Oligofluorenols:Diastereoisomeric Effect,Photooxidation Behaviors And Green Emission Defects' Photophysical Identificatoin

Based on the background of fluorene-based semiconductors,in-depth research on revealing the molecular structure,structure-property relationship,photo-oxidation behaviors and the origin of green emission band have been made.In the past studies,polyfluorenes were used as models.However,the structure-property relationship could not be accurately clarified due to the ambiguous molecular structure of polymers.Moverover,the oxidation pathway of polyfluorenes was mostly theoretically speculated and lacked the basis of real molecular model.In addition,there are still controversies on the mechanism of the origin of the g-band.In this paper,hydroxyl group was introduced at the 9-position to enrich the weak interaction.Using bifluorenols as a model,the diastereoisomers are compared and characterized.The diastereoisomeric effects on absorption,emission,electrochemistry,self-assembly,spontaneous amplification radiation and microcrystalline laser have been investigated.On this basis,the dynamic behavior of fluorenols photooxidation was explored including the effects of solvent and configuration on photooxidation behavior,which provided a theoretical basis for the reveal of photooxidation pathway of polyfluorene materials.Model fluorenols molecules with g-band caused by aggregation defect were introduced along with model molecules with fluorenone defects and bend defects were introduced.The three molecules were compared to further uncover the unique optical properties of different origins of the green band.Firstly,9,9'-diphenyl-9H,9'H-[2,2'-bifluorene]-9,9'-diol?DPFOH?with a flexible rotation bonding between two fluorenols and chiral carbons at 9-site was employed as a desirable model to reveal diastereomeric effects of fluorene-based organic semiconductors.Two Diastereomers,rac-DPFOH and meso-DPFOH were isolated and unambiguously elucidated by X-ray crystallography.The diastereomeric effects on photophysical properties,self-assembly and microlasing behaviors of them has been studied.The results showed that both have similar behaviors of electrochemical characteristics and optical properties in solution or film states.However,the diastereomers had great effect on the microcrystal morphologies and microlasing behaviors.Rac-DPFOH and meso-DPFOH self-assembled into one-dimensional?1D?rod-shaped and two-dimensional?2D?plate-shaped microcrystals,respectively.The 1D microrod of rac-DPFOH exhibited microlasing behavior at 400 nm with a threshold of 211.6 W/cm²,whereas the individual 2D microplate of meso-DPFOH with a broad structureless emission band centered at420 nm did not show similar phenomenon due to the change of molecular arrangement and supramolecular interaction.The results indicate that diastereomeric effects play a significant role in organic functional materials and provide a new approach to rationally design molecules for microlasing applications.Based on the first chapter,it is found that the surface of DPFOH single crystal grown by the slow evaporation method under sunlight is accompanied with the green emission.And it is speculated that the fluorenol-based material is prone to photodegradation reaction.Using naked-eye observation combined with thin-layer chromatography,it was found that both daylight and 365 nm UV lamp irradiation induced photodegradation of DPFOH solution.This photodecomposition phenomenon and results were not affected by configuration differences.The stability of DPFOH in the solvent was ranked from good to bad:THF>toluene>CH₂Cl₂>CHCl₃.In the THF solution,DPFOH solution did not become yellowish when observed by the naked eye.The photodegradation products and photolysis paths are complicated.Therefore,further research is needed on the photodecomposition path.To uncover the unique photophysical properties of undesired appearance of g-band emission in fluorene-based materials,Trifluorenols with aggregation defect,along with other two well-defined oligofluorenes corresponding to other two types of“defects”attributed to:keto defect and chain-entanglement defect,respectively,have been systemically investigated.Firstly,the optical properties of defect molecules in different states were studied.The g-band induced by aggregation defect was absent in the dilute solutions and films doped at 0.01wt.%with?poly?methyl methacrylate??.Then,the solvent effect of g band emission caused by keto defect and chain-entanglement defect has been compared.Different from g-band of chain-entanglement defect,the green emission of keto defect showed a strong dependence on solvent.Thirdly,energy transfer between stable blue emission material poly?diarylfluorene?s and the keto or chain-entanglement defect molecules was illustrated.Compared with chain-entanglement defect,at low proportions of the keto defect molecule?0.1%?,the spectra showed signs of defect emission.Fom the point of synergistically molecular attractor–repulsor theory,oligofluorenols model molecules were designed and synthesized.Diastereomeric effects of fluorene-based organic semiconductors have been systemically investigated and the understanding of the structure-property relationship of fluorene-based materials was deepened.These investigations not only provide insight into understanding the photophysics of oligofluorenes but also supply a new strategy to explore defects in semiconductor polymers,which will aid in the development of effective approaches to obtain stable pure blue OLEDs based on polyfluorenes.