Accessing Stable Pyridinium Nanohoops with Low-Lying LUMOs by Pyridine-Directed C-H Activation

Designing stable organic molecules with tailored electronic properties is key to advancing next-generation electronic materials. Cycloparaphenylenes (CPPs) and related nanohoops have emerged as promising p-type semiconductors, yet their use as n-type materials remains limited due to inherently high-lying LUMO levels. While pyridine-containing CPPs can be alkylated to lower the LUMO, the resulting pyridinium species are often unstable and unsuitable for practical applications. In this study, we report two complementary synthetic strategies to access bridged-pyridinium[n]CPPs as stable, n-type organic materials. A bottom-up approach enables intramolecular cyclization to form the bridged architecture, confirming the stability of this new motif. To streamline access and facilitate structural diversification, we developed a second strategy employing pyridine-directed C-H activation-marking the first example of pyridine-directed C-H functionalization directly on the CPP backbone. The resulting pyridinium-functionalized nanohoops were thoroughly characterized. Electrochemical and spectroscopic studies revealed low reduction potentials (as low as-0.84 V) and red-shifted optical absorption edges (up to 770 nm) for the doubly functionalized di(bridged-pyridinium)[10]CPP 5. Density functional theory calculations corroborate the observed trends in LUMO energies and support the utility of these molecules as n-type building blocks. Together, these results establish bridged-pyridinium[n]CPPs as a new class of stable, tunable materials with potential for organic electronic applications.