Capturing the geometry of the emissive state of a Cu(i) red emitter through strong intramolecular stacking forces

Abstract

Condensation of ethylenediamine with fluorenone produces a diimine with two terminal fluorophores and a flexible central backbone, N,N′-bis-fluoren-9-ylidene-ethane-1,2-diamine (flen). The diimine reacts with [Cu(MeCN)₄]BF₄ or CuI to produce a homoleptic compound of the stoichiometry [Cu(flen)₂]BF₄ or [Cu(flen)₂][CuI₂] respectively. Both complexes emit in the red part of the spectrum, with a maximum around 720 nm and excited state lifetime of 0.2 μs in solution. The crystal structures of the complexes reveal almost an identical Cu(I)-diimine core where the predominant forces are the intramolecular π–π interactions between the fluorenone aromatic systems resulting in considerably distorted coordination environments. The negligible difference in the emission maximum between solution and solid state indicates that the vibrationally relaxed excited state, assigned as MLCT/IL, adopts a structure closely similar to the crystallographically determined one.