Synthesis And Fluorescent Properties Of Aryl Radicals Derived From N-Heterocyclic Carbenes

In general,organic radicals with open shell electronic structures are non-luminescent and can quench the emission of other fluorescent groups or materials.Until 2006,the first example of luminescent organic radical was synthesized and characterized.Subsequently,experiments and theoretical calculations show that the OLED devices constructed with free radicals as luminescent materials are expected to achieve 100% internal quantum efficiency.Since then,the luminescent radicals have been extensively investigated.Although some achievements have been achieved in the study of luminescent organic radical,there are still many challenges to further developing luminescent organic radicals,such as:1)Luminescent organic radical species is quite limited,till now,stable luminescent radicals are mainly confined in perchlorotriphenyl methyl radical derivatives.In addition,the existence of chlorine atom can lead to the decay of radicals under irradiation with light.2)Although the emission wavelength of radicals can be modulated by chemical modification,the emission colors of radicals reported in the literature are still mainly orange or deep red.3)The study on the luminescence mechanism of radicals is not thorough,and there is a lack of effective theory to guide the design and synthesis of luminescent radicals.Based on our research in the field of N-heterocyclic carbene,we synthesized new luminescent organic radicals by combining N-heterocyclic carbene and chromophore,which enriched the types of luminescent radicals and expanded the luminescent range of radicals.1.Four stable radicals 2a-2d were obtained by a simple two-step synthesis strategy by using N-heterocyclic carbene with electronic properties and 4-iodide-N,N-diphenylaniline as starting materials.The formation of radical species was confirmed by EPR,and the solid molecular structure of radical 2a-2c was further determined by X-ray single crystal diffraction analysis.UV-Vis absorption spectra and DFT calculations show that the electronic properties of N-heterocyclic carbene have obvious effects on the electronic properties of radicals.With the increase of?-accepting capabilities of N-heterocyclic carbene,the maximum absorption peak of the radical is redshifted and the energy of single electron occupied orbital(SOMO)is decreased.In addition,fluorescence emission spectra showed that the maximum emission wavelength of radical 2a-2d in dilute solution was significantly blue shifted compared with that reported luminescent radicals in the literature.Among them,the maximum emission wavelength of 2a-2c is less than 530 nm,and the photoluminescence quantum yields of 2a and 2b is more than 30%.The DFT calculation shows that the emission of 2a-2d is mainly assigned to the radiative transition from?-SUMO to?-SOMO,and it has obvious intramolecular charge transfer(CT)luminescence properties.2.Furthermore,the influence of chromophore on luminescence properties was investigated by substituting carbazole group for triphenylamine group.A similar synthesis method is used for the synthesis of carbazole-based radicals 4a-4c,and the molecular structure of 4a-4c was determined by X-ray single crystal diffraction analysis.The DFT calculation shows that the energy of single-electron occupied orbital of radicals 4a-4c lower than radicals 2a-2c.In addition,the emission wavelength of carbazole-based radicals 4a-4c was further blue shifted and the maximum emission wavelength less than 485 nm.3.Polyradicals 6a-6c were obtained by using tetrakis(4-iodophenyl)ethene and N-heterocyclic carbene as the starting material.X-ray single crystal diffraction analysis indicated the carbon-carbon double bond in the radicals 6a-6c opened to form a carbon-carbon single bond,and the fluorescence was completely quenched.Subsequently,when AgPF₆ is added to the THF solution of the radicals 6a-6c,the carbon-carbon double bond in the tetraphenylethylene unit can be reconstructed.In summary,the purpose of this study is to synthesize novel fluorescent radicals by combining chromophores unit and N-heterocyclic carbenes,screen out suitable N-heterocyclic carbene precursors,and study the effects of different chromophore on fluorescence characteristics of radical.The results of experiment and theoretical calculation provide the basis for the design and synthesis of fluorescent radicals.