Constructions Of Functional π Systems Based On Propargyl Radicals

Organic radicals are a class of molecules that have been widely studied so far.They not only are key intermediates in organic reactions,but also play a significant role in the applications as organic functional materials.With the development of radical chemistry,a series of organic π-radicals have been synthesized and investigated.Unlike the closed-shell molecules in which π-electrons exist in the bonding orbitals,there is one or more unpaired electrons in their electronic structures.Due to the open-shell character of these molecules,they have exhibited unique optical,electronic and magnetic properties and potentially applied in organic electronics,energy storage,and spintronics.However,these molecules are highly active,making their synthesis and characterization a challenging task.Therefore,the developments of new synthetic methods and stabilization strategies are important for construction of stable organic open-shell π-systems.In-depth study of organic radicals helps to control their reactivity,and further guide the molecular design in the field of organic functional materials.Therefore,starting from the active propargyl and fulvenallenyl radicals,we develop 9-ethynyl-fluorene radical using both kinetic and thermodynamic stabilization strategies.Based on 9-ethynyl-fluorene radical,a series of novel π-conjugated molecules have been synthesized,and the relationship between molecular structures and properties are revealed.In addition,we also investigate the stabilizing effect of propargyl substituents on organic open-shell structures,and their physical properties are further studied.The specific research contents are as follows:(1)We designed and synthesized 9-ethynyl-fluorene radicals with different substituents,and further studied their reactivity.Both experimental and theoretical results indicate that combining spin density distribution and the effects of steric hindrance,9-ethynylfluorene radicals can selectively undergo head-to-tail,head-to-head dimerization.In addition,we have also found that 2,4,6-trimethylphenyl-substituted 9-ethynylfluorene radicals can undergo rearrangement and cyclization reactions at room temperature,resulting in unique 3,4-dimethylenecyclobutene derivatives.(2)A series of cyclobutenyl o-quinodimethane-bridged π-conjugated molecules were designed and synthesized by π-extension of 9-ethynylfluorene radicals.Single crystal structure analyses indicate that a close overlap between the π moieties at the terminals along the inner edge of the spiral is generated,suggesting a strong intramolecular π-π interaction.Two enantiomers were successfully resolved,and their Coton absorption reached 600 nm.Combined with theoretical calculations,we investigated the redox behavior of these compounds,which revealed that the cyclobutadiene resonance structure had a significant contribution to the structure of its charged redox species.(3)We designed and synthesized a series of helical-shaped cyclobutenyl o-quinodimethane-bridged indeno[1,2-b]fluorene diradicals.Based on their ESR data as well as theoretical calculations,these helical π-conjugated diradicals exhibited large diradical characters and a extremely small singlet-triplet energy gap.Besides,we also observed an unusual ESR signal corresponding to the forbidden transition of ΔMs = 2.Furthermore,the two open-shell diradicals are susceptible to oxidation in an aerobic solution,eventually forming a furan-bridged π-conjugated structure.(4)We designed and synthesized a tripropargyl-substituted truxene triradicals.The product of ESR intensity and temperature increases with temperature,indicating a quartet ground state and a small doublet-quartet energy gap,which was supported by and theoretical calculations.In addition,their infrared region(IR)spectra also revealed the contribution of the allenyl resonance struct ure.