Study on the Aggregation Structures of Organic Conjugated Molecules and Their Optoelectronic Properties

Author:Zhou Jia Dong

Supervisor:jie zeng qi


Degree Year:2019





The intrinsic relationship between molecular stacking structure and optoelectric properties has been widely studied since the last century,which is accompanied with the development of exciton theory.In this paper,we explore the degree of intermolecular coupling and predict different exciton motions in M-,J-and H-aggregation modes on the basic principle of photochemistry.The molecular aggregation modes depend on their chemical structure.We use diffraction and spectral charateristics to determine the molecular stacking structures,and investigate on the exciton localization in M aggregate for mb-PBI,exciton diffusion in J aggragte for A-D-A molecules and exciton separation for D-A molecules.With the extension of material system,the limitation of exciton theory becomes more obvious,which needs amount of experimental suppourt.Therefore,establishing the intrinsic relationship between molecular aggregation modes and optoelectric properties is conducive to the improvement and integration of disciplines,which helps to understand the intrinsic property of materials and the working mechanism of optoelectric devices.It would provide guidinglines for the novel design of efficient materials and the fabrication of stable devices.In chapter 2,we synthesized a perylene bisimide derivative,and confirmed that its stacking mode was the first example of magic-angle aggregation.The growth mechanism of crystal was explored by controlling the tradition.Based on the exciton theory,M aggregate,where the molecule stacks parallel with a slip angle with ~54.7° between the neighboring molecules,makes optically allowable transition at the lowest splilting level,as well as restricted radiationless resonance energy transfer for its very small orientation factor.In addition,the aggregate shows a large red shift in the absorption and fluorescence spectra.The excitation spectra and theoretical calculations indicate that the effective conjugation contains at least three molecules.In the power-dependent transient and steady-state fluorescence spectra,we attribute the unique optical property to the formation of Freankel-CT excitons,of which the localiztion energy is up to 120 meV.The terminal barriers limit the excitonic delocalization,which is equivalent to radiation dipole in a resonant cavity,leading to the increasing radiation rate.Meanwhile,the large intermolecular π-π orbital overlap is beneficial for the charge transport.Therefore,we could achieve the high solid-state luminescence efficiency and charge carrier mobility simultaneously by constructing magic-angle stacking mode.In chapter 3,single crystals structures of some planar A-D-A type fused-ring cumpounds were summaried in order to explore the relationship between the molecular fragments and their aggregation patterns:(i)the rotational effect of aromatic rings in the side-group could increase the variaty of molecular conformation;(ii)the extended central fuse ring would lead the multiple π-π interaction with a complex stacking pattern;(iii)as for the introduction of halogen atoms at the terminal group,fluorine have a relatively neglected effect on intermolecular arrangement,while the chlorine atoms leads an obvious interaction with central conjugated ring.Then we take ITIC as an example,and explore the electronic excitation process of monomer and J-type dimer in detail.The J-type dimer shows the anisotropy of Coulomb and CT coupling,where the π→π* transition occurs along the molecular axis and the differential density centerize on the H-type D-A fragments.Finally,the exciton diffusion length was evaluated to be over 100 nm by exciton-exciton annihilation experiments.In chapter 4,the supramolecular self-assembly process of D-A type compound(HBC-PBI)was explored by using spectroscopy,X-ray scattering and electron microscopy.The aggregation process contains two steps: oligomer formation and polymerization(nucleation-elongation mechanism).The oligomer contains about five units with aligned donor-donor and acceptor-acceptor units through self-recognition.The coooperactive growth process of oligomers occurs with the entanglement of alkyl chains and forms lamellar structure.Finally,by pumping different molecular fragments with selected excitation wavelength,the aggregate shows the electron transfer and recombination with time of about 7 and 2995 ps,and the hole transfer and recombination time of about 10 and 3674 ps.