| π-Conjugated organic polymers have attracted considerable attention owing to their interesting properties as materials for charge transport, for non linear optics, for electronics and optoelectronics. Since the first poly(phenylenevineneyl) (PPV) materials was reported by the research group in Cambridge, PPV oligomers as their model compounds and as active materials themselves have induced great interest. Typically, distyrylbenzene (DSB) and its derivatives are widely investigated, which results in an increased understanding of the important conjugated polymers; meanwhile DSB is also a bright pure blue emitter. However, unfortunately, the excellent luminescent properties of DSB (ca. photoluminescence (PL) efficiency over 90 %) are only obtained in dilute solution, but in the solid-state DSB emits very weakly. This solid-state fluorescence quenching behavior has been associated with the formation of intermolecular aggregates, especially the side-by-side arrangements known as H-aggregates. This means that the stacking of DSB molecules plays a determining role in the physical properties of these materials. In order to adjust the morphology and enhance the PL efficiency, we introduced the biphenyl core into the cruciform system and constructed a series of PPV cruciform oligomers with a 2,5,2′,5′-substituted biphenyl center. Through this chemical construction, a morphology change from a crystalline to amorphous glass has been realized in cruciform oligomers, which is desirable for OLED applications. Meanwhile, the biphenyl core, which is a relatively free rotation center, can provide proper flexibility to those cruciform molecules. Because of the big substituted groups at the meta- and ortho-position of the biphenyl, the rotation of the biphenyl will be partly depressed, which endows the molecular with appropriate rigid/flexible abilities. Thus during film-forming process, the molecular stacking induced repulsion can trigger the mild rotation of the arms in this kind of cruciform PPV oligomers along the biphenyl bond to adjust its conformation for compact stacking, which will benefit their film forming ability during vacuum deposition or solution spin coating. Base on those two points, the cruciform PPV oligomers with a biphenyl core is very suitable for the OLEDs applications, which can result in the high performance devices.Firstly, the cruciform TSB with a biphenyl center and HSTP with a terphenyl center have been synthesized. They both show very high PL efficiency in the solid-state (TSB: 19 %; HSTP: 47 %). As a comparison, their model linear compound, DSB, only shows a very low PL efficiency in the solid-state (8 %). In the further device fabrication, the devices based on those two materials both show strong blue emission, especially for HSTP based blue OLEDs show a luminescence efficiency about 4.88 cd/A, which is one of the best reported blue OLEDs. The cruciform oligomer TSB also shows larger loading ability to the guest molecules and it can be used as a blue host materials. In the experiment, the blue PPV oligomer DPA-DPDSB had been used as the corresponding guest material. The PL efficiency of DPA-DPDSB/ TSB guest/ host films arrive the high level of 80 %, which is approaching the PL efficiency of the guest DPA-DSB in dilute solution (82 %), indicating that the host TSB can sufficiently disperse the guest DPA-DSB with little aggregation. The organic light-emitting devices using DPA-DPDSB (4 wt.%) doped TSB as blue emitting layer show the maximum efficiency of 18.33 cd/A. Combining rigid backbone structure and mild flexible property, the cruciform host TSB exhibits large loading ability to the guest DPA-DPDSB, and they can form a'solid solution'film and dramatically enhance the performance of the guest/host devices.In following experiment, the cruciform PPV oligomers with different functional groups and further optimize the properties of the materials. Some other applications, such as two-photon absorption, had also been extended. DPA-DSB is a well-known compound with large two-photon absorption section and strong fluorescence in solution, but its easy crystallization characteristic leads to the formation of incontinuous crystalline phases during vacuum-deposition process, which greatly limits its applications as solid-state device. We construct a cruciform dimer of DPA-DSB, named as DPA-TSB, which can efficiently suppress crystalline and intermolecular interaction. The neat solid of DPA-TSB shows strong green-blue fluorescence as excited by steady-state absorption as well as two-photon absorption. The solid of DPA-TSB exhibits a PL efficiency (ηsolid) of 29 % and a solid-state two-photon action cross section (δηsolid) of 954 GM, which is much higher than its model compound DPA-DSB (ηsolid = 16 % andδηsolid = 150 GM). Based on the high PL efficiency, good film-forming ability and strong two-photon absorption, DPA-TSB exhibits great superiority for applications in solid-state optical devices. Based on the cruciform oligomer (bpy-DPA-TSB) with a bipyridine functional group, the phosphorescent rhenium (I) complex had been introduced into the cruciform system, in which a donor-acceptor molecule had been formed and can be used in organic photovoltaic application. There are some advantages for this new phosphorescent cruciform oligomer bpy-DPA-TSB-Re. Firstly, the phosphorescent metal complex always has a long lifetime due to the nature of triplet state, thus the rate of charge separation may be faster than that of recombination in this kind of material, which may further enhance the carrier separation. Secondly, the incorporation of the transition metal complexes into the polymers backbone can broaden the absorption spectra of copolymers due to the newly generated MLCT (metal to ligand charge transfer) absorption band. As a result, the photovoltaic device based on bpy-DPA-TSB-Re showed power conversion efficiency about 0.8 % (blended with BCPM), which is an encouraging result for application of such molecular donor-acceptor ensembles to solar cells.
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