Engineering optical anisotropy in paramagnetic organic crystals: Dichroism of nitronyl nitroxide radicals

Abstract

Six aryl- and pyridine-substituted nitronyl-nitroxide radicals were synthesized and characterized to investigate their optical anisotropic properties. Single-crystal X-ray diffraction analysis revealed molecular packing organized by either halogen and hydrogen bonding or hydrogen bonding alone. Single-crystal electronic absorption spectra in the visible region of three studied radicals exhibit pronounced linear dichroism, while single crystals of other radicals do not demonstrate this property. Time-dependent DFT and ab initio calculations were employed to determine the transition dipole moment (TDM) vectors corresponding to the long-wavelength absorption bands. For all radicals, these vectors are found to be practically parallel to the O⋯O direction of the nitronyl-nitroxide chromophore. Correlation between the dichroic properties and crystal structure was established through comprehensive analysis of TDM vector orientations relative to the crystal surface. The strongest dichroic effect was observed in crystals where all projections of the TDM vectors onto the illuminated face are parallel to each other, while weaker or absent effects correspond to non-parallel arrangements. This study constitutes the first systematic investigation of linear dichroism in paramagnetic organic crystals, thereby establishing new avenues for developing multifunctional materials that respond to both optical and magnetic stimuli.