Long-Lived Open-Shell States of Boron-Locked Anilido-Pyridines: Spectral, Electrochemical, and DFT Study

BF₂-rigidified anilido-pyridines have attracted considerable interest due to their photophysical properties, whereas a comprehensive study of the electronic structure and stability of their open-shell redox states has not been performed previously. To gain deeper insight into the electronic and spectral properties of the ion-radical states, as well as spin density distribution, a new series of variously furnished BF₂-rigidified anilido-pyridines was synthesized via the modular approach and studied using a wide variety of spectral, electrochemical, and DFT methods. The discovered remarkable stability of the radical-cationic state (the ESR-estimated half-life time achieves 6 h at r.t.) opens a possibility for multiple repetitive cycling between the fluorophoric neutral state and radical-cation, which causes fluorescence quenching, thus giving rise to intrinsic electrofluorochromic properties. The BF₂-locked anilido-pyridine furnished with a nitro group exhibits redox-ambipolarity, i.e., reversible both oxidation and reduction, which is extremely rare for this type of dyes. The ESR-estimated half-life of the radical-anions is 30 h at r.t. The results of the UV-vis spectroelectrochemical study of the radical-anions were rationalized using quantum-chemical calculation of the energy and localization of the electronic transitions. Intrinsic electrofluorochromism and redox-ambipolarity are complemented by low-temperature phosphorescence (the lifetimes of the triplet state were determined), which emphasizes a diversity of properties of the novel BF₂-rigidified anilido-pyridines.