| Organic optoelectronic materials have been widely used in the application oforganic light-emitting diodes (OLEDs), field-effect transistors (FETs) andphotovoltaic cells (PVC) due to their advantages of low cost, low toxicity,processability and flexibility. However, compared with inorganic semiconductors,organic materials usually have much lower mobility, which limited their applicationsin organic optoelectronic devices. Over the past two decades, designing new materialswith high charge mobility has been a formidable task and now many molecularsystems have been achieved mobilities higher that10cm2V-1s-1in thin film at roomtemperature and even higher values for single crystal forms. To further improve thecharge mobility of organic materials and then achieve superior device performance,researchers have designed many different molecular systems experimentally;accordingly, theoretical calculation is becoming a powerful tool to study the chargetransport properties of organic semiconductors and predict the drift mobility.On the other hand, organic-transition metal complexes have drawn large attentiondue to their superior photo physical properties and electrochemical property. Therehave been considerable interests in this kind of materials since they can achieve theinternal quantum efficiency as high as100%. Typically, the most widely used emittermaterials for phosphorescent OLEDs are Ir and Pt complexes. Whereas due to theirhigh cost, unevenly distributed and produce toxic waste, more abundant andcost-efficient metals such as Cu, Ag and Au have received much more attention.Among this group, copper complex have attracted increasing interest due to their richstructural and photo physical properties. However, because of their relatively weakspin-orbit coupling (SOC), the quantum yields are usually low. Recently, severalresearch have shown that specific Cu (I) complexes exhibit a pronounced thermallyactivated delayed fluorescence, making it possible to harvest both singlet and tripletexcitons.In this dissertation, based on quantum chemistry theory, we explored the ground state and excited state geometry, reorganization energy, frontier orbitals, ionizationpotential and electron affinity, transfer integral, charge carrier mobility and absorptionand emission spectra properties of several organic molecular systems throughtheoretical methods. We are herein aiming to design novel organic materials with highcharge mobility and Cu (I) complexes with thermally activated delayed fluorescenceproperty, and establish structure-property relationship through theoretical calculation,expecting to shed light on designing new organic optoelectronic materials andimprove the performance of electronic devices experimentally. Here, we chooseseveral typical types of organic optoelectronic materials and investigate the chargetransport property and photo physical property in detail. Specifically, it is divided intofour parts as follows:Part I: The electronic structure and charge transport property of9,10-distyrylanthracene (DSA) and its derivatives with high solid-state luminescentefficiency were investigated by using density functional theory (DFT). The impact ofsubstituents on the optimized structure, reorganization energy, ionization potential (IP)and electronic affinity (EA), frontier orbitals, crystal packing, transfer integrals andcharge mobility were explored based on Marcus theory. It was predicted that DSA andits derivatives show both high charge mobility and high solid-state luminescentefficiency. For DSA, the lower reorganization energies and the higher transferintegrals lead to the hole and electron mobilities to be0.21and0.026cm2V-1s-1,respectively. The calculated results showed that the charge transport property of thesecompounds can be significantly tuned via introducing different substituents to DSA. Itwas found that the hole mobility of DSA was0.21cm2V-1s-1while the electronmobility was0.026cm2V-1s-1, which were relatively high due to the lowreorganization energies and high transfer integrals. When electron-withdrawinggroups were introduced into DSA, DSA-CN exhibited hole mobility of0.14cm2V-1s-1which is in the same order of that of DSA. But the electron mobility of DSA-CNdecreased to8.14×10-4cm2V-1s-1due to the relatively large reorganization energy anddisadvantageous transfer integral. For TFDSA, it is also in favor of hole transport.And the hole mobility (which is0.029cm2V-1s-1) decreased one order of magnitudecompared with DSA and DSA-CN due to the small hole transfer integral. Otherwise,TFMDSA benefits to electron transfer because of its high electron transfer integral,and its electron mobility is0.011cm2V-1s-1。 The effect of electron-donating substituents was also investigated by introducingmethoxy group, tertiary butyl, methyl into DSA. DSA-OCH3and DSA-TBU showmuch lower charge mobility than DSA resulting from the steric hindrance ofsubstituents. On the other hand, both of them exhibited balanced transport properties(for DSA-OCH3, the hole and electron mobility is0.0026and0.0027cm2V-1s-1; forDSA-TBU, the hole and electron mobility is0.045and0.012cm2V-1s-1) because oftheir similar transfer integrals for both hole and electron. While the charge mobility ofTMDSA is relatively lower than the other derivatives, which is10-4-10-3cm2V-1s-1.DSA and its derivatives were supposed to be one of the most excellent emissivematerials toward organic electroluminescent applications because of their high chargemobility and high solid-state luminescent efficiency.Part II： The electronic and charge transport properties of a series ofmethylchalcogeno-substituted acene derivatives are investigated via quantumchemical calculations. To gain a better understanding of the role of methylchalcogenosubstitution, the results for bis (methylchalcogeno) anthracene and bis (methylthio)pentacene are compared with those for their parent oligoacenes, that is, anthraceneand pentacene. The introduction of methylchalcogeno groups in acenes can changethe molecular arrangement from herringbone packing to π-stacking, resulting theenhanced intermolecular electronic coupling. The introduction of methylchalcogenogroup can increase the stability of their anionic compounds and lower the injectionbarriers of electrons in organic electronics devices. The methylchalcogeno groups alsochange the charge transport properties by enhancing π stacking andchalcogen-chalcogen interactions. The halcogeno atom can involve in the formationof the frontier orbitals, which is good for the π-stacking interactions.The S-S and Te-Te interaction can also improve the intermolecular electroniccouplings. The calculations predict that the electron mobilities of bis (methylthio)anthracene and bis (methyltelluro) anthracene are0.68and0.48cm2V-1s-1,respectively, which increase by a factor of2to3with respect to that of anthracene. Itis demonstrated that the introduction of methylchalcogeno group is an effective wayfor the promotion of electron mobility of acene-based materials.Part III：The electronic and charge transport properties of peri-xanthenoxanthene(PXX) and its phenyl-substituted derivative (Ph-PXX) are explored via quantumchemical calculations. To gain a better understanding of the physical properties of PXX, a comparative study is performed for its analogue, that is, anthanthrene. Herewe mainly focus on the impact of heteroatom oxygen and phenyl group substitutionson the charge transport property. By employing Marcus electron transfer theorycoupled with an incoherent charge hopping and diffusion model, we estimate thecharge mobilties of PXX and Ph-PXX. Our calculated results indicate that theintroduction of a heteroatom (oxygen) at the reactive sites of anthanthrene canstabilize the extended π-system and improve the efficient charge injection inelectronic devices. The phenyl substitution of PXX makes a remarkable change ofcharge transport characteristics from a p-type semiconductor to an n-typesemiconductor, which shed light on molecular design for an n-type semiconductorthrough simple chemical structural modification.It is well known that a common approach to design electron transporting materialsfrom typical hole transporting material is to functionalize p-type semiconductors withstrong electron withdrawing substituents, such as perfluorophenyl, carbonyl, andcyano groups. In this part, we show that the conversion from p-type materials (PXX)to n-type materials (Ph-PXX) can be realized through simple chemical modifications,e.g., phenyl substitution. The phenyl groups afford dual roles in the crystal state:(1)change the molecular packing arrangements from slipped stacking toon-top-of-each-other stacking, leading to the characteristic of transfer integralchanged from being in favor of hole to electron;(2) provide strong intermolecularinteraction energy to stabilize dimer configuration.Part IV: In this section, the thermally activated delayed fluorescence (TADF)property of a series of neutral Cu (I) complexes were investigated based on hybridfunctional B3LYP, compared with long range functional ωB97X-D, using6-31G*basis set. The ground state and excited state geometry changes, frontier orbitals,excitation energy, thermally activated delayed fluorescence and natural transitionorbital (NTO) analysis as well as spin-orbital coupling for all the compounds werepredicted through density functional theory (DFT) method. The calculated resultsshowed large difference between the two methods. The hybrid functional B3YLPshowed that these compounds possess TADF property, while the results obtainedfrom ωB97X-D functional exhibited much larger energy separation between S1and T1states. Compared with the results of absorption and emission energies obtained fromthe literature, the results from B3LYP functional agreed much better compared withthose gained from ωB97X-D functional. In this system, compounds4,7and8are promising to be used as TADF materials based on B3LYP/6-31G*level in OLEDsapplication because they show smaller singlet-triplet energy gap. Typically, forcompound7, the energy separation between S1and T1states is as small as0.06eV. |
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