-π stacking interaction when compared with its neutral analog. Two are better than one: A doubly-bridged naphthalene diimide (NDI) cyclophane has been synthesized. It shows efficient π-orbital overlap between two rigid cofacial NDI units. The resulting through-space electronic delocalization in the neutral as well as reduced states induces emergent photophysical, electrochemical, and magnetic properties both in solution and in the solid phase. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.