Synthesis and Properties of Cycloparaphenylene-2,5-pyridylidene: A Nitrogen-Containing Carbon Nanoring

The first synthesis of a nitrogen-containing cycloparaphenylene, cyclo[14]paraphenylene[4]2,5-pyridylidene ([14,4]CPPy), has been achieved. A palladium-catalyzed stepwise assembly of 2,2'-bipyridine, benzene, and L-shaped cyclohexane units, followed by NaHSO(4)/o-chloranil-mediated aromatization, successfully provided [14,4]CPPy. While the absorption and fluorescence properties of [14,4]CPPy were somewhat similar to those of cycloparaphenylenes (λ(abs) = 344 nm, ε = 7.3 × 10(4) cm(-1) M(-1), λ(em) = 427 nm, Φ(F) = 0.80), it was found that [14,4]CPPy possesses an interesting halochromic property.