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Reports: DNI451737-DNI4: Supramolecular Interactions of pi-Acidic Naphthalenediimide Receptors with Anions

Sourav Saha, Florida State University

Using the funds provided to us by ACS-PRF we have discovered that depending on the Lewis basicity of anions and electron-accepting ability of pi-acidic receptors, such as naphthalene diimides (NDIs) and perylene diimides (PDIs), in aprotic solvents they can interact with each other through four different modes of electronic interactions: (i) strong Lewis basis anions (OH–, F–) reduce the ¹-acidic receptors to corresponding radical anions and dianions via thermal electron transfer (ET), (ii) less Lewis basic anions (AcO–, H2PO4–, Cl–) does so via photoinduced ET (PET), (iii) non-Lewis basic anions (Br–, I–) form diamagnetic charge-transfer complexes with these pi-acids, and (iv) charge-diffuse anions (TfO–, ClO4–) form weak anion–pi and CH–anion contacts. Upon ET anions behave as sacrificial agents.

Although anion–pi-acid complexes have been known for more than a decade, formal ET from Lewis basic F– and OH– anions remained obscured, because it was presumed that, since fluorine is the most electronegative element, F– anion cannot be an electron donor. This assumption ignored the fact that, owing to its small ionic radius and large electronic reorganization energy and charge density, F– is one of the most Lewis basic anions, which by definition should have a high HOMO level.  As a result, when F– is poorly solvated in aprotic solvents, it can act as a good reducing agent, but in aprotic solvents, it becomes so heavily solvated and stabilized—F– has large hydration energy—that it loses its reducing ability. Our research has elicited this fundamental knowledge.

While the ET and CT interactions produce anion-specific spectroscopic signals (UV-Vis and EPR), enabling naked-eye detection of Lewis basic anions, we have exploited the nonchromogenic anion–pi and CH–anion interactions for anion template-directed synthesis of a N,N'-dipyridyl-NDI (DPNDI) based non-catenated square-grid metal–organic framework, which contains large cavities (20 x 20 A2) filled with ClO4– anions.