Synthesis And Optoelectronic Properties Of Sulfur-Containing Donor-Acceptor Type Organic Small Molecules

Organic light-emitting materials have been widely applied in organic light-emitting diodes?OLEDs?,organic photovoltaics?OPVs?,organic field-effect transistors?OFETs?and optoelectronic sensors due to their great performance of light-weight,low-cost,flexibility and tunable optoelectronic properties.Up to now,OLEDs have been gradually commercialized into screens of televisions and computers or mobile phones and have brought a brand new visual experience to people.Usually,the organic electroluminescent devices strongly depend on their condensed films,whose optoelectronic properties are closely related to the molecular structures and the strength of the intermolecular interactions.Therefore,it is one of the most necessary conditions for the preparation of efficient OLEDs via reasonably designing the molecular structures and regulating the intermolecular interactions to achieve high solid-state luminous efficiency.Sulfur,in the III period and VI main group of Periodic Table of Elements,owns two lone pair electrons and an empty d orbital with the outermost electron structure of3s²3p⁴.When formed into chemical bonds,sulfur is easily oxidized to sulfoxide or sulfone.After oxidation,the vacant d orbital of sulfur and the p orbital of adjacent oxygen can form the d-p?feedback bond.And to some degree,the conjugated length can be increased to reduce the LUMO energy,which contributes to the electron injection and transmission.The sulfur-containing heterocyclic materials can be full-color displayed on OLEDs,also applied in the fields of OPVs and bioimaging among Donor-Acceptor?D-A?type organic small materials.In this thesis,we design and synthesis several sulfur-containing D-A type organic molecules via various ways to obtain high solid-state luminescent efficiency.And the relationship between the molecular structures and optoelectronic properties has been studied in details and we also applied the materials in OLEDs.In addition,a series of atropisomers'pure crystals of sulfur-containing organic molecules are also obtained and it is found that the atropisomers have good stimuli responsiveness to pressure and temperature.The detailed research progress is listed as follows:In Chapter 2,we use tetraphenylethylene groups to connect with different sites of thiophene to synthesize four positional isomers with aggregation-induced emission property.They all have high solid-state luminous efficiencies(?_M1=37.39?,?_M2=67.42?,?_M3=18.93?,?_M4=62.50?).From the crystal structure,3,4-position thiophene derivative,M2,showed quite a twisted structure in space and had a strong intermolecular interaction.Besides,we systematically studied the effect of different sites of thiophene on optoelectronic properties through detailed characterizations of photophysical,thermal and electrochemical properties.The material with more planar arrangements and longer conjugations,like 2,5-position thiophene,M1,could be more beneficial for the carrier injection and transportation in devices.Therefore,the non-doped device with M1 as the emitting layer exhibited the best results of maximum external quantum efficiency?EQE?of 4.49??almost 5??and maximum luminescent efficiency of 65210 cd·m^-2 when fabricating blue OLED devices,which would be the high level among the best values of OLEDs based on TPE derivatives.In Chapter 3,we introduced nitrogen heteroatoms to get a highly efficient acceptor,benzothiadiazole and could adjust the band gaps via increasing the number of benzene rings and introducing selenium atom of the same main group,then the luminescent behavior could be controlled so as to obtain four donor-acceptor type sulfur-and selenium containing organic luminous materials.Four materials all have high solid-state luminours efficiencies(?_{d TPE-SBT}=23.10?,?_dTPE-SeBT=13.36?,?_{d TPE-SNz}=37.39?,?_dTPE-SeNz=17.23?).After a series of characterizations,it is found that the introduction of heavy atom selenium could reduce the band gap well and make it red-shifted to deep red-light zone.The spin-coating film of dTPE-SeNz emitted at the peak of 676 nm.However,the introduction of selenium also quenched the fluorescence and reduced the solid-state luminous efficiency.We have fabricated the non-doped devices based on four materials as the emitting layers.dTPE-SBT as the emitter showed the best electroluminescent performance among the four materials.The maximum EQE is 2.45?and the maximum brightness is 27860 cd·m^-2,almost twice higher than the values of the literature.And the electroluminescent spectra were in red and deep red area with the emission peaks of 616 nm and 663 nm,respectively,in the non-doped devices based on dTPE-SNz and dTPE-SeNz as the emitters.When we combined the thermally activated delayed fluorescent materials and traditional host materials together,then fabricated and optimized several doped red OLED devices,which obtain the EQE of 1.3?of a deep-red?654 nm?device and 3.6?of a red?610 nm?device,respectively,almost twice as much efficiency higher than those of nondoped red devices.In Chapter 4,we designed and synthesized a cruciform molecule with space conjugation to change molecular arrangements to achieve high solid-state luminous efficiency,one is NBT with linear type,and the other is dNBT with cruciform and space-conjugated type.And the study showed that two materials are atropisomers of the optical isomerism.By varieties of characterizations,it is found that dNBT with cruciform type could effectively inhibited the non-radiative transition process to enhance the solid-state luminous efficiency.It made it easy to separate the pure atropisomers and obtain different colors of aggregations and four polymorphs with crystalline-induced emission enhancement properties by means of different methods of purification?recrystalline,physical vapor transport,sublimation and so on?.As for stimuli response,pressure-induced phase transformation and thermally-induced phase transformation existed in the NBT polymorphs.Green crystal phase could be changed suddenly to yellow one under small pressure?0.22 GPa?,while yellow crystal phase could be ground into green powder with a spoon.The blue crystal phase could be turned into green molten phase when heated up to the melting point?210°C?,and the green molten phase can be converted into blue crystal when slowly freezing.Also,we found chiral signals in all the four crystals that partly dissolved in good solvents because of the inadequate racemization in atropisomers.The chiral signal of dNBT was stronger than the ones of NBT polymorphs.Then we discussed why there were no signals in completely dissolved solutions or other aggregations in details.These applications have developed a lot in atropisomers.In conclusion,it could be an effective way to obtain the materials with high solid-state luminescence efficiency via synthesizing the molecules with the aggregation-induced emission properties,introducing highly efficient acceptors or changing molecular arrangements.And through the detailed analysis in the varied materials,we have got a further understanding on the structure-property relationship of the sulfur-containing D-A type organic molecules.Those materials were eventually fabricated into the corresponding organic electroluminescent devices and achieved pretty good results.And the pure crystals of atropisomers showed great potentials in the fields of information storage and temperature control sensing at the same time.