Crystal structures, magnetic and non-linear optical properties of methoxyphenyl nitronyl–nitroxide radicals

Abstract

The three isomers (para, meta, and ortho, hereafter denoted P, M and O) of 2-(methoxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide have been synthesized and their crystal structures solved. The lattice parameters and space groups are: P, a= 9.997(2)Å, b= 19.347(3)Å, c= 9.322(4)Å, β= 128.67(2)°, Cc, Z= 4; M, a= 6.391(1)Å, b= 11.230(1)Å, c= 10.228(1)Å, β= 107.78(1)°, P2₁, Z= 2; O, a= 12.203(6)Å, b= 15.677(6)Å, c= 14.670(3)Å, Pbca, Z= 8. The exchange couplings between the radical molecules are deduced from an analysis of the linewidths of the electron paramagnetic resonance (EPR) spectra at X- and Q-band, and are found to be |J|= 0.2 cm^–1 for P, |J|= 1.6 cm^–1 for M and |J|= 0.35 cm^–1 for O. The properties of M and P as frequency-doubling materials are briefly investigated on the basis of second-harmonic generation (SHG) experiments on powders: both are found to be non-phase-matchable materials.