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

## TL;DR

Researchers synthesized a new compound and studied its complex with another molecule, finding stable structures and fast electron transfer processes.

## Contribution

The study reveals structure-property relationships in supramolecular complexes involving di(3-pyridyl)ethylene derivatives and identifies fast back electron transfer in specific isomeric complexes.

## Key findings

- A highly stable pseudocyclic complex forms between (Z)-2 and (E)-1 with logKD·A = 8.48.
- Back electron transfer is approximately two times faster in (E)-1·(Z)-2 compared to (E)-1·(E)-2.
- Quantum chemical calculations suggest the photorelaxation pathway for the complexes.

## Abstract

The Z-isomer of N,N’-diammoniopropyl derivative of di(3-pyridyl)ethylene was synthesized. The structure and stability of complexes between this non-planar weak acceptor (A, (Z)-2) and a planar strong donor, the E-isomer of bis(18-crown-6)stilbene (D, (E)-1), 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 (logKD·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 (E)-1 with (E)- and (Z)-2 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 (E)-1·(Z)-2 than in closely related (E)-1·(E)-2. Meanwhile, it is ~67 times slower in complex (E)-1·(E)-2 than in the isomeric complex based on N,N’-diammoniopropyl derivative of (E)-di(4-pyridyl)ethylene. Quantum chemical (DFT, TD-DFT) calculations suggest the actual photorelaxation pathway for the complexes under study.