Persistent Room‐Temperature Radicals from Anionic Naphthalimides: Spin Pairing and Supramolecular Chemistry

Abstract

N‐Substituted naphthalimides (NNIs) have been shown to exhibit highly efficient and persistent room‐temperature phosphorescence from an NNI‐localized triplet excited state, when the N‐substitution is a sufficiently strong donor and mediates an intramolecular charge‐transfer (ICT) state upon photo‐excitation. This work shows that, when the electron‐donating ability of the N‐substitution is further increased in the presence of a carbanion or phenoxide, spontaneous electron transfer (ET) occurs and results in radical anions, verified with electron‐paramagnetic resonance (EPR) spectroscopy. However, the EPR‐active anion is surprisingly persistent and impervious to nucleophilic and radical reactions under anionic conditions. The stability is thought to originate from an intramolecular spin pairing between the N‐donor and the NI acceptor post ET, which is demonstrated in supramolecular chemistry.

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

“…As can be expected, the single‐electron transfer process readily takes place between a deprotonated CA donor (or carbanion) and the NNI‐Br acceptor and gives rise to a colored solution as has been recently verified in both intramolecular and intermolecular systems . However, it is discovered for the first time here that the dramatic color difference among different donor CAs (in the presence of the same acceptor molecules) points to the fact that the radical species are not contributed from the NNI‐Br acceptor alone and are consistent with the radical pair model.…”

“…The application is based on our recent discovery that when an organic anion, such as a carbanion, comes in close contact with an electron‐deficient aromatic compound, such as N‐substituted naphthalimide (NNI), a persistent radical pair (PRP) forms . The pair apparently lacks reactivity towards any reactions generally involving anions or radicals in the ambient condition.…”

“…To explore the possibility, a series of CAs with known p K a values were first selected with photos showing various colors in DMSO solutions in the presence of NNI‐Br (Scheme S1 showing the synthetic route) and excess t BuOK (Figure a), resulting from the formation of PRPs. The elaborate mechanism for the observed coloration has been recently described by us in a previous publication . The protocol for duplicate experiments is described as follows: to DMSO solution (20 mL) of CAs (2.5×10 −4 mol L −1 ) was added excess t BuOK, followed by the addition of NNI‐Br (equivalent molar ratio vs. CA) into the system.…”

“…However, it is discovered for the first time here that the dramatic color difference among different donor CAs (in the presence of the same acceptor molecules) points to the fact that the radical species are not contributed from the NNI‐Br acceptor alone and are consistent with the radical pair model. In addition, the 1 H NMR spectra of the CA/NNI‐Br mixtures under the basic condition lost nearly all of the intensity, suggesting that both organic components are paramagnetic in the presence of t BuOK in solution (Figure S1) . The representative UV/Vis absorption spectra of these radical pairs are shown in Figure c (Table S1).…”

Persistent Radical Pairs Between N-Substituted Naphthalimide and Carbanion Exhibit pKa-Dependent UV-vis Absorption

“…The application is based on our recent discovery that when an organic anion, such as a carbanion, comes in close contact with an electron-deficient aromatic compound, such as Nsubstituted naphthalimide (NNI), a persistent radical pair (PRP) forms. 23 The pair apparently lacks reactivity towards any reactions generally involving anions or radicals in the ambient condition. More interestingly, the PRP exhibits an absorption spectrum in the visible range.…”