Research On Syntheses, Structures And Property Of Novel Radical Cations

Radicals are atoms, molecules and ions with unpaired electrons, e.g. biradical molecule O2. Research on radicals has close relationship with reaction mechanism, structure chemistry and materials science. Due to the high reactivity, most of radicals are short-lived. How to stablize radicals therefore becomes a crucial issue in this field. It is important for people to understand the nature of chemistry reaction and develop new materials. This thesis gives a short summary of the research of radical cations. We have stabilized some new radical cations by using weakly coordinating anions and studied their structures and property.1. With salt AgAl((OC(CF3)3)4),9,10-Dialkoxyanthracenes were oxidized to radical cations and stabilized in solution, which dimerize upon crystallization and come back to radical cations upon dissolution. The identity and stability of radical cations have been unequivocally confirmed by EPR and UV-vis spectroscopy. The experimental results are supported by theoretical calculation. This work has provided conclusive evidence for reversible σ-dimerizations of persistent organic radical cations. Study of such reversible process may have an impact on exploring mechanism of oxidative oligo-and polymerization of aromatic systems.2. With SbF6-,an aniline radical cation TBA"+(TBA=2,4,6-tri-tBuC6H2NH2) was successfully stabilized both in the solution and solid state, and its structure was characterized by EPR spectra, electronic absorption and X-ray crystallography. This compound undergoes reversible isomerization upon heating or cooling in the solid state, which is unprecedented for a simple phenyl ring system and implies that the valence electrons of the radical cation can be controlled by temperature. The work provided the first example of an aniline radical cation in the solid state, which may be used as a stable intermediate for model studies.3. Upon one-electron oxidation, the parent benzidine (BZ) and its methylation derivatives (TMB) form radical-cation71dimers, whereas that modified with bulkier isopropyl groups (TPB) becomes a monomeric free radical. By increasing the chain length,π stacks of π-dimers were obtained for the radical cation of4,4’-terphenyldiamine (DATP). This work leads to a steric control synthesis and hence provides a systematic study of benzidine radical cations. The crystal structures of BZ·+and TMB·+offer conclusive evidence for π-dimer formation of oxidized benzidines, and that of DATP·+provides the first example of π-stacked oxidized p-phenylene oligomers. The single-crystal conductivity measurements show monomerized or π-dimerized radicals (BZ·+, TMB·+, and TPB·+) are nonconductive, whereas the π-stacked radical (DATP·+) is conductive. This implies that%stacks and π interactions are important for conduction in DATP·+and might also play an important role in the conduction of p-doped p-phenylene polymers, in addition to the traditional model that conduction results from bipolaron (or polaron) migration along polymer chains.4. A series of dialkoxy p-terphenyl oligomer radical cations have prepared and characterized. Their stacking structures and conductivity were effected by temperature and solvent. The complexes crystallize as nonsolvates and solvates to afford insulators and semiconductors. They differ in the presence or abserence of interaction between dimers. Their stacking modes are also effected by anions and the end groups. Such supermolecular isomerism of an oligomer radical cation is unprecedented in the field and might play a crucial role in understanding conductivity through π-π interaction for organic conductors.5. Sterically unprotected thiophene/phenylene co-oligomer radical cations BPnT·+(n=1-3) have been synthesized by using the polyfluoroalkoxy aluminate anion [Al{OC(CF3)3}4]-.The work has demonstrated that polyfluoroalkoxy aluminate anions are efficient tools for stabilizing and solubilizing sterically unprotected thiophene/phenylene co-oligomer radical cations, and they might do the same for other types of unsubstituted oxidized (co-)oligomers. The conductive meso-helical stacking, formed by cross-overlapping radical chains of BP2T+, is distinct from previously reported face-to-face overlaps of sterically protected (co-)oligomer radical cations. The isolation of such radical species together with their structural determination may have an impact on exploring interchain conduction of p-doped oligomers and polymers, and will open up a new avenue for electrical conductors.