Synthesis and Reactivity of Triazaphenanthrenes

Pyridonaphthyridines (triazaphenanthrenes) were prepared in 4 steps and in 13-52% overall yield using Negishi cross-couplings between iodopicolines and 2-chloro-pyridylzinc derivatives. After chlorination, Gabriel amination and spontaneous ring-closure, the final aromatization leading to the triazaphenanthrenes was achieved with chloranil. This heterocyclic scaffold underwent a nucleophilic addition at position 6 leading to further functionalized pyridonaphthyridines. The influence of these chemical modifications on the optical properties was studied by steady-state and time-resolved optical spectroscopy. While the thiophene-substituted heterocycles exhibited the most extended absorption, the phenyl- and furan-substituted compounds showed a stronger photoluminescence, reaching above 20% quantum yield and lifetimes of several nanoseconds.