Construction Of Perylene Bisimide With Magic-Angle Stacking In Crystal And The Study Of Their Photoelectric Properties

In photoelectric devices,photoelectric materials usually realize their functions in condensed state.Therefore,the study of the effect of molecular aggregation structure on molecular photoelectric properties is the basis of the developing of high-performance optoelectronic functional materials and fabricating the device of excellent performance,Therefore,the research on the relationship between molecular aggregation structure and molecular photoelectric properties is the theoretical basis of developing high-performance photoelectric functional materials and further realizing high-performance devices.The monomers of organic π-conjugated molecules usually exhibit mirror-symmetric absorption and fluorescence spectra in the UV-visible region.When it comes to molecular aggregates,the interaction between molecules causes significant changes in absorption and fluorescence spectra,including spectral shift,spectral shape,etc..Meanwhile.the excited state energy tends to transfer in the close-packed system,which will exert significant influence on the fluorescence efficiency of aggregates.The common molecular aggregation modes include H-aggregates and J-aggregates,which cause blue shift and red shift of absorption spectrum respectively.These two patterns of aggregates are accompanied by the molecules arranging in parallel,however,distinctive in the slip degree along the transition dipole direction.In J-aggregates,adjacent molecules slip less than 54.7 °.With the increase of slip angle,H-aggregates are formed.The particular angle 54.7 °,which is the demarcation point of H-and J-aggregates,is called as Magic Angle.When the neighboring molecules slip approaches to the Magic Angle,a "transition state"(M-aggregate)is formed.According to the characteristics spectral effects of H-and Jaggregates,it can be inferred that the electron spectra of such special "transition state" aggregate should be similar to that of molecules,which is expected to achieve high solid-state luminous efficiency.Moreover,the strong π-π interaction of M-aggregates is conducive to realizing high carrier mobility.Theoretically,M-aggregation is the optimal aggregation mode with high solidstate luminous efficiency and high carrier mobility.Perylene imide(PBI)and its derivatives are one of the functional modules with good photoelectric performance and excellent(photothermal)chemical stability,which are commonly used to study the regulation of molecules from chemical structure to aggregation structure.Herein,we concentrates on the construction of special M-aggregates molecular crystals to study the relationship between molecular packing structure and photoelectric function,and finally achieve high solid-state luminescence efficiency of strong π-π packing system.In chapter 2,we synthesized a series of N-benzyl substituted perylene bisimide derivatives based on M-aggregates and obtained high-quality single crystals by physical vapor transfer and liquid-liquid diffusion methods for the structural analysis.Among them,the introduction of branched alkoxide chains at the ortho-position of N-benzyl group greatly improves the solubility of the black pigment,which is conducive to the preparation and processing of the crystalline and orientated films.Furthermore,the tunability of the substituents at ortho-position of benzyl benzene ring is beneficial to control the sliding distance of adjacent molecules along the short axis and the long axis,and to realize the fine regulation of the packing structure of molecules in the crystal.We summarized the single crystal structure of this series of imide molecules and analyzed the driving force of the stacking structure of imide molecules from the perspective of quantum chemistry,which mainly comes from the π-π interaction between the planar perylene imide skeleton(dispersion force).In addition,the slip along the main axis of the molecule or rotation perpendicular to the plane of the molecule depends on the steric hindrance effect between the N-substituents.The permutation orientation of the thin film was determined according to the crystal morphology and grazing incident X-ray diffraction,i.e.the long axis of the molecule was tilted on the substrate.In chapter 3,based on parallel slipped π-π stacking and face-to-face rotational π-πarrangement,the intermolecular aggregation structure was further quantified by the π-πinteraction distance between molecules,the transverse displacement and longitudinal displacement.The total exciton coupling in the crystal structure is determined by theoretical calculation as the sum of long-range Coulomb coupling(JCoul)and short-range CT coupling(JCT).The theoretical results show that the Coulomb coupling between adjacent molecules is very weak in all molecular crystals with M-aggregates structure,however,significantly differences in CT coupling.The latter is hypersensitive to the transverse offset of adjacent molecules and decreases with the increases of transverse displacement,and the interference condition changes from in-phase to out-phase.The face-to-face rotational π-π stacking exhibits both small Coulomb coupling and CT coupling.Finally,the energy level splitting resulted by exciton coupling brings out the characteristic absorption and emission spectra of such π-conjugated material.Such π-π stacking interaction is also reflected in the change of the color of powder crystal(from black to brown to red)on the macroscopic property and the carrier migration characteristic was studied by single crystal field effect transistor.The calculation of exciton coupling related to PBI dyes structure is further improved,which is beneficial to directly predict the properties of molecular aggregates including spectral characteristics,color changes,carrier transport and so on according to the single crystal structure of PBI dyes.In chapter 4,the single crystal structure and crystal luminescence of N-alkoxybenzyl substituted PBI derivatives were studied in detail.This series of compounds are weakly emissive in polar,aprotic solvents.In sharp contrast,the fluorescence quantum yields of polycrystalline films increase from 2% to 60%.The single crystal structure analysis showed that these molecules contain typical M-aggregation structure with strong π-π interaction between adjacent molecules.According to experiments and theoretical calculations,the main pathway of fluorescence quenching in such M-aggregate crystals with strong π packing comes from the photo-induced excited state electron transfer(PET)process between the PBI receptor(A)and the alkoxybenzyl donor(D),which is closely related to the molecular conformation.The difference in luminescence efficiency of this series of PBI molecules in the crystal state can be attributed to the following two factors,(1)The effect of dihedral angle(θ)between benzyl and alkoxy on the energy levels of charge transfer states(CT);(2)The effect of strongπ-π interaction between molecules on the energy level of locally excited states(LE).The strongπ-π interaction between molecules greatly reduces the energy level of LE state.In the system with large dihedral angle θ and strong intermolecular π-π interaction,the energy level of LE state is close to that of CT state.The thermal activation process can lead to the inversion of LE state and CT state in the crystal,the lowest excited state is LE state,showing high fluorescence quantum yield in solid state.In this paper,the researches on the structure and photoelectric properties of a series of Maggregates crystals show that M-aggregates can effectively inhibit fluorescence quenching,and it is an ideal aggregation mode to realize high solid-state luminous efficiency and high mobility.