Highly Stable Supramolecular Donor–Acceptor Complexes Involving (Z)-, (E)-di(3-pyridyl)ethylene Derivatives as Weak Acceptors: Structure—Property Relationships

The <i>Z</i>-isomer of <i>N</i>,<i>N</i>'-diammoniopropyl derivative of di(3-pyridyl)ethylene was synthesized. The structure and stability of complexes between this non-planar weak acceptor (A, (<i>Z</i>)-<b>2</b>) and a planar strong donor, the <i>E</i>-isomer of bis(18-crown-6)stilbene (D, (<i>E</i>)-<b>1</b>), were studied using X-ray diffraction, ¹H NMR spectroscopy, and optical spectroscopy, including ¹H NMR and spectrofluorimetric titrations. In MeCN, the components form a very stable pseudocyclic bimolecular complex (log<i>K</i>_D·A = 8.48) due to homoditopic coordination of the ammonium groups of the acceptor to the crown moieties of the donor through numerous hydrogen bonds. Intrasupramolecular photo-driven electron transfer (ET) in the isomeric complexes of (<i>E</i>)-<b>1</b> with (<i>E</i>)- and (<i>Z</i>)-<b>2</b> was studied using steady-state absorption and fluorescence spectroscopy with time-resolved pulse absorption spectroscopy. It was found that back ET is approximately two times faster in complex (<i>E</i>)-<b>1·</b>(<i>Z</i>)-<b>2</b> than in closely related (<i>E</i>)-<b>1·</b>(<i>E</i>)-<b>2</b>. Meanwhile, it is ~67 times slower in complex (<i>E</i>)-<b>1·</b>(<i>E</i>)-<b>2</b> than in the isomeric complex based on <i>N</i>,<i>N</i>'-diammoniopropyl derivative of (<i>E</i>)-di(4-pyridyl)ethylene. Quantum chemical (DFT, TD-DFT) calculations suggest the actual photorelaxation pathway for the complexes under study.