The Influences Of Supramolecular Interactiontions On Optoelectronic Properties Of Organic π-Conjugated Materials

OrganicÏ€-conjugated polymer/ oligomer is a class of promising functional organic materials, with the qualities of light weight, flexible, excellent processability and tunable optoelectronic properties, which are widely applied to organic light emitting diods (OLEDs), organic field effect transistors (OFETs), photovoltaic cells (PCs), organic lasers and chemical & biological sensors. During the past 20 years, chemists continually devoted their effort to construct and develop the system of organic optoelectronic materials. Based on a huge number of materials, structure-property relationship of organic optoelectronic materials are gradually optimized. Main chainÏ€-conjugated structure, substituent, molecular conformation, molecular electronic structure, purity, supramolecular interactions and film morphology are all key factors for the optical, electronic and optoelectronic properties ofÏ€-conjugated materials.Recently, supramolecular chemistry ofÏ€-conjugated materials have attracted increased interest, which provides an efficient tool to explain many optoelectronic phenomenon and construct nanostructures with distinct size, shape and function. Supramolecular chemistry, a term introduced by Jean-Marie Lehn, is"chemistry beyond the molecule", that is the chemistry of molecular assemblies using noncovalent bonds. Its main features are: (1) weak noncovalent forces can become much stronger when they cooperatively combine together. (2) supramolecular self-assembly ofÏ€-conjugated systems can attribute many new features beyond the single molecules. The main noncovalent forces include: metal-ligand, coulomb, hydrogen bond,Ï€-Ï€and van der Waals interactions. In crossing with material science, supramolecular chemistry plays an important role in creating new functions of materials and extends the application field ofÏ€-conjugated systems to nanoscience and biology. This thesis is mainly focused on studying the influences of supramolecular interactiontions on optoelectronic properties of organicÏ€-conjugated materials. On one hand, we utilize the principles of supramolecular chemistry to investigate some optoelectronic phenomenon in existing systems; on the other hand, proper supramolecular interactions are introduced at the molecular level to construct nanostructures for supramolecular electronics application.Chelation between metal ions andÏ€-conjugated polymers composed of chelating ligands in the backbones have attracted much attention recently because it provides an easy way to tune up the optical and electronic properties of the conjugated polymers. We studied the interactions between metal ions and polyfluorene (PF)-type copolymer containing metal ion binding sites, such as 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen), showed a strong ionochromic effect and demonstrated a new approach to sensitive, selective and highly reversible ion-responsive polymers for metal ion sensors. Furthermore, aimed to reveal the effect of metal ion binding amount on the optical property of conjugated polymer, a series of PF-based copolymers (P)(bpy)x (x=0.1, 0.2, 0.5) that contain different amount of bpy units in backbones were synthesized, and their optical properties as zinc²⁺ binding in solution were studied. Most metal ions would quench the fluorescence of polymers; however, zinc²⁺, the closed-shell metal ion, was found to change the emission color. We found that zinc²⁺ did not participate in the formation of frontier molecular orbitals (MOs) but induced a redistribution of the electron cloud density of the frontier MOs, indicating a charge-transfer (CT) excited-state characteristic in the zinc(II) chelating compound. By adjusting the bpy content in the polymer backbone, the emitting color of the polymer solution could be tuned from initial blue to green, white and violet when it interacted with zinc²⁺ ions due to the incomplete energy transfer. The counteranions of zinc²⁺ ions were also found to influence the optical properties of copolymers greatly.Biphenyl is the building block of manyÏ€-conjugated systems, so it is significant to study the molecular conformation and intermolecular interactions of biphenyl-basedÏ€-conjugated molecules. An intramolecular [²⁺2] cycloaddition at room temperature in a swivel-cruciform molecule 2,5,2',5'-tetra(4'-N,N-diphenyl- amino-styryl) biphenyl (DPA-TSB) based on a biphenyl center was reported. By 1H NMR spectra analysis and theoretical calculation, it is found that the biphenyl center is a special tether for its non-linear assemblage and relatively free rotation, which results in a swivel-cruciform conformation and the conformational multiformity in DPA-TSB. The conformational transfer of the biphenyl core can construct the ortho-substituted double bonds in DPA-TSB to the configuration as required by the thermal [²⁺2] cycloaddition (an orthogonal stacking fashion ofÏ€orbitals and short distance) and result in an intramolecular cycloaddition product under very mild condition. DPA-TSB as a model compound based on distyrylbenzene (DSB), a typical organic light emitting material, can be used to investigate the physical and chemical processes of DSB in solid state. It is considered that this [2+2] cycloaddition may be a newly observed ageing and fatiguing process of the thin film devices.Recently, hydrogen-bond directed self-assemblies ofÏ€-conjugated system have been extensively studied. The synergetic combination ofÏ€-stacking and hydrogen bonding is found important for the future development of supramolecular electronics based on such hybrid systems, where long-range order, high charge carrier mobility and thermal stability are prerequisites. In this paper, a series of bisurea-end capped oligo(p-phenylenevinylene)s (OUPVs) have been synthesized and fully characterized. In OUPVs, the urea groups (hydrogen-bond motifs) are covalently linked with OPVs (Ï€-conjugated moieties) without any spacers to unify the two forces into interlocked hydrogen bond andÏ€-Ï€interactions; the urea-urea hydrogen bonds are enabled in theÏ€-stacking direction to construct balanced supramolecular interactions. Through careful tuning and design, urea-urea associations at both ends of theÏ€-conjugated moiety can provide a forceful framework for"face-to-face"Ï€-Ï€stacking due to the synergetic combination of different interactions at the molecular level. A broad range of analytical methods have been used to study the aggregation behavior of the molecules. It is found that the OUPV molecules can self-assemble into supramolecular wires with lengths up to tens of microns that exhibit intense electronic coupling, high molecular ordering and extraordinary thermal stability.