Enhancement of fluorescence and photostability of luminescent radicals by quadruple addition of phenyl groups

Quadruple addition of phenyl groups to a diphenylpyridylmethane skeleton was achieved. The derived (3,5-difluoro-4-pyridyl)bis(2,6-dichloro-3,4-diphenylphenyl)methyl radical (Ph4-F2PyBTM) displayed superior luminescence properties to the previously reported organic diphenylpyridylmethyl radicals in various organic solutions at room temperature, and it reached 33% photoluminescence quantum efficiency in PMMA film. With the introduction of methoxy groups, quadruple adduct (MeOPh)4-F2PyBTM and triple adduct (MeOPh)3-F2PyBTM were isolated. The excited state of these radicals showed an intramolecular charge transfer character and efficient fluorescence only in nonpolar solvents. The radicals retained not only persistency at the ground state but also durability under photoirradiation, with a 6 to 14 fold improved photostability with respect to the original PyBTM.