ESR Relaxation Studies on Orbitally Degenerate Free Radicals. II

Abstract

Careful linewidth and saturation studies were performed on several orbitally degenerate hydrocarbon free radicals in liquid solution as a function of temperature, solvent, and counterion, and these are compared with similar studies on nondegenerate hydrocarbon free radicals. The anomalous T1's and T2's attributed to the orbital degeneracy are found to be strikingly independent of all these variables. This is most clearly evidenced for the coronene and triphenylene anion radicals. The results for the benzene and cyclooctatetraene anions (alkali metal prepared) are complicated by a further contribution to T2-1 which increases with temperature. This contribution is correlated with counterion and solvent dependent ion-pairing effects. The intrinsic temperature-independent T 2 -l's for the orbitally degenerate free radicals are surprisingly well correlated with the small but anomalous deviations of the g values for these radicals from the Segal, Kaplan, and Fraenkel experimental fit to Stone's theory of g values. This is taken as positive evidence for the role of anomalous spin-orbit interactions in the mechanism(s) yielding the anomalous relaxation times. Their independence of temperature, solvent, and counterion suggests that the relaxation mechanism(s) is largely intramolecular.

Tuning Magnetic Interactions Between Triphenylene Radicals by Variation of Crystal Packing in Structures with Alkali Metal Counterions

“…It was also found that the spectral resolution of the monoanion 1 ·– generated by the Na-induced reduction is affected by temperature and coordinating solvents. Several years later, the Freed group reported the EPR study of the Li-induced reduction of 1 in DME, revealing similar results to those obtained with Na metal . However, all earlier works have been focused on solution behavior of in situ generated triphenylene monoanion-radicals that could not reveal geometry perturbation or specific coordination environment of 1 ·– .…”

“…47 This attracted significant attention of the scientific community to investigation of the reduction processes of 1 by EPR spectroscopy. 41,48,49 The EPR study of mono-and doubly-reduced triphenylene was first reported by the Boer group in 1967, confirming that both exhibit radical behavior. 48 It was also found that the spectral resolution of the monoanion 1 •− generated by the Na-induced reduction is affected by temperature and coordinating solvents.…”

“…During the following decades, reduction processes of triphenylene have been broadly studied in solution by UV–vis and IR spectroscopy − and electrochemistry. − Due to D 3h symmetry, the HOMO and LUMO of 1 are doubly degenerate, allowing a potential triplet ground state upon a 2-fold reduction . This attracted significant attention of the scientific community to investigation of the reduction processes of 1 by EPR spectroscopy. ,, The EPR study of mono- and doubly-reduced triphenylene was first reported by the Boer group in 1967, confirming that both exhibit radical behavior . It was also found that the spectral resolution of the monoanion 1 ·– generated by the Na-induced reduction is affected by temperature and coordinating solvents.…”

“…Several years later, the Freed group reported the EPR study of the Li-induced reduction of 1 in DME, revealing similar results to those obtained with Na metal. 49 However, all earlier works have been focused on solution behavior of in situ generated triphenylene monoanion-radicals that could not reveal geometry perturbation or specific coordination environment of 1 salt exhibits the semiconductor-like properties. 41 Recently, S ̌tefancǐc et al reported the first X-ray structure characterization of the monoreduced triphenylene crystallized from DME as [{K + 2 (DME)}(1 •− ) 2 ] ∞ .…”