Theoretical Study On The Stability And Photophysical Properties Of Organic Radicals

Neutral organic radicals are a kind of open-shell molecules with an unpair electron.Organic radicals are generally considered to be short-lived and highly active substances,because the presence of single electron on the highest single-occupied orbital(SOMO)makes them highly active and prone to dimerization,decomposition,disproportionation,etc.In 1900,the first relatively stable neutral organic radical-triphenylmethyl radical(TPM)was reported by Moses Gomberg,which opened the relevant research on stable radical chemistry and was a milestone.From then on,more and more stable radicals were reported successively.Since 2015,the research group of Professor Li Feng at Jilin University has reported a radical-based doublet light-emitting diode,which has set off an upsurge in the study of the doublet luminescence properties of organic radicals.In addition to the extensive research in the field of organic light-emitting diodes(OLED),they also exhibit broad applications in the fields of organic magnetism,fluorescent probes,spintronics,carrier transport,organic field effect transistors(OFET),etc.So far,though the great progresses have been achieved in the research of organic radical based OLED materials,there are still some urgent problems to be solved.Firstly,there are fewer kinds of organic radicals for stable and efficient light emission.To date,triarylmethyl-based carbon radicals are currently the only stable and possess roomtemperature luminescent properties,such as perchlorotriphenylmethyl(PTM),tris(2,4,6-trichlorophenyl)methyl radical(TTM),pyridine diphenylmethyl radical(Py BTM)and diphenylmethyl radical(BTM)derivatives.The poor stability of organic radicals is the key factor limiting their further development in OLEDs.Therefore,it is of great significance to explore the factors affecting the stability of radicals through theoretical calculation methods.Secondly,there are relatively few theoretical studies on the relationship between the structure and luminescent properties of radicals,especially for the excited-state properties.In addition,the unique single electron property of organic radicals makes them exhibit excellent bipolar charge transfer properties.Therefore,we conduct a systematic theoretical study on the stability,photophysical properties and charge transport properties of neutral organic radicals with quantum chemistry methods.The research results clarify the main factors affecting the stability of organic radicals,the relationship between structure and luminescence properties and predict the bipolar charge transport properties of organic radicals,which provide a theoretical basis for the wide application of organic radical materials.The main research contents of this article include the following three parts:(1)In view of the stability of organic radicals,we mainly studied the triphenyl methyl radical(TPM)and its derivatives.To better understand the effect of steric hindrance effect,substituent effect and electron delocalization effect on the stability of radicals,geometric structure,electronic structure,spin density,radical stabilization energy(RSE),ionization potential and electron affinity have been analyzed by density functional theory(DFT)in details.The results show that the steric hindrance of chlorine atom,the introduction of substituents,the smaller dihedral angle between the donor group and the radical skeleton and the larger conjugated substituents are beneficial to improve the thermodynamic stability of the organic radicals.This study provides a useful theoretical guidelines for the development and design of high stability organic radicals.(2)In order to further understand the properties of the excited state of stable and efficient luminescent radicals,we took the diphenylmethyl radicals as an example to theoretically explore the luminescence process of radicals.Based on the experimental molecules Cz BTM and Py ID-BTM,we explained the reason for the difference in quantum efficiency and designed two new organic radical molecules by linking BTM with different substituents α-carboline and diazacarbazole.The geometrical structure,electronic structure,absorption and emission properties,radiation and non-radiation deactivation processes of excited states,redox ability of the four radical molecules were studied by DFT and TDDFT methods.It is found that the small D-A dihedral angle,the weak electron donating ability and the small recombination energy can help suppress non-radiative processes,thereby increasing the fluorescence quantum efficiency.Through the analysis the properties of the excited states of organic radicals,we can gain a deeper understanding of the reason why the marked difference in quantum efficiency.In this work,we provide more physical understanding of the relationship between the structure and properties of the organic radicals.(3)Based on three stable non-luminescent carbon-based radicals,we investigate their applications in charge transport.In order to have a deeper understanding of the charge transport properties of organic radicals,we used density functional theory to study the geometric structure,electronic structure,recombination energy,redox ability,and carrier mobility.The calculation results show that the organic radicals we studied all exhibit bipolar charge transport properties.This work has thoroughly explored the charge transport properties of organic radicals and provided an effective theoretical hints for the development of bipolar organic semiconductor materials.