Luminescence Mechanism And Regulation Of Triarylmethyl Radical And Its Derivatives

With the development of science and technology,people have higher and higher requirements for the functionality of materials.Meanwhile,all kinds of new materials arise.In recent years,stable doublet luminescent radicals as a new class of material with great application prospect,have emerged in the research of organic light-emitting devices.By avoiding of the spin-forbidden triplet process in conventional luminescence material,their theoretical quantum efficiency can reach 100%.However,in practice,radical species with both high luminous efficiency and sufficient stability are still very limited.In addition,the luminous mechanism of doublet radical,as well as the intrinsic relations between their electronic structure and photophysical property are still unclear,which greatly hindered the further development and utilization of doublet radical material.Herein,we have employed high-level quantum chemistry method,including the complete active space self-consistent field(CASSCF),complete active space second-order perturbation theory(CASPT2)and time-dependent density functional theory(TD-DFT)methods,to explore the luminescence mechanism of triphenylmethyl radical and the dependence of the luminescent properties on the electronic structure.On the basis of mechanism understanding,we carried out molecular design and found the relationship between substitution groups and the luminescent properties and proposed the regulation strategy by using donor-acceptor(D-A)substitution groups.We hope the findings can provide theoretical guidance in developing new doublet radical luminescent materials.The main contents are summarized as follows:(1)A high-level theoretical study at the multi-reference ab initio CASSCF,MS-CASPT2 and TD-DFT level of theory has been carried to uncover the luminescent mechanism.In addition,introducing isoelectronic replacement to design radical molecules.The results show that the triphenylmethyl radical is supposed to be excited from the ground state(Do)to the seventh(D7)and eighth(D8)doublet excited states under light irradiation,owing to the strongest absorption peak resulted from the D0?D7 and D0?D8 excitations.Then,the molecule nonradiative decays to the lowest excited state(D1)via ultrafast internal conversion,and eventually goes back to D0 and gives rise to fluorescent emission.Subsequently,the replacement of central carbon atom by nitrogen and boron atoms and their influence on the electronic structure and color of luminescence were elucidated from the perspectives of spatial and electronic effect,respectively.It provides a new idea for the design of carbon-based radical luminescence system with excellent performance.(2)On top of the mechanistic study,we carried out two doublet luminescent radical,TTM-1Cz and CzBTM,by introducing various substituents on different sites of the molecule.The results show that the introduction of electron-donating groups in the carbazole unit reduces the torsional angle between the D-A systems,increases the molecular rigidity,reduces the bond dissociation energy of precursor molecules,and expands the degree of spin delocalization of the system.As a result,the stability of the radical is improved.Meanwhile,the energy gap between SOMO-LUMO decreases,the absorption and emission spectra show red shift,and the excited state transition dipole moment increases.On the contrary,the introduction of electron-withdrawing groups on the carbazole unit reduces the degree of spin delocalization,reduces the planarity of the molecule,increases the steric hindrance,and enhances the shielding effect on the central radical,and thus the stability of the molecule is improved.In the meantime,the energy gap between SOMO-LUMO increases,the absorption and emission spectra shift blue,and the radiation rate of radicals increase.In a short,properly combining the electron-donating group with the electron-withdrawing group can tune the electronic structure of the system through conjugation effect and induction effect.At the same time,it can realize the regulation of photophysical properties.The above results not only contribute to the understanding of the mechanism of luminescence of the doublet radical,but also provide a theoretical reference for the design of new luminescent radical molecular.Meanwhile,questions such as how to reduce the probability of non-radiative transition of radicals by means of regulation and at the same time enhance the thermodynamic and kinetic stability of radicals,are remaining to be two of the important problems in the design of novel and efficient radical molecules,which need to be solved in future studies.