Aromatic donor–acceptor (D–A) interactions are a type of non-covalent bond between a donor (electron-rich) and an acceptor (electron-deficient) aromatic molecules. Aromatic molecules feature one or more rings with ‘delocalized’ electrons. The aromatic D–A interactions are widely used for building supramolecular structures, which are assembly of molecules formed by non-covalent bonds like building blocks. The supramolecular structures have smart properties such as external stimuli-responsiveness and self-repairing. The stability and smart properties of supramolecular architectures formed by D-A interactions depend on characteristics of the D–A interactions in various circumstances. Shogo Amemori and colleagues from Kanazawa University have now investigated the solvent effects on a strength of D–A interactions. Their findings provide important insights into the origin of aromatic D–A interactions and new molecular design of supramolecular architectures.
The researchers worked with organic compounds known as pyromellitic diimide (PMDI) and pyrene (Py) derivatives as acceptors and donors, respectively. The strength of the D–A interaction between the PMDI and Py derivatives was evaluated by the association constant of the D–A interaction, in which a higher association constant means stronger D–A interactions.
Amemori and colleagues used solvents with a variety of polarity for the evaluation of solvent effect on the D-A interaction. Polarity is related to the separation of charge (into positive and negative components) within a molecule; in low-polarity molecules, the charge separation is small. As most previous studies focused on low-polarity solvents belonging to the class of aliphatic molecules, the scientists explored another class of compounds, the so-called poly(dimethylsiloxanes) (PDMS) or oligo(dimethylsiloxane) (ODMS). These molecules consist of backbone of silicon-oxygen bonds fully covered organic groups, and, as solvents, generally lead to low-polarity.
The main finding of the researchers is that the siloxanes lead to higher association constants of the D–A interactions between PMDI and Py derivatives compared to that of hexane as aliphatic solvents. As to the origin of this effect, the scientists point to the ‘incompatibility’ between the aromatic molecules (PMDI and Py) and the siloxane molecules.
Amemori and colleagues point out that further investigation using various donor and acceptor molecules is necessary. Nevertheless, the result that D–A interactions are stronger in PDMS and ODMS solvents than in aliphatic solvents, possibly due to the generally poor solubility — quoting the scientists — “may help elucidate the origin of aromatic D–A interactions in low-polarity solvents”.
Absorption spectra and photographs of mixtures containing Py and PMDI in PDMS, ODMS(hexamethyldisiloxane), n-hexane and chloroform (Concentration of Py and PMDI are 11 mM and 2 mM, respectively) . Darker color of solutions and a higher absorbance represent a higher association constant.
Title: Poly(dimethylsiloxane) and oligo(dimethylsiloxane) solvent effects on aromatic donor–acceptor interactions
Journal: Chemical Communications
This work was supported by JSPS KAKENHI Grants 18K14187 (Early-Career Scientists).