Design,Synthesis,and Characterization Of Novel Organic Conjugated Phenolamine Radicals

Organic neutral radical is a special organic compound in metastable state,which contains one or more unbounded single electron.Because organic neutral free radicals have high reactivity and instability,they are easy to form dimers or react with oxygen,so research about their instability has been a hot topic in this field.Organic neutral free radicals can generally enhance the stability of free radicals by conjugation effect or steric hindrance effect to form persistent or stable free radicals.At present,a large number of stable organic neutral free radicals have been reported,such as triphenylmethyl radicals and their derivatives,polycyclic aromatic hydrocarbon biradicals,nitroxide radicals,phenoxy radicals and so on.Organic neutral radicals have many unique properties,such as quasi-reversible redox property,strong near infrared absorption,magnetism and excellent electrical properties.They can be used in optoelectronic devices,organic ferromagnetic materials and other fields.Therefore,it is of great significance to design and synthesize stable organic radical materials.The research was aim to improve the stability of organic neutral radicals through molecular aggregation and conjugation effects,and to explore the stability mechanism and electrical properties of free radicals.Based on triphenylamine framework,phenolic hydroxyl groups were introduced into para-position of benzene ring in triphenylamine,strong electron-withdrawing groups were used as acceptors and electron-pushing groups as donors,D-A-D type and D-π-D type organic conjugated phenolamine radicals were designed.We also adopted a low-cost,simple and feasible two-step synthesis routes to synthesize them.At the same time,the sources and stabilization mechanism of organic conjugated phenolamine radicals were systematically studied.The results showed that organic conjugated phenolamine radicals mainly came from phenoxy radicals.When the conjugated phenolamine radical is in the solid state,free electrons can delocalize in the whole molecular skeleton to form a variety of quinoid-type resonance structures and aromatized inorganic acid resonance structures,and the molecular aggregation and conjugation effects stabilize the radicals.When the organic conjugated phenolamine radical is in solution,and the radical signal is pretty weak due to the easy free rotation of the molecules.Based on the above experiments,we proposed Aggregation-induced radical(AIR)to explain the enhancement of free radical signal in the solid state.The VT ESR test and theoretical calculations showed that the organic conjugated phenolamine radicals had the singlet electronic ground-state structure.Because the energy difference between the singlet state and the triplet state is small,the singlet state is easily excited to the triplet state.Conductivity and single hole transporting mobility tests showed that the free radical molecules had higher charge transporting properties than their methoxy precursors.Theoretical calculations showed that the planarity of the organic conjugated phenolamine radicals were enhanced,resulting in better electron delocalization,which was good for improving conductivity.It is worth mentioning that the HOMO level of some D-π-D and D-A-D type organic conjugated phenolamine radical materials are matched with that of indium tin oxide(ITO).The solubility test showed that the organic conjugated phenolamine radical materials were typical excellent alcohol soluble materials.Therefore,the organic conjugated phenolamine radical materials reported in this paper can be used as hole transporting materials in the solution processing of photovoltaic devices.