Hierarchically Structural Tunability Of Polyfluorenes And Their Optoelectronic Property Based On Conformational Transistion

As a mostly potential application of deep-blue emissive conjugated polymer,polyfluorene（PFs）are widely used as active materials in organic light-emitting diodes（OLEDs）,organic laser and organic field-effect transistor（OFET）.In general,hierarchical structure of polyfluorenes in condensed structure is key factor to dominate their optoelectronic property and applications.According to traditional polymer physic theory,hierarchical structures of conjugated polymer are defined as preliminary structure（chemical structure）,secondary structure（conformation and morphology of single chain）and tertiary structure（condensed structure）.As the fundamental parameter,preliminary structures of polyfluorenes,involved molecular structure and stereoisomeric configuration,are dominated the initially the electronic structure and effective electron delocalization.Due to the liner-type topological structure,chain conformational and morphological behaviors of polyfluorenes are seriously effect on the intrachain arrangement,charge transport and exciton disffusion behavior.As the key role of tertiary structure,interchain arrangement play a key tole in controlling interchain charge behavior and photophysical processing in the solid states.Compared to the non-planar conformational superstructure,planar conformation of polyfluorenes acts as the most desired situation to provide the excited matters and obtain a favorable condition and electron landscape to function as semiconducting behavior.Meanwhile,the non-planar conformational chain of polyfluorenes（PFs）in solid states may induce the formation of physical defect,which may completely suppress in the hierarchically ordered structure of polyfluorene based on the planar conformation.In this thesis,we will paid more attention to investigate the formation of physical defect structure in non-planar conformational film of polyfluorenol（PPFOH）and further explore the dynamic behavior of planar conformation in polydiarylfluorenes（PODPF）toward inhibiting the effect of defect structure on the optoelectronic of polyfluorene and their devices.The main content of the dissertation includes the following four parts:Firstly,we are focused on investigating the origin of aggregation induced green-band emission in polyfluorenol（PPFOH）,which present a non-planar conformational chain in various states.Exciton behavior is crucial to the exploitation of light-emitting conjugated polymer（LCPs）for plastic optoelectronics,which are easily trapped by the intrinsical defect structures.In this section,we set a supramolecular polyfluorene（PPFOH）as an example to systematically investigate its photophysical behavior to check the role of defect emission in LCPs.Hydroxyl units at 9-position enable PPFOH to present a diverse aggregation-induced defect structure in various states.More interestingly,accroding to time-resolved photoluminescence analysis,the well-ressolved feature emission behavior from individual chain for PPFOH diluted DMF solution is effectively avoided the influence of fluorenone formation.However,the residual green-band emission at 500～600 nm is easily observed in the PL spectra of PPFOH in diluted toluene solution obtained delay 1.5 ns,associated with interchain or intrachain defect structure.Remarkedly,efficient energy transfer from high-band individual chain to defect green-band emission is found in both two type PPFOH film spin-coated from DMF and toluene solutions.Interestingly,the fast energy transfer happaned for the film obtained from DMF solution（200 ps）than those of toluene ones（600 ps）.Meanwhile,compared to stable green-band emission in PPFOH film obtained from toluene solution,red-shifted green-band emission（about 11 nm）of PPFOH film coated from DMF solution exposed to saturated DNT vapour also confirmed the different defect structures in the solid states.Therefore,this defect structures assisted by interchain aggregation are serious influenced on the photophysical property of LCPs in solid states.Secondary,in this section,we demonstrated the photoexcitation dynamics of exciton migration and energy transfer in self-dopedβ-conformational film of polydiarylfluorenes（PODPF）via time-resolved PL spectroscopy and low-temperature optical analysis.Compared to the first generationβ-conformational polyfluorene（PFO）,energy transfer occurred in our polydiarylfluorenesβ-conformational films are in a time about～150 ps,much longer than those of PFOβ-conformation（<5 ps）,which may associate with the effective intrachain energy transfer（few hundred picoseconds）rather than interchain F（?）rster ones（a few picoseconds）.Similar to PFO,PODPFβ-conformation also show well-resolved vibronic emission peaks at 20 K,attributed the oriented and rigid conformation.Interestingly,residual 0-0 band emission of non-planar conformation（435 nm,2.85 e V）at 20 K also further confirmed the exciton migration from amorphous state toβ-conformational domain in PODPF films.Therefore,stable dual amplified spontaneous emission（ASE）behavior of self-doped films at 461 nm（2.69 e V）and 483 nm（2.57e V）,original from the individual amorphous andβ-conformational domain,respectively,also indicated the slower rate of energy transfer than PFO ones.Exciton behavior can be precisely tuned via controlling chain conformation and arrangement.Third,compared to the only periodic arrangements in conventional crystalline nanostructure,uniform condensed structure at chain-level will promote electronic landscape to obtain excellent optoelectronic behaviors,which are desirable for deep-blue polymer light-emitting diodes（PLEDs）and polymer lasers.In this section,we constructed a range of robust hierarchical uniform crystalline nanowires of wide bandgap bulky polydiarylfluorenes via a simple“one-step”interface self-assembly for light-emitting optoelectronic application.Interestingly,these nanowires show a robust deep-blue emission with a high fluorescence quantum yield of approximately 35%,which also simultaneously presented a highly crystalline structure with excellent anisotropic emission behavior.Meanwhile,the steric hindrance interactions of the bulky groups enable these nanowires to effectively suppress interchain aggregation and obtain excellent intrachain photophysical properties.Interestingly,efficient deep-blue nanowires-based PLEDs were fabricated with an intrachain excitonic behavior（maximum luminance:1036 cd/m²）.Amazingly,compared to controlled ones,ourβ-conformational hierarchical uniform nanowires present a crystalline-enhanced emission（Enhanced about 33%）and low-threshold（280 m W/cm²）lasing behavior,associated with the local excellent exciton energetic landscape.To the best of our knowledge,this is the first example of using steric wide bandgap LCPs self-assembled nanowires for deep-blue organic nanolasers by constructing hierarchical uniform crystalline structure.Finally,high quality and larges-cale deep-blue polymer light-emitting diodes（PLEDs）are also fabricated based on the PODPF materials via brushed processing technology.Spin-coated technology is a common film-form processing to systematically investigate the fundemantal optoelectronic property of conjugated polymers,which also present a limitation of device area,associated with the incontious and nonuniform film morphology.Instead,brush-coating technology is introduced to construct a large-scale uniform film with an efficient deep-blue emission.Firstly,we revealed the effect of the solvent on the film morphology via brush-coating.Compared to the toluene solution,film obtained via using CHCl₃ had a smooth film surface and continuous morphology.And large-scale film can be also constructed with a uniform deep-blue emission and high efficiency of>40%.More interestingly,exciton lifetime is also similar to those of spin-coated film,indicated the slight influence of interchain rearrangement induced by the slow solvent evopration.Besides,large-scaleβ-conformational PODPF films are also manufactured via brush-coating and film-post thermal annealed processing.High fractions ofβ-conformation in this large-scale film are calculated about 20%,which can induce a feature emission ofβ-conformation PODPF.Finally,large-scale efficient deep-blue PODPF Pled are also obtained with a higher EQE of1.65%,which is much higher than those of amorphous film obtained via brush processing（1.00%）andβ-conformational film fabricated via spin-coated and annealing processing（1.16%）,associated with the synergistic effect of the formation of planar conformation and orientation in brush-coating and annealed processing processing.In this regard,brush-coating processing is a convenient strategy to construct a large-scale deep-blue emissive film for optoelectronic deivces.In summary,the origin of green-band emission in polyfluorene is initicially investigated to reveal the key role of defect structure with a distributed narrow-band in solid structure.Subsequently,to suppress the effect of defect structure,the enhancement of deep-blue emission in polyfluorene via hierarchically ordered strategy based on planar conformational chain are further confirmed,to discover that stable emission center ofβ-conformation domain allow for stable and efficient deep-blue emission,associated with the efficient energy from non-planar and planar conformation.What is more,theseβ-conformation chains also enable PODPF nanowire to obtain excellent deep-blue electroluminescent emission and robust lasing behavior.Finally,large-scale deep-blue PODPFβ-conformational PLEDs are also fabricated based the brush-coating and annealed processing,which present a high device efficiency and brightness.