Trapping of a photoinduced metastable paramagnetic state of a molecular material is one of the most fascinating phenomena of molecular bistability, due to its potential uses in memory devices, sensors, or radiation detectors. This thesis focuses on two photomagnetic systems that are very different but show conceptual similarity - spin-crossover (SCO) complexes and sigma-dimers of organic radicals. Despite the long history of research on SCO in complexes of 3d⁴-3d⁷ transition metal ions, the design and preparation of SCO complexes for surface functionalization, the prediction of spin state behavior, and the combination of SCO with other properties, such as conductivity or ferroelectricity, in a single material are current emerging areas associated with many challenges. In this thesis, we demonstrate that 2,2′-biimidazole offers a convenient platform for modification of SCO complexes for surface functionalization. The ligand can be conveniently alkylated at the protonated nitrogen atoms, preserving essentially the same ligand filed strength. We suggest a simple approach to predicting the spin state of tris-homoleptic diimine complexes with the Fe(II) ion. We also show an effective way to incorporate TCNQ*[superscript δ]⁻ organic radical as the conducting moieties into SCO complexes. These complexes offer a rare example of highly conducting photomagnets. The light-induced magnetism has been studied extensively in spin crossover coordination compounds and charge transfer complexes. Prior to the present work, however, no such studies existed on organic systems in solid state form. Here we demonstratefor the first time the photoinduced splitting of hypervalent 4-center 6-electron S***S−S***S bridged σ-dimers of bisdithiazolyl radicals, 8-fluoro-4-ethyl-4H-bis[1,2,3]dithiazolo[4,5-β: 5',4'-e]pyridin-3-yl, β-dimer phase (β-FBPEt) and 8-fluoro-4-methyl-4H-bis[1,2,3]dithiazolo[4,5-β: 5',4'-e]pyridin-3-yl, (β-FBPMe). The results are very encouraging, as we have been able to generate the S=1/2 radical forms of these materials under irradiation. Moreover, these radicals exhibit an unprecedented stabilty in the solid state; one of them convert back to the diamagnetic σ-dimer (S=0) at the temperature as high as 242 K. The dimer-to-radical photoconversion, which we dubbed the LIRT (light-induced radical trapping) effect, has been demonstrated by magnetic susceptibility measurements, optical spectroscopy, and single crystal X-ray diffraction.
