Engineering Optical Anisotropy in Paramagnetic Organic Crystals: Dichroism of Nitronyl Nitroxide Radicals

Six aryl- and pyridine-substituted nitronyl-nitroxide radicals were synthesized and characterized to investigate their optical anisotropy properties. Single-crystal X-ray diffraction analysis revealed molecular packing organized either by halogen and hydrogen bonding or by hydrogen bonding alone. Electronic absorption spectra of these radicals in single crystals were recorded in the visible region, revealing pronounced linear dichroism in three compounds, while the others displayed no such 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 were found to be nearly 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 TDM vector projections were parallel to each other on the illuminated face, while weaker or absent effects corresponded to non-parallel arrangements. This study represents the first systematic investigation of linear dichroism in paramagnetic organic crystals, opening new possibilities for multifunctional materials responsive to both optical and magnetic stimuli.