Expansion of Photostable Luminescent Radicals by Meta‐Substitution

Polychlorinated pyridyldiphenylmethyl radicals having substituents meta to the position bearing the carbon‐centered radical (α‐carbon) are synthesized. All of them are stable in ambient conditions in solutions and fluorescent in cyclohexane. The fluorescence of the radicals with bromo, phenyl, 4‐chlorophenyl, or 2‐pyridyl substituents are enhanced in chloroform, while the emission of the radicals with 2‐thienyl or 2‐furyl substituents are quenched in chloroform. DFT and TD‐DFT calculations indicate that the first doublet excited states of the former are locally excited, while the first doublet excited states of the latter are charge transfer states from the π‐electron‐donating substituent to the accepting radical. The latter also show much higher photostability under 370‐nm light irradiation compared with the first reported photostable fluorescent radical, (3,5‐dichloro‐4‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl radical (PyBTM), with pronounced bathochromic shifts of the fluorescence. To clarify the role of meta‐substituents on stable luminescent radicals, six novel stable radicals (mBr2−F2PyBTM, mPh2−F2PyBTM, mClPh2−F2PyBTM, mPy2−F2PyBTM, mTh2−F2PyBTM, mFu2−F2PyBTM) were synthesized. All of them are stable in solutions at room temperature and fluorescent in cyclohexane. When the meta‐substituent becomes highly electron‐rich, the fluorescence was red‐shifted, and quenching and photostability were enhanced.