Fluorescent Supramolecular Organic Frameworks Constructed by Amidinium-Carboxylate Salt Bridges

We report here a set of fluorescent supramolecular organic frameworks (SOFs) that incorporate aggregation-induced emission (AIE) units within their frameworks. The fluorescent SOFs of this study were constructed by linking the tetraphenylethylene (TPE)-based tetra(amidinium) cation TPE⁴⁺ and aromatic dicarboxylate anions through amidinium-carboxylate salt bridges. The resulting self-assembled structures are characterized by fluorescence quantum yields in the range of 4.6∼14 %. This emissive behavior is ascribed to a combination of electrostatic interactions and hydrogen bonds that operate in concert to impede motions that would otherwise lead to excited state energy dissipation. Single-crystal X-ray diffraction analyses revealed that the length of the dicarboxylate anion bridges has a considerable impact on the structural features of the resulting frameworks. Nevertheless, all SOFs prepared in the context of the present study were found to display emissive features characteristic of TPE-based AIE luminogens with only a modest dependence on the structural specifics being seen. The SOFs reported here could be reversibly "broken up" and "reformed" in response to acid/base stimuli. This reversible structural behavior is consistent with their SOF nature.

Hydrogen bond-Driven Self-Assembly between Amidinium Cations and Carboxylate Anions: A Combined Molecular Dynamics, NMR Spectroscopy, and Single Crystal X-ray Diffraction Study

A combination of molecular dynamics (MD), NMR spectroscopy, and single crystal X-ray diffraction (SCXRD) techniques was used to probe the self-assembly of para- and meta-bis(amidinium) compounds with para-, meta-, and ortho-dicarboxylates. Good concordance was observed between the MD and experimental results. In DMSO solution, the systems form several rapidly exchanging assemblies, in part because a range of hydrogen bonding interactions is possible between the amidinium and carboxylate moieties. Upon crystallization, the majority of the systems form 1D supramolecular polymers, which are held together by short N-H⋅⋅⋅O hydrogen bonds.

Tuning the electronic coupling in Mo2-Mo2 systems by variation of the coordinating atoms of the bridging ligands

Three novel [Mo2]-bridge-[Mo2] complexes were synthesized by a convergent assembling reaction of the dimetal precursor Mo2(DAniF)3(O2CCH3) (DAniF = N,N'-di(p-anisyl)formamidinate) with the bridging ligands terephthalamidine, terephthalamide and dithioterephthalamide. The structures of these compounds, [Mo2(DAniF)3]2[μ-1,4-{C(E)NH}2-C6H4] (E = NH (), O () or S ()), were determined, either by X-ray crystallography or (1)H NMR spectroscopy, to be the analogues of the terephthalate bridged dimolybdenum dimer. These compounds are structurally and electronically closely related by having the same structural skeleton and similar bonding parameters, which allowed us to analyze the differences between N, O and S atoms on the bridging ligand in promoting electronic interaction between the two [Mo2] units. In the electronic spectra, the metal to ligand charge transfer absorption bands, attributed to the HOMO (dδ) → LUMO (pπ*) transition, was red shifted as the variable atoms change from N to O to S. The mixed-valence species (+), (+) and (+), generated by one-electron oxidation of the neutral precursors and measured in situ, exhibited characteristic intervalence absorption bands, for which the energy and half-height bandwidth decreased from (+) to (+). Therefore, in comparison to O atoms, S atoms are capable of enhancing the electronic coupling between the two [Mo2] units, and the incorporation of N atoms to the bridging ligands slightly diminished the metal-metal interaction. The molecular structures and spectroscopic properties of these compounds were simulated by theoretical calculations at DFT level on the simplified models, which gave results consistent with the experimental observations.