Many-Body Green's Function Theory Studies On The Excited-State Properties Of Photosynthetic Systems And Photocatalytic Materials

Light is an inexhaustible source of clean energy from nature and the basis for the survival of human and all kinds of living things.Research on the use of light energy varies from macro to micro,from experiment to theory.The use and research of light are mainly divided into two systems:one is the biological system and the other is the photovoltaic material.Biological systems use sunlight to produce organic matter and oxygen through photosynthesis.The structures involved in photosynthesis include light harvesting center ??LH2?,light harvesting center I?LH1?and reaction center?RC?,all of which contain chromophore molecules carotenoids and chlorophyll,as well as some proteins and water environment.The reactions that occur during photosynthesis include energy transfer,charge transfer,and redox reactions.Due to the complexity of plant structure,LH2 in photosynthetic bacteria with a simple structure is a good research object.There are many types of photovoltaic materials,such as transition metal oxides.Graphene-based carbon-nitrogen materials?g-C₃N₄?have become a new research hotspot due to their high stability and wide range of raw materials.In this work,the many-body Green's function theory is used to study the dynamics of excited state and the excitation mechanism of g-C₃N₄ in photosynthetic system,including the energy transfer from carotenoid to chlorophyll,the potential energy surface of carotenoid itself,the electron transfer mechanism between g-C₃N₄ and Cdot heterojunction,the excited-state behavior of B and O co-doped g-C₃N₄.Through theoretical calculations,we explain the experimental behavior of the new state enriching the excited-state model of carotenoids,and the improved principle of g-C₃N₄ providing new ideas for the future scientific research and experimental direction.The main contents and conclusions of this paper are as follows:1.Carotenoids not only absorb light,but also transfer the absorbed energy to chlorophyll.However,it has been debated which excited states of carotenoids are involved in the energy transfer process,and the huge difference between theoretical calculation and experimental measurement of the energy transfer rate has not been well explained.Previous studies suggest that the new state between the ground state So and the absorption peak S₂ that participates in the energy transfer of carotenoids is 1Bu^-,but the low energy value of 1Bu^- cannot transfer energy to the chlorophyll Qx state and dependence of 1Bu-on conjugation length does not match experiment.We calculate the energy absorption in the ground state,but did not find a new absorption state between the ground state S₀ and the absorption peak S₂.However,the energy transfer does not occur in the absorption process of the ground state but in the relaxation process of the excited state S₂.We used a variety of methods to obtain the relaxation structure of S₂,and calculated the excitation energy of the relaxed structure by the many-body Green's function theory and the high-precision EOM-CCSD method.We found that a new excited state?named S_y?appears under the S₂ state.The symmetry of S_y is Ag+,and the energy?about 0.5 eV higher than that of S₂?is lower than the experimentally determined 1Ag⁺ state,so we name S_y symmetry nAg⁺.We studied the excited-state relaxation of carotenoids with conjugation length N = 5-13,and found that the variation of S_y state with the conjugate length N is consistent with the experiment.Based on our proposed new excited-state model,the simulation of the two-dimensional electron spectrum is in good agreement with the experiment.In terms of structure,we find that methyl has little effect on the excitation energies of the S₂ state and the new state S_y,but has a great influence on the dark state 2Ag^- and 1Bu^- energy.The difference in excited state energy can affect the direction and rate of energy transfer.Finally,we calculated the energy transfer rate of S_y.The energy transfer takes place mainly through the S₂ state of the carotenoid absorption peak,and energy transferred by S_y is about one-tenth that of the S₂ state.The large difference in theoretical and experimental energy transfer rates did not result from the absence of a new state,but the low calculated values for coupled terms.2.Carotenoids,a kind of polyene containing functional groups such as methyl,are often in slightly twisted all-trans configuration in living organisms,and also become cis-forms in certain solutions.Carotenoids evolve from all-trans to cis configuration with some energetic changes in the excited state,and some low-intensity states have also been found to possess high absorption intensity.These properties are important aspects for the application of carotenoids,but there is no systematic study of the excited states for different configurations of carotenoids.Some study uses the configuration of carotenoids to account for the emergence of new signals in experiments;which leads to a new controversy.Therefore,in this paper,we have studied the potential energy surfaces of carotenoids with different configurations from all-trans to cis.We found that protonation alters the absolute energy of the excited state of carotenoids,but does not change the number of potential wells in the potential energy surface.The shape of the carotenoid potential energy surface is not only related to the conjugation length N,but also the rotation center.The shapes of the potential energy surfaces obtained by different rotation centers are different.The addition of methyl groups can adjust the bandgap both in cis and in all-trans configurations,but not the same for different conjugation lengths and different configurations.This not only clarifies the factors that affect the energy of the excited state,but also provides a method for changing the energy of the excited state quantitatively.3.Since g-C₃N₄ was found to produce hydrogen in the sunlight,the explanation of the hydrogen production mechanism and the improvement of hydrogen production efficiency of g-C₃N₄ have been the research hotspots.It has been found experimentally that when g-C₃N₄ and Cdot are coupled together,the hydrogen production rate can be greatly increased,but the mechanism is not clear.Some theoretical studies focus on shape and size of the Cdots,hoping to find a suitable shape and size.However,the model they used was a-H saturated graphene fragment,which is quite different from the carbon quantum dots experimentally saturated with functional groups such as-OH,-CHO and-COOH.The model we used was close to the experimental structures to explain the experimental phenomena and to investigate the effect of the functional groups on the Cdots.We find that the hydrogen production efficiency of the g-C₃N₄/Cdot heterojunction is enhanced because it is a type-? heterojunction,which distributes electrons and holes in two different species after irradiation,reducing electron-hole recombination rate and therefore increasing the effective utilization of electrons and holes.By changing the kinds of functional groups,we found that the HOMO and LUMO energies of Cdot increased when the functional groups were reduced?the electronegativity became weaker?,while the HOMO and LUMO energies of Cdot decreased when the functional groups were oxidized?electronegativity increased?.By changing the kinds of functional groups,different kinds of type-? heterojunctions can be formed.When the electronegative capacity of functional groups on Cdot is weak,excited states of electrons accumulate on g-C₃N₄ and holes accumulate on Cdot,so H₂ is generated on g-C₃N₄ and O₂ is generated on Cdot.When the electronegativity of functional groups on Cdot is strong,the excited holes accumulate on g-C₃N₄ and the electrons accumulate on Cdot,so O₂ is generated on g-C₃N₄ and H₂ is produced on Cdot.This rule has the guiding significance to change the electronic properties of Cdots,which is beneficial to the development of more Cdot-containing heterojunctions.4.Doping is a widely used means to modify the material,which can effectively improve the hydrogen production efficiency of g-C₃N₄.However,single-element doping may creat a half-occupied state which is a recombination center for electrons and holes.Passivated doping with n-type and p-type elements can avoid the generation of the half-occupied state.We doped g-C₃N₄ with B element and O element with the atomic ratio of 1:1.It was found that the passivated g-C₃N₄ not only increased the absorption in the visible range,but also greatly enhanced the separation rate of electrons and holes.However,these effects are controlled by the relative positions of B and O.Only when B and O are in different g-C₃N₄ units forming a planar heterojunction,the above-mentioned enhanced effect can be achieved.The important role of microstructure in the co-doping of g-C₃N₄ has been verified,which provides a reference for the experimental operation.