Tuning Aggregation Structure And Photophysical Properties Of Wide-bandgap Fluorene-based Polymers

Wide-bandgap fluorene-based polymers have been widely applied in optoelectronic devices,such as polymer light-emitting diodes,organic lasers and field-effect transistors,because of their versatile modification,deep-blue emission and high photoluminescence quantum efficiency(PLQE).However,due to its complicated and changeable aggregation behavior,fluorene-based polymer semiconductors are still fall short of expectations in material properties and device applications.There are many key scientific issues,such as poor blue emission stability,low self-assembly ability,variable condensed structure and single function,etc.,which need to be solved urgently.In this dissertation,based on synergistically molecular attractor-repulsor theory(SMART),we investigated the self-assembly ability and explored the origin of green-band emission through supramoelcular method,systematically realized controllable conformational transition,further improved the efficiency and stability of blue light emission.The main research contents are as follows:Firstly,we proposed the supramolecular functionalization strategy to introduce the hydroxyl groups into the polyfluorenes,and investigated the effect of intra and interchain hydrogen-bond interaction on the assembly behavior and opto-electronic property.We explore the preparation of well-defined spherical assemblies of supramolecular conjugated polymer during the process of “one step” solvent evaporation induced interface self-assembly(SEIS)and confirm the universality of amphiphilic polymers in terms of zero-dimensional assembly.PPFOH sub-microspheres with different sizes have been obtained by controlling the precursor solution concentration.The fascinating size-dependent green band emission properties of spheres provide a convincing evidence of aggregation mechanism.Secondly,poly(9,9-dioctylfluorene-2,7-diyl)(PFO)was used as the prototype mode to investigate the aggregates evolution in nondilute 1,2-dichloroethane(DCE)solution.The aggregate structures in solution would dominate the structural development in the film-casting process,and influence the morphology and optical property of corresponding spin-coated film.The results reveal the structure-function relationship between the planar conformation and aggregation behavior.We optimize the film spin-coating technology via critical aggregate time and obtain high-quality β-conformation film which show excellent film morphology,high crystallization and high fluorescence quantum yield.The formation of β-conformation may suppress the g-band emission and improve the stability to some extent.Subsequently,coupling spectroscopic visualization methods with high spatial resolution optical techniques,we systematically explored the fluorescence dynamics and anisotropic property of polyfluorenes exhibiting diverse conformation(amorphous and β-conformation)behaviors in inhomogeneous films on submicrometer spatial and picosecond temporal scales.The excitons of the β-conformation decay faster than those in the amorphous state.Besides,β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation.Finally,ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers.FLIM and FAIM provide a direct visualization to screen the self-dopant in β-conformation films.On the basis of summarizing supramolecular synergistic effect manipulating chain aggregation,condensed structure and morphology as well as optoelectronic property,we demonstrate a supramolecular chiral oligofluorenol with a hierarchical uniform crystalline structure by precisely controlling molecular stereoisomerism.This well-defined architecture shows unique robust and ultrastable crystalline-enhanced emission with PLQE 90% higher than that of the pristine one.Meanwhile,the structural rigidity,dense packing and well-aligned conformation in the crystalline structure also result in excellent morphological stability(restricted molecular motion)and resistance to chemical oxidation,further improving blue emission behavior.Finally,the nanosheet laser with deep-blue emission was manufactured,attributed to the 0-1 vibronic band of the quasi-planar conformation.This is the first systematic investigation of the value of molecular stereoisomerism to tune hierarchical condensed structures and explore their useful optoelectronic properties.Finally,we extend the material system to a series of polymer.We prepared 2D nanosheets via mixed solvent method successfully.The nanosheets exhibit highly ordered molecular chain arrangement and unique photophysical behavior.Through the selection of polymer types,good solvents and poor solvents,the universality of the method for preparing 2D nanostructures was also verified.Our strategy paves the way of applications in eco-friendly,low-cost organic optoelectronic devices.In conclusion,we systematically investigated the aggregation behaviors in solution,film and nanostructure state,realized the hierarchical assembly at the molecular level of fluorene-based polymers,improved the fluorescence efficiency and stability.In this regard,this work provided a new strategy to improve and multi-functionalize the opto-electronic property of electronic device.