Persistent Radical Pairs between N-Substituted Naphthalimide and Carbanion Exhibit pKa-Dependent UV/Vis Absorption

A new strategy was devised for estimating and screening pK(a)values among different carbon acids under ambient conditions by using the UV/Vis absorption spectrum of persistent radical pairs (PRPs), which are generated from an N-substituted naphthalimide (NNI) derivative in the presence of various carbanions in organic solutions. The electron paramagnetic resonance (EPR) spectroscopy was used to examine the presence of radicals. Unexpectedly, it was discovered that the UV/Vis spectrum of PRPs reveals a distinct linear relationship between the PRP absorption and the pK(a)value of a corresponding carbon acid, which is likely due to the energy difference among different RPRs. The finding may offer organic chemists an alternative reference to conduct carbanion-mediated reactions in various organic solutions.