Theoretical Study on Modulating Electronic Process during Energy and Materials Conversion Using Chemical Group Modification

Author:Liu Ran

Supervisor:jiang jun


Degree Year:2019





Organic functional materials have broad application prospects in the field of energy and material conversion.It is an effective way to obtain functional materials with new properties by analyzing the structure and properties of the system and modifying the structure of the system by physical or chemical means.Thanks to the rapid development of computer hardware performance and theoretical simulation algorithm,which is developed from the density functional calculations(DFT),seize the key structure elements from the atomic scale;cooperate with each other with the experiment.This rational design and prediction for the modification of the material structure has become an indispensable research method to develop new organic ftmctional materials.The essence of the change of the material property is the change of its electronic structure.This paper is to study the electronic structure of organic molecular systems through the DFT calculation.Specifically,the electronic structure,photophysical(Chapter 3)and photochemical(Chapter 4)properties of the organic molecules can change by the modification of the chemical groups.This paper will be combined with specific examples to illustrate the related point.This thesis consists of four chapters.The practical application background and theoretical knowledge background of the work are introduced in the Chapter 1 and Chapter 2,respectively.Chapter 3 is our work on the promoting the intersystem crossing of fluorescent molecule via single functional group modification;Chapter 4 is about the modulating electron transfer process in an organic reaction via chemical group modification of the photocatalyst.Chapter 1 mainly introduces the development of organic materials.According to our works,it covers two parts.The first part is about materials luminescence.Albeit organic luminescent materials have the similar basic principle of emission,there are many ways to regulate luminescence.In particular,the modification of the intermal structure in the materials is more operable and flexible.The second part is organic photocatalytic redox reactions.Compared with the inorganic semiconductor or organometallic compound photocatalyst,pure organic photocatalyst has the advantages of high flexibility,cheap and environment friend.Their catalytic performance can be comparable to that of metal or semiconductor catalysts in many reactions.Therefore,it has become a new hot spot in the research of catalysts.Similarly,it is an important means to develop organic photocatalysts by adjusting their properties via their different structures.In Chapter 2,we mainly introduce the theoretical development of the density functional theory,which is based on the first principles and has been developed by a series of reasonable approximations to solve the practical problems.Specifically,the DFT can be used to study the electron density and structural properties of the system after the development of approximation of adiabatic,average field and the theorems.Among them,the Kohn-Sham equation makes the complicated Schrodinger equation to the simplified form.After the approximation of the exchange correlation function,all physical and chemical properties can be predicted from the obtained ground state density.The TDDFT is developed by the application of the time-dependent perturbation to the accurate simulation of the electronic structure of excited state.And the nature of a system in a given environnent can be obtained by the application of the corresponding restriction condition added to DFT.The quantitative software packages are the bridge from DFT to practical application.Chapter 3 is the study of the relationship between molecular structure and molecular luminescence,specifically,promoting the intersystem crossing of fluorescent molecule via single functional group modification.The intersystem crossing of the intermolecular plays an important role in the luminescence of various materials.The traditional method to enhance the probability of intersystem crossing is to use heavy metal doping.The metallic luminescence substance is not only toxic,but also very expensive,which limits the application range of materials.In our work,we propose a novel mechanism by which the intersystem crossing of pure organic fluorescence luminescent molecules is enhanced via the modification of chemical groups.In pure organic molecules,the electron-donating groups changed the conformation in the relaxation of the lowest single excited state and constructed the corresponding charge-transfer state.This subtle change can not only increase the intersystem crossing rate,but also significantly reduce the fluorescent rate.Simulation calculations and the experiment data shows that this excited system with charge transfer property has a high probability of intersystem crossing,which promotes the molecular phosphorescence emission of the system.Chapter 4 i5 about the modulating electron transfer in an organic reaction via chemical group modification of the photocatalyst.The electron transfer process from the catalyst to the reactant is a key step of photocatalytic reaction.And the difficulty of electron transfer process from the catalyst molecule to the reactant molecule is directly related to the electron affinity energy of the catalyst molecules.We studied the frontier orbital distribution of the system and the one-dimensional potential energy curves of Marcus electron transfer model.The results show that different chemical groups make the difference of electronegativity of catalysts,the distribution of frontier Kohn-Sham orbitals and the one-dimensional potential energy curves.So the electron transfer rates vary greatly.It is known that the less electron binding energy of the catalyst molecules,the smaller the difference of the Gibbs free energy during the reaction(△G0),and the larger the electron transfer rate in the process.So,we indicate that the electron binding energy of the catalyst and lG0 Oan bD used as the descriptors of the electrol transfer rate to guide the design and research of functional catalysts.And chemical groups can effectively change the two terms,and regulate the performance of the catalyst.