NIR Emission and Acid-Induced Intramolecular Electron Transfer Derived from a SOMO–HOMO Converted Non-Aufbau Electronic Structure

Some organic radicals violate the Aufbau principle and have unique electronic structures in which the energy level of the singly occupied molecular orbital (SOMO) is formally lower than that of the highest occupied molecular orbital (HOMO). We synthesized a novel organic radical with a SOMO–HOMO converted electronic structure, TPA-R<sup>•</sup>, a novel donor–acceptor hybrid of triphenylamine (an electron donor), and a stable polychlorinated diphenyl­(4-pyridyl)­methyl radical (an electron acceptor). TPA-R<sup>•</sup> exhibited fluorescence in the near-infrared region (λ<sub>max</sub> = 910 nm) in cyclohexane from a polar intramolecular charge-transfer excited state. Cyclic voltammetry, absorption spectroscopy, and density functional theory calculation revealed the inversion of the SOMO and HOMO levels in the electronic structure of TPA-R<sup>•</sup>. Addition of trifluoromethanesulfonic acid to TPA-R<sup>•</sup> caused a two-step change. Protonation initially occurred on the diphenylpyridylmethyl radical moiety to form TPA-[RH]<sup>•+</sup>. This protonation induced large modulation in the frontier orbitals, while the SOMO–HOMO converted electronic structure was conserved. Further addition of the acid caused unprecedented intramolecular electron transfer from the triphenylamine moiety to the protonated radical moiety, generating [TPA]<sup>•+</sup>-RH. TPA-[RH]<sup>•+</sup> and [TPA]<sup>•+</sup>-RH could be switched by changing the acidity of the solution. These results constitute the first example of the multistep switching behavior stimulated by a single external stimulus in the SOMO–HOMO converted non-Aufbau electronic structure and demonstrate its great potential for realizing unique molecular photonic and electronic functions.