Tuning Aggregation Behavior And Opto-electronic Properties Of Fluorene-based Polymer Via Supramolecular Approach

For convenient synthesis and multifunctionality, fluorene-based polymers, especialpolyfluorenes (PFs), can serve as one of the most promising polymer semiconductor for plasticdevices. However, complicated aggregation behaviors and their induced diversity film morphologyor opto-electronic properties has become a difficult and hot spot in the research of fluorene-basedpolymers. In this doctoral dissertation, combining the latest development in supramoelcularelectronic, we have incorporated the supramoelcular unit (flexible chain, nitrogen substitution,hydroxyl unit, carbazole) into the polyfluorenes via supramolecular functionalization andinvestigated their effect on the molecular aggregation behavior via studying the supramoleulcargelation processing, to explore the origin of green band emission of polyfluorenes, further improveand multi-functionalize the opto-electronic property of polymer semiconductors and their plasticsdevices via controlling green band emission. First, poly(9,9-dioctylfluorene)(PFO) was used as theprototype mode to investigate molecular aggregation behavior in the gelation or non-gelationsolvents. We have observed that the gelation processing of PFO in1,2-dichloroethane (DCE) wouldpromote the formation of β phase, which may improve the polymer light-emitting diodes (PLEDs)performance (current efficiency:3.5times higher than amorphous states) and the stability ofelectroluminance (EL) spectra. The prerequisite to achieve β-phase of PFs is van der Waals forcesof side chains strong enough to overcome the steric repulsion and polarize polymer backbone. ThePFO-β sheets self-assembled into cluster in the gels can induce the green band emission at550~560nm, suggesting the aggregation was the origin of green band emission. In order to check theaggregation resulting in green band emission and further explore the effect of supramoelcularaggregation of backbone chain of PFO on the formation of β phase, we have designed and prepareda series of the nitrogen heteroatomic PFO with a different ratio of nitrogen substitution,PF8-co-DAF8, confirmed that the formation of C-H…N hydrogen-bonding interaction woulddisrupt the original balance and prevented the β phase forming, but they also easily self-assembledinto gels in the toluene and DCE at room temperature. For the strong intermolecular interaction, thephotoluminance (PL) spectra of PF8-co-DAF8with the heteroatomic ratio of50%exhibitedstronger green emission than those of PFO. And considering the supramoelcular coordinatedinteraction, the EL color of PF8-co-DAF8with the heteroatomic ratio of20%would be tuned fromblue, nearly white and green via supramoleuclar approach. Meanwhile, we have introduced hydroxyl group into the polyflurenes (PPFOH) to enhance inter-and intrachain interaction andinvestigated the effect of interchain hydrogen bonding interaction on the aggregation behavior andopto-electronic property. The strong green band emission in the PL spectra of the concentratednon-gelation solvents and films would effective confirm the aggregation mechanism of green bandemission, attributed to the hydrogen bonding and π-π stacking interaction of PPFOH in theaggregate processing. And this hypothesis also can be supported by the evidence that synergisticeffect would be significantly enhanced in the PPFOH nanoparticles. Large effect of numbermolecular weight (Mn) on green emissions of the organogels and superstructured thin films has beenobserved. The emission colour of PPFOH thin films and PLEDs can be tuned flexibly from blue toyellow via selecting different type of solvents and Mn. From the above discussion, in considerationof the inherent drawback of stronger aggregation and poor environmental stability (oxidation) ofPFO, PF8-co-DAF8and PPFOH, we rationally designed and prepared two fluorene-basedmolecular building blocks containing the bulky groups at9sites with the feature of steric repulsionsand alkyl groups (PODPF) or carbazole (PCzODPF) at4sites with the van der Waals attractiveforces via key Baeyer-Villiger rearrangement. Their thin-films both exhibited excellent spectralstability under thermal annealing in air and N2at200C, indicated that the bulky groups in9-positon of fluorene served as the element to improve the thermal and chemical stability againstaggregation and oxidation, which is essential for the organic device. Besides thepoly(9,9-dialkylfluorene)s (PDAFs) and their derivatives, we have first constructed a novel β-phasepolyfluorenes (PODPF), which the β-phase morphology was achieved in toluene solution,organogels, and films spin-or drop-coating from toluene solution as well as in amorphous thin filmsby annealing process. Grazing incidence X-ray diffraction (GIXD) analysis suggested that thebalance of the steric hindrance interaction and van der Waals interaction were favorable for theformation of β-phase. Furthermore, the π-stacked carbazole group at4-possition of fluorene(PCzODPF) would disrupt the balance and exhibited stronger gelation ability in the toluene that canself-assemble into nanofiber to realize laser and waveguide. Therefore, we have first systemiclytuned the aggregation behavior (gelation processing), and opto-electronic property offluorenes-based polymers via supramoelcular approach (the effect of van der Waals forces, π-πstacked interaction, heteroatom interaction and hydrogen-bonding interaction on theself-organization), and provided a new strategy to improve and multi-functionalize theopto-electronic property of electronic device. With the development of third generation heteroatom-contained polymer semiconductor, inter-and intramolecular supramolecular interactionwould be an important ingradient to improve and multi-functionalize the opto-electronic property ofelectronic device. In this regard, this work would provide a novel concept and route to explore therelationship between molecular arrangement, film morphology and opto-electronic property ofpolymer semiconductors and their based devices.