Study Of System Interactions On Luminescence And Catalytic Mechanism

Materials provide the material foundation for development of human civilization.Waves of efficient,economical and practical new materials help the advance of human societies.Therefore,research and development of new materials are of strategic significance.Fortunately,theoretical models and computational studies offer powerful tools for understanding the properties by the internal mechanism and electronic structures.In particular,with recent advances in high performance computing and sophisticated electronic structure theories,the ability of first-principles electronic structure calculations and molecular dynamics modelling has been rapidly increasing.Predicting and understanding the properties of materials from the atomic and molecular scale,in conjunction with complementary experiment for rational design and synthesis,has become an efficient way for the new material development.In addition,systems interaction between different molecules or materials is a pivotal bridge between the structure and properties.Rational comnbination of different molecules or materials based on the interaction relationship,could pave the way to the development of new functional material with high performance.In this dissertation,we presented our first-principles study using density functional theory?DFT?calculation.It is divided into four chapters.Introduction of this dissertation and theory employed in the work are laid out in Chapter 1 and 2,respectively.The main work is narrated in Chapter 3 and 4.Specifically,Chapter 3 is about our study on the effects of intermolecular interaction??-? coupling?on molecular luminescence;and Chapter 4 is about our study on the effects of charge polarization induced by the interaction between different materials on catalytic mechanism.Chapter 1 contains two sections according to our later works.The first is the research status of luminescence in aggregates.Compared with the modification of the internal structure in the molecules.the luminescence in aggregates with the intermolecular ?-? coupling interaction is more operable and flexible.Thus it has aroused intensive concerns and research efforts.Although the basic principle of emission is similar with monomers,due to the intermolecular interactions,luminescence in aggregates possess some specific performances with unique internal mechanism.This provides the possibilities for the regulation of molecular luminescence process.The second is the research status of catalytic reaction by metal semiconductors and two-dimensional materials.The heart of catalytic chemistry is centered on the design of catalysts.Metal catalyst is superior in catalytic performance with high cost and low stability,while semiconductor catalyst exhibited high stability,but the catalytic activity is relatively low.Thus rational comnbination of different materials based on the interaction relationship,could achieve synergistic effects with low cost,high efficiency and catalytic activity.In addition,two-dimensional non-metallic catalysts exhibited advantages in high thermodynamic stability and engineerability,good optical performance,low cost and high catalytic performance,therefore,it sparked intense interest and been extensively studied in many reactions.In Chapter 2,we briefly introduce the density functional theory?DFT?,which includes the theoretical framework,development,time-dependent density functional theory?TD-DFT?and the application.DFT is based on quantum mechanics,and the basic variable is electron density.By solving the Kohn-Sham equation,the interacting multiparticle system is transformed into a non-interacting single electron system and approximated by the appropriate exchange correlation function As a result,all properties of the system can be included in the unique functionals of the ground state density.In the TDDFT,time-dependent perturbation is applied to simulate the electronic structure of excited state under linear response approximation.The implementation of DFT is through the computation of software packages.Chapter 3 is mainly about the study of intermolecular interaction on molecular luminescence,including two works:The first work is aggregation-induced intersystem crossing.Intersystem crossing?ISC?in nanoscale systems,plays an important role in various opto-electronic applications.However,the developed methods to promote ISC rate including chemical modifications and heavy metal atoms additions to molecules are still very limited and especially inconvenient,which seriously hinders many important photo-electron conversion related applications.In our work,our first-principles simulations lead to a convenient "aggregation-induced intersystem crossing"?AI-ISC?mechanism which utilizes the energy splitting of excited states due to inter-molecular interactions,so as to improve the energy matches between singlet and triplet states and thereby substantially enhance ISC rate.Our simulations together with experimental tests demonstrate that enhanced ISC rate substantially promoted molecular phosphorescence in the aggregated systems of originally fluorescent dye molecules.Meanwhile,the emission spectra experience a red shift along with the aggregation,providing a convenient knob to tune the phosphorescence wavelength.The second work is self-assembly organic dots with distinctive fluorescence behaviors.Cooperated with experimental group,we have designed new organic dots in free and aggregated states.In aqueous solution,the dots,exhibit a very high fluorescent quantum yield much larger than those of photoluminescent carbon dots to date.While in solid phase,with quantum yield greatly diminished,it emits lights in different colors under different excitation.Theoretical simulations confirm the fluorescence of the fluorophore in solution complies with the push-pull emission mechanism when undesirable ?-? stacking and H-aggregation in solid state are eliminated by the hydrogen bonding with solvents.Whie in solid state,due to the ?-?stacking and H-aggregation,energy splitting ocuurs in the aggregates,and the vibration relaxation is suppressed,suggesting the breaking of Kasha's rule.Thus the fluorescence is much attenuated,and the emission wavelengths in different colors under different excitation are different.Chapter 4 includes three parts,which is mainly about the study of charge polarization induced by the interaction between different materials on catalytic mechanism:The first work is about combining photocatalytic hydrogen generation and capsule storage by graphene-based composite materials.In this work,we proposed a multi-layer structure where a carbon nitride?g-CxNy?is sandwiched between two graphene sheets modified by different functional groups?GR_F?.First-principles calculations demonstrate that due to the charge polarization between g-CxNy and GR_F,such system can harvest light and deliver photo-generated holes to the outer GR_F sheets for water splitting and proton generation.Driven by electrostatic attraction,protons penetrate through graphene to react with electrons on the inner g-CxNy to generate hydrogen?H2?.The produced hydrogen is completely isolated and stored with a high-density level within the sandwich,as no molecules are capable of migrating through graphene.The proposed GR_F-CxNy-GR_F system has achevied the combination of photocatalytic hydrogen generation and capsule storage.The second work is catalytic reduction of 4-nitrophenol by nitrogen-doped graphene.Three different types of MOF materials have been exploited as hard templates to obtain metal-free porous carbon materials with various nitrogen dopant forms and contents,degrees of graphitization,porosities,and surface areas;furthermore,these materials possess different types of nitrogen species,depending on the parent MOF structure.The MOF-templated carbon materials are very active toward the catalytic reduction of 4-NP to 4-AP in the presence of NaBH4 under ambient conditions.The PCN-224 templated material outperformed all other counterparts because of its excellent activity,stability,and recyclability.Theoretical investigations have suggested that the content and type of the nitrogen dopants are important for the catalytic conversion.Three types of nitrogen species,especially the pyrrolic nitrogen species,have shown a strong ability to absorb 4-NP ions,activate the nitro groups,and deliver energetic charges.Thus,the pyrrolic nitrogen species makes the greatest contribution to the high catalytic performance,which is in consistent with the high pyrrolic nitrogen contents and excellent catalytic efficiency in experiment.The third work is about photocatalytic water splitting by atomically controlled TiO2-Pd@Pt cocatalysts.In collaboration with an experimental group,we have designed atomically controlled Pd@Pt quasi-core-shell cocatalysts and deposited them on n-type semiconductor,namely anatase TiO₂ nanosheets.The well-designed cocatalysts play a dual role during photocatalytic water splitting,both of which are dependent on the Pt shell thickness:improving charge separation by promoting electron trapping with interfacial charge polarization beween Pd and Pt,and enhancing H₂O adsorption onto the Pt surface by increasing the electron density and lattice strain.Taken together,the improved charge separation and molecular activation dramatically boost the overall efficiency of photocatalytic water splitting.