Study On The Synthesis,aggregation Structure And Photoelectric Properties Of Perylene Imide Derivatives

Conjugated organic molecules have been widely used in solar cells,light-emitting diodes,field effect transistors and fluorescent probes.The aggregation of molecules has a great influence on their photoelectric properties.Scince the organic conjugated molecules are usually used in solid state in these devices,it is quite important to study the effect aggregation behavior between molecules on the optoelectronic properties,such as absorption,solid-state fluorescence,exciton,hole and electron transporting properties,etc.Perylene bisimide（PBI）is an archytype organic conjugated molecule,which possesses good stability and photoelectric properties.Moreover,the strong intermolecular interactions between PBI molecules induce different stacking modes,which can enrich the properties of aggregates.In this dissertation,PBI dyads that connecting two PBI units through covalent bonds were synthesized and the aggregation behaviors and their optoelectronic properties were investigated in details.By adjusting the substituents at imide positions,the different stacking modes between molecules were observed,the application of these PBI dyads in organic solar cells was explored,and the relationship between structure and performance were summarized.Firstly,we synthesized dPBI（1-1）and reported the single crystal structure for the first time.The thickness and shape（rhombic or hexagonal）of the two-dimensional crystal can be controlled by choosing different solvents and volatilization rate.The surface morphology of PBI dyad slice crystals was characterized by scanning electron microscopy（SEM）and atomic force microscopy（AFM）.The crystallographical investigation clearly indicated that PBI units are packed in two different patterns—one is rotary columnarπ–πstacking（i.e.H-type stacking）and the other one is discrete dimeric slippedπ–πstacking（i.e.J-type stacking）,and the two different stacking moieties are isolated from each other.The lamellar crystals show relatively low photoluminescence efficiency of 12%at room temperature,while it dramatically increases to～90%at low temperature（80 K）.Both of the rotary and slip stacked moieties are emissive and the non-radiative energy transfer processes between them are sup-pressed at low temperature,which ensure the highly efficient excimer-like long-lived fluorescence.The spectral and theoretical analysis show that the H-type stacking possesses relatively higher excitonic energy relative to the J-type stacking.The energy transfer from the H-type stacking moieties to the J-type moieties is allowed at room temperature but suppressed at low temperature.The restricted non-radiative energy transfer processes together with the enhanced Herzberg-Teller effect at low temperature result in the highly efficient long-lived excimer-like emission.This work provides a new perspective on improving solid-state luminescence by tuning the molecular stacking modes.Secondly,we synthesized another two PBI dyads,dPBI（1-2）and dPBI（2-2）with different branched alkyl chains at imide positions.The UV/Vis absorption spectra,fluorescence emission spectra,X-ray diffraction spectra and micrographs were used to study the aggregation behaviors of these two dyads.It was found that the aggregation behaviors were affected by the branched structure of the alkyl chains at imide positions of these PBI dyads.When the branched position is far away from PBI core,J-aggregation structure with stronger intermolecular interactions and lower excited-state energy is easily formed.Although the crystallinity of dPBi（1-2）is better than that of dPBI（2-2）,the device performance of organic solar cells using dPBi（1-2）as the electron acceptors is modest,indicating that the diversity of the aggregation of the acceptor materials is not suitable for organic solar cells.Thirdly,ring condensation reaction of PBI molecules was used to synthesize the derivative of peryleneimide（P₂B）.Through the analysis of UV/Vis absorption spectra and ¹H NMR spectra,it is found that P₂B can form aggregates only at a higher concentrated solutions,which is far less than that of PBI.P₂B is further successfully functionalized by introducing bromine atoms at bay area andP₂B dyad is synthesized C-C coupling reaction,which has been applied as the acceptor materials of organic solar cells.