Theoretical Study On The Aromaticity In Radical Chemistry And Singlet Fission Materials

Aromaticity is an important concept in chemistry that has attracted the attention of a large number of experimental and theoretical chemists.It has been demonstrated that aromaticity can bring additional stability to compounds,whereas antiaromaticity often leads to unstable compounds.According to the Hückel’s and Baird’s rules,compounds are usually aromatic in only one state(the ground or excited state).Through density functional theory(DFT)calculations,Zhu group discovered for the first time in 2018 that the 16-electron osmapentalene is aromatic in both the lowest singlet and lowest triplet states,and termed it adaptive aromaticity.In addition,Zhu group also found that the concept of adaptive aromaticity is applicable to the field of singlet fission through a joint study with the experimental group,which is expected to aid in improving the efficiency of solar cells.Moreover,radicals(ions)tend to exhibit unique photochemical and photophysical properties due to the presence of unpaired electrons,and have a wide range of applications in many fields,including organic synthesis and materials science.Note that the presence of unpaired electrons increases the reactivity of radicals(ions),complicating their selectivity.Thus,the design and preparation of highly stable radicals(ions)become one of the hottest topics in chemistry and materials.This thesis investigates the application of aromaticity in the field of radical chemistry and singlet fission materials using DFT calculations,and theoretically demonstrates that the concept of antiaromaticity helps to prepare radicals(anions)with relatively high thermodynamic stability by taking advantage of aromaticity criteria and radical stabilization energy calculations.The quantitative analysis of intramolecular interactions and bonding in conjunction with frontier molecular orbitals suggests that the distribution of unpaired electrons in radicals(anions)affects their thermodynamic stability significantly.Among them,unpaired electrons tend to be populated in the ring of the radical(anion)formed by the antiaromatic compound,releasing antiaromaticity and increasing thermodynamic stability.In addition,it is found in this thesis that the concept of adaptive aromaticity not only satisfies the thermodynamic requirements of the singlet fission process,but also reduces its thermal loss due to the moderate excitation energy E(T1),suggesting that the concept of adaptive aromaticity can help experimental chemists in the design and preparation of new singlet fission materials.