Effects of hydrocarbon substituents on highly fluorescent bis(4-phenylphenyl)pyridylmethyl radical derivatives

Abstract

Eight new stable luminescent radicals are reported. We previously reported that the photoluminescence quantum yields (PLQYs) of diphenylpyridylmethyl radicals is dramatically increased by the addition of mesityl (2,4,6-trimethylphenyl) groups. By changing the mesityl groups to other hydrocarbon-substituted phenyl groups, the issue of the fluoresence efficiency and photostability of these radicals could be clarified. Contrary to our expectations, changing 2,4,6-trimethyl to 2,4,6-triisopropyl did not produce a positive effect, other than some improvement in purification. Nevertheless, a PLQY of more than 60% was a significant value. The addition of methyl substituents at the meta-positions drastically quenched the fluorescence in dichloromethane, while maintaining bright fluorescence in chloroform. Removing one of the ortho-methyl groups somewhat decreased the fluorescence efficiency but greatly improved the photostability. By using a phenyl substituent instead of a methyl group, more stabilization under photoirradiation could be achieved.