Electronic Excited States Of Covalent Organic Frameworks Studied By Many-Body Green's Function Theory

Burning fossil fuels to provide energy has caused severe global environmental pollution,which has become a threat to human survival and development.Excessive greenhouse gas is produced and released into the atmosphere,making global warming,causing the polar ice to melt and sea level to rise.How to adsorb carbon dioxide and other harmful gases generated after combustion has been a hot issue for people.Exploring new energy resources that can replace fossil fuels is an effective way to solve the energy crisis.Hydrogen and methane are clean and pollution-free energy.However,safety,storage and transportation issues restrict their development.Since the 21st century,organic framework materials have developed rapidly.Since these materials exhibit large surface area and light weight,they could be used to absorb gases such as CO2,H2 and CH4.In 2005,Yaghi and co-workers are the first to successfully synthesize covalent organic frameworks(COFs).Reversibility of the reaction also ensures that the product reaches the lowest energy after multiple equilibrations,forming highly ordered pore structures.So far,a large number of COFs have been synthesized experimentally,and their properties have been characterized by some experimental techniques.In this work,first-principles many-body Green's function theory(MBGFT)is used to study several kinds of COFs.GW method and BSE equation based on MBGFT are used to study the band structure,absorption spectrum and excited state of COFs,which may provide some useful information for further studies on complex COFs.This thesis has the following main results.1.Since COFs discovery,two-dimensional covalent organic frameworks that can achieve high-efficiency luminescence have become hot research topics.The sp2 carbon-conjugated COFs have good luminescent efficiency in experiments,but the internal mechanism of luminescent has not been discussed.We used GW method and BSE equation to study the electronic gap,optical gap and optical absorption spectrum of two kinds of sp2 carbon-conjugated COFs,including their bulk phases in the AA-and AB-stacked mode and their single-layer materials.The AA-stacked bulk COF has an indirect band gap.If the geometry is changed to the AB-stacked mode,the band gap of COF becomes direct.From the energy dispersion of indirect exciton calculated by BSE,we may exclude the possibility that light emission in COFs is caused by phonon-assisted recombination of indirect excitons.Light emission in COFs might result from recombination of direct excitons at defect sites.By studying the distribution of photoelectron and hole of the lowest singlet excited state and the second lowest singlet excited state in the bulk,the exciton migration direction of the luminescence COFs is obtained.Changing the stacking mode of COFs from AA to AB can convert the band gap from indirect to direct and therefore may enhance the fluorescence quantum yield.The above analysis of the luminescent mechanism of the material has certain guiding significance for the design of better luminescent materials in the future.2.The imine-linked COFs are stable and performance easy to control,which is one of the main ways to synthesize functional COFs.However,C-N in imine-linked COFs can generate energy dissipation under the guidance of light,which is generally non luminous or poor luminous material.It is the first time to synthesize COFs of solid-state imine luminescence in experiments.The mechanism of COFs needs to be discussed in theory.We used the large pore structure in the experiment to study the internal luminescence mechanism of imine-linked COFs.In the configuration optimization,the material of the small pore structure is twisted seriously,and the large pore structure is easier to get a stable and reasonable configuration.Using GW method and BSE equation we studied the band structure and optical absorption spectra of two kinds of imine-linked COFs.The calculated absorption spectra agree with the experiments.Bulk IMDEA-COF-1 is a direct-band-gap semiconductor,while bulk IMDEA-COF-2 is an indirect one.This may explain why the former is luminescent while the latter is not as reported in experiments.The introduction of-OH group leads to the red shift of absorption peak.Therefore,for this kind of COFs,the absorption peak can move to the high wavelength direction by introducing electron donor group,which provides a reference for the design of modified COFs in the future.