| Organic luminescent materials have wide applications in many fields, such as organic light-emitting diodes, organic solid-state lasers, chemo-sensors and bio-imaging. In the past few decades, the topic of how to design the organic luminescent materials not only with ideal emission color but also with high quantum yield was deeply investigated. According to the principle of “Structures determine properties”, it can be easily concluded that the luminescent behaviors of organic luminophores are determined by the molecular structures. Consequently, in the initial research studies, researchers focused on the molecular structure-property relationships and modified the molecular structures to gain high-performance organic luminophores. In this period, researchers accumulated some experience about the material design. For example, large π-conjugated skeletons or molecular frameworks with strong polarity might lead to red emission. On the contrary, molecular structures with low conjugation and polarity are more likely to be blue emissive. In a practical application, materials usually exist in solid form. However, many luminophores with ideal emission in solution cannot function well in aggregated state. In the recent studies, researchers found that the luminescent properties of organic luminophores were not only decided by the molecules’ chemical structures but also highly affected by the molecular conformation and intermolecular interactions when the molecules aggregated into solid state. Accordingly, it is very important to investigate the molecular “structure-property” relationships of organic luminophores. Here the molecular “structures” are not limited to the molecules’ chemical structures. They also consist of the molecular conformations and inter-/intra-molecular interactions in the aggregated state. The goal in this dissertation is to design some new organic luminescent materials with high quantum yield, and then investigate their “structure-property” relationships as guidance for the design of new high-performance organic luminophores. In this thesis, we designed four molecular systems. All of them displayed high fluorescent quantum yields and very clear “structure-property” relationships of organic luminophores in the crystalline state. Besides, we also investigated the potential applications of the prepared highly fluorescent organic crystals.1. In chapter II, we designed a series of unsymmetrical 1,3-diaryl-β-diketones 1?6 with different substituents or substituted positions. Large amounts of crystals were prepared using solvent diffusion method. All the crystals showed similar size, morphology and color, indicating that different substituents or substituted positions did not affect the crystallinity of these molecules. The photophysical properties of 1?6 in different states were fully characterized and the measured data showed that 1?6 displayed similar emission behaviors in solution while in crystalline state 1?6 showed distinctively different emission properties. To be specific: when the substituent locates in a para-position relative to the carbonyl group, the crystals would be highly emissive(1?4: ΦF = 0.39?0.53); when the substituent locates in a meta-position relative to the carbonyl group, the crystals would be weakly emissive or non-emissive(5: ΦF = 0.17; 6: ΦF ~ 0). This also demonstrates that substituted positions have great impact on emission behaviors in crystalline state. The analysis of the crystal structures shows that all the crystals 1?6 have the same molecular packing structure. The difference of the crystal structures lies in the molecular conformations, which leads to different fluorescence quantum yields of the crystals. Detailed investigation on the emission behaviors and the crystal structures demonstrates that the more planar conformation the molecules in the crystal takes, the brighter emission the crystal will display. Besides, crystals 1?5 display amplified spontaneous emission(ASE) with minimum full width at half maximum(FWHM) of 3?7 nm, indicating their potential as laser media in organic solid-state lasers.2. In chapter III, we designed the 2’-hydroxychalcone derivatives based on the unsymmetrical 1,3-diaryl-β-diketone structure in the previous chapter. The hydroxyl group near the dimethylamino substituted phenyl group was removed to improve the molecular polarity and promote the excited-state intramolecular proton transfer(ESIPT) process. The as-synthesized 2’-hydroxychalcone derivatives thus display very large Stokes’ shift. Using very simple crystallization process, we obtained three deep red/near-infrared(NIR) emissive crystals 1?3(λem = 680–716 nm; ΦF = 0.25–0.32) with centimeter size in large amounts. For a deep understanding of the emission mechanism of crystals 1?3, another four compunds 4?7 were synthesized as comparison. The crystal structures and the emission behaviors of these compounds demonstrate that there are several factors determining the emission behaviors of the crystals: the molecular polarity, the ESIPT activity, the molecular conformation and the molecular packing structure in the crystal. Reasonably combining all the above factors, we obtained crystals 1?3 with highly efficiency deep red/NIR emission. On the other hand, the comparison of the emission behaviors between 1?3 and 4?7 also indicates that the small modification on the molecular skeleton might lead to great change of the emission behavior. Besides, crystals 1?3 display typical ASE properties with very low threshold values, indicating their high potential as NIR laser meda.3. In chapter IV, by introducing a fluorine atom into the 2’-hydroxychalcone skeleton, we prepared the 4’-fluoro-2’-hydroxychalcone derivative. Using different crystallization methods, we prepared two polymorphs based on this single molecule: centimeter-sized highly NIR emissive crystal 1R(α phase, λem = 702 nm, ФF = 0.41) and mini-sized relatively weak orange emissive crystal 1O(β phase, λem = 618 nm, ФF = 0.05). Both 1R and 1O display typical ASE character, demonstrating that the single-molecule multicolor ASE has been realized based on this simple chalcone derivative. Besides, β phase crystal can transform in to α phase through a single-crystal to single-crystal(SCSC) mode accompanied with the ASE switching.4. In chapter V, we designed a series of pyrazole derivatives 1?5 using the unsymmetrical 1,3-diaryl-β-diketones in the second chapter as main reactant. These compounds display a single emission band peaking at near ultraviolet(NUV) region(λem = 370–380 nm, ΦF = 0.05–0.08). However, in the crystalline state, these compounds display special dual-peak emission(enol form: λem = 430–437 nm; keto form: λem = 554–610 nm. ΦF = 0.09–0.16) which can be ascribed to the local emission and ESIPT emission. In other words, crystallization process can induce the keto emission and we name this extraordinary phenomenon as crystallization-induced keto emission(CIKE). The two emission bands have appropriate ratios which endow the crystals with nearly pure white emission. By adding some acetic acid(HOAc) in the crystallization process, we prepared the acidized crystal 1·HOAc(λem = 433 nm, ΦF = 0.17) with deep blue emission. The difference of the emission behaviors and crystal structures between 1?5 and 1·HOAc indicates that the intramolecular hydrogen bond determines the keto emission by controlling the ESIPT process. By fuming with HOAc and heating processes, the emission color of the crystalline solid can be switched between deep blue and white.In summary, four types of solid-state organic luminescent materials with different emission colors were designed and their crystals were prepared. We deeply investigated their photophysical properties in various states, especially in the crystalline state. These materials clearly disclosed the structure-property relationships of organic luminescent materials, providing some guidance for the design of high-performance... |
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