Theoretical Investigation On Electronic Absorption And Fluorescence Spectra Of Rubrene And Its Derivatives

In recent years,organic semiconductor materials have achieved rapid development in the field of semiconductor devices due to their low cost,simple preparation,flexibility,and light weight.Especially in organic sensors,organic memories,organic thin film transistors and other areas have a broad application prospect.Accordingly,the research of organic semiconductors has attracted extensive attention.Rubrene,5,6,11,12-tetraphenyltetracene,is a small molecule organic semiconductor,and have widely been used in organic filed-effect transistors(OFETs),organic light-emitting diodes(OLEDs)and organic solar cells(OSCs),etc.because of its significant advantages in electron mobility and luminous efficiency.The hole mobility of rubrene is as high as 40 cm2V-1s-1,which is good for the mobility of organic materials.However,rubrene is particularly sensitive to photo-oxygen,and there are still many deficiencies in photoelectric properties,therefore,the design of high-performance rubrene derivatives is the focus of current research.New materials are often designed by introducing certain substituents into the molecule,but different substituents have different effects on material properties,so the effect of substituents on material properties needs to be investigated.However,using experimental methods to study the effect of substituents on material properties is not only time consuming but also wastes resources,while theoretical calculations solve this problem.Theoretical calculation have been widely used to study the electronic structure,optical propertie and electrical propertie,and it has made remarkable progress.In this thesis,the effect of substituent species and positions on the structure,frontier molecular orbital and optical properties were investigated by the density functional theory(DFT)and time-dependent functional theory(TD-DFT)methods.The influence of solvent polarity on the structure and optical properties of rubrene and tetracene were also studied.The main research contents and conclusions in this thesis can be draw as follows:(1)The ground state,frontier molecular orbital,absorption spectra,fluorescence spectra and reorganization energy of rubrene and its eighteen derivatives were calculated by DFT and TD-DFT methods.Then,the effects of substituents on the structures and spectra properties were studied.The results indicate that introducing different substituents in different positions of rubrene changes the partial bond lengths and bond angles of the molecule.The introduction of electron-donating groups-CH3,-OCH3 and electron-withdrawing groups-CF3,-NO2 groups in the side benzene chain of rubrene decreases the twist of the molecule,as a result,the degree of conjugation of the molecule was increasing to some extent.The Introduction of the electron-donating groups-CH3 and-OCH3 in the ortho positions of the tetracene backbone of rubrene increases the twist of the molecule,however,when introducing-CH3 and-OCH3 at the relative positions of the tetracene backbone,the twist of the molecule is reduced.Moreover,the effect of the introduction of electron-withdrawing groups-CF3 and-NO2 on the tetracene backbone of rubrene on the molecular torsion is opposite to that of the electron-donating groups.(2)When introducing substituents in t:he side benzene ring of rubrene,blueshift occurs in the electron absorption spectra,whereas redshift is observed in the fluorescence spectra.Introducing substituents in the different positions of tetracene backbone have different effects on the spectra.By analyzing the frontier molecular orbital of rubrene and its derivatives,it is found that introducing the electron-withdrawing groups into rubrene decreased the HOMO(Highest Occupied Molecular Orbital)and LUMO(Lowest Unoccupied Molecular Orbital)energies,while the introduction of electron-donating group increased the HOMO and LUMO energies.The introduction of the electron-donating groups-CH3 and-OCH3 into the rubrene molecule reduces the ionization potential,electron affinity and extraction energy of the disubstituted rubrene derivatives,which will help the transmission of holes,but when introduction of electron-withdrawing groups-CF3 and-N02,the ionization potential,electron affinity and extraction energy are increased,which is beneficial to the electron transmission.(3)The molecular structure,frontier molecular orbital,absorption and fluorescence spectra of rubrene and tetracene in vacuum,cyclohexane,benzene,THF,methanol,acetonitrile and water solvents were calculated by DFT and TD-DFT methods.The effect of solvent polarity on the structures and properties of rubrene and tetracene was investigated.It is found that the influence of the solvent on the structure of rubrene is concentrated on the tetracene backbone,and the bond lengths and bond angles of rubrene and tetracene increase or decrease with the increase of solvent polarity.The frontier molecular orbital analysis shows that the HOMO and LUMO energies of rubrene and tetracene decrease with the increase of solvent polarity.The results of spectra calculations show that the solvent leads to redshift of the absorption spectra,and the redshift degree decreases with the increase of polarity compared with the spectra in vacuum;while the solvent leads to a large redshift of the fluorescence spectra,and the degree of redshift also increases with the increase of solvent polarity.