Study on the Heteroatom Influence in Pyridine-Based Nitronyl Nitroxide Biradicals with Phenylethynyl Spacers on the Molecular Ground State

Abstract

Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV−vis spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra with exact half line spacing as compared to monoradicals (giso = 2.0067) with isotropic line spacing |aN|= 7.36 G. This indicates strong, intramolecular exchange coupling (J ≫ 7 × 10-4 cm-1; J/aN ≫ 1) of the biradicals with in the limit of EPR. The temperature dependence on the Δms = ±2 signal intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or its near-degeneracy with the singlet state. UV−vis studies of 1−5 show characteristic differences in the extinctions of n−π* transitions around 600 nm. Both biradicals 1 and 2 were crystallized in monoclinic space groups C2/c and P21/a with the intraradical distances 1.54 and 1.47 nm, respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8) method to calculate the singlet−triplet (ΔEST) energy difference and the spin density distribution. Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not alter the triplet ground-state spin multiplicities supporting the obtained experimental results.