Abstract

The crystal structures of two kinds of nitronyl nitroxide biradical with a phenolic substituent, 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (1a) and 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (2a) have been solved. The biradical 1a belongs to the monoclinic system with space group P2₁/n, a = 11.806(4) Å, b = 25.330(5) Å, c = 7.337(7) Å, β = 104.65(4)°, and Z = 4, while the biradical 2a to the orthorhombic system with Pbca, a = 20.206(4) Å, b = 40.405(4) Å, c = 11.888(3) Å, and Z = 8. From the magnetic susceptibility in the crystalline solid state, both 1a and 2a are found to have a triplet (S = 1) ground state with the intramolecular ferromagnetic interaction of 2J/k_B = 26.0 ± 0.5 K for 1a and 12 ± 1 K for 2a. The triplet ground states are confirmed by the EPR measurements on the isolated molecules in diluted glassy solutions. The singlet–triplet energy gaps of the biradicals with a phenolic hydroxy substituent are found to be reduced as compared with a parent non-substituted biradical.