Dual Acid Responses of Stable Luminescent Diphenylpyridylmethyl Radicals With Additional Pyridyl Groups

Polychlorinated diphenylpyridylmethyl radicals, exemplified by (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), have garnered significant attention as stable radical emitters. Due to their halogenated pyridyl groups, these radicals also exhibit weak basicity and acid-responsive properties. To further explore and expand the acid-responsive behavior of PyBTM, we substituted additional pyridyl groups (3-pyridyl, 4-pyridyl, and 2-methoxy-3-pyridyl) for chlorine atoms at the para-positions relative to the methyl radical center. This modification yielded six novel, stable luminescent radicals. Spectroscopic analysis revealed distinct changes in absorption and emission profiles upon titration with a Brønsted acid (p-toluenesulfonic acid), a Lewis acid (tris(pentafluorophenyl)borane), and metal cations (Ag⁺, Zn²⁺, Cu²⁺). Complementary ¹H NMR and DFT studies suggested that the 3-pyridyl and 4-pyridyl substituents act as stronger bases compared to PyBTM, whereas the basicity of the 2-methoxy-3-pyridyl is attenuated. The observed optical changes, including the emergence of new absorption bands and emission quenching, were rationalized using TD-DFT calculations.