Study Of Preparation,structure And Properties Of Pyrene Type Optoelectronic Materials

As one large polycyclic aromatic hydrocarbon,pyrene shows the deep-blue light,high photoluminescent(PL)efficiency and carrier mobility,and is easy to be chemically modified.Our research group has carried out a lot of work on the design and preparation of pyrene derivatives.However,the further systematic research on the relationship between the structure and properties still shows the necessity.Because the main properties of the pyrene derivatives are their outstanding optoelectronic characteristics,it should be necessary to use the optical and electric spectroscopy to study them.Nowadays,although the optical spectroscopy methods such as absorption and photoluminescent spectroscopy studies have became the general and necessary methods,only the cyclic voltammetry is used for the primary study.However,although the electric spectroscopy – admittance spectroscopy method possessed the matured hardware basis,high sensitivity and strong power,it's application to the organic electronics still maintains the beginning step,and the systematical research should be needed.So this dissertation carried out research of preparation,structure and properties study on the photoelectronic pyrene derivatives as the followings.1.In order to give an insight of the special spectra of isolated ? systems,a series of pyrene-fluorene derivatives,with conjugated or non-conjugated pyrene groups,were systematically studied.Besides absorption of non-conjugated pyrene itself and main conjugation chain,all the pyrene-non-conjugated large conjugated materials show a characteristic sharp absorption peak at ~352 nm.Through systematical investigation,this special absorption peak was attributed to the intramolecular through-space interaction between the two isolated large ? systems,which was then defined to be ?-? hyperconjugation.And from quantum calculation,this hyperconjugation does shorten the length of single bond.Through systematical research,the condition of this ?-? hyperconjugation is proved to be that the non-conjugated pyrene must have projection on fluorene plane.And then this effect is applied to explain the improved injection ability of the materials by bringing out one simple hopping model.Because of the ?-? hyperconjugation,there are three tunneling hopping ways in DP1,and there is only one tunneling hopping way in DP2,which is the reason why DP1 shows higher carrier injection ability than that of DP2.2.Four starburst molecules based on pyrene with truxene cores,Py T,Py ET,PFT and PFET,have been designed and synthesized via Pd(0)-catalyzed Suzuki coupling and Sonogashira coupling reaction,respectively.And their optoelectronic properties were investigated.Because of the introduction of non-conjugated pyrene group,the ability of hole injection and transporting were increased.The investigation also proved that when the 9-pyrenyl-9-phenyl fluorene group and the corresponding main conjugated structure existed,the ?-? hyperconjugation effect originated from the non-conjugated pyrene still existed in the truxene systems,and leads to the optoelectronic difference between the four compounds.3.In order to investigate the effect of free radical,the dimethyl substituted pyrene-fluorene derivative DMPPF was designed and synthesized.This compound could be pyrolyzed under 200 ?.During the OLED device fabrication,DMPPF pyrolyzed and led to free radical.Under the catalysis of In atom from ITO electrode,the free radical further led to serious generation of molecule aggregation or fluorenone,which should be the reason why the OLED showed the red-shift and unexpected EL spectra.And the possible mechanism for transfer of free radical and molecule aggregation had also been suggested.Through the careful comparative research,this report showed that small amount of free radicals still existed in the emitting device,which could be stabilized by the conjugation of intramolecular aromatic rings and the weak coordination of the electron-rich large conjugated molecules.This could explain the low performance of the device.4.Analysis of thermal molecule motion of organic semiconducting pyrene-fluorene derivatives and its potential influence on the OLED has been conducted.The first-order phase transition of MP2 and MP3 has been attributed to the activation energy of rotation and translation.In EHOP2 and EHOP3,the effect of the long-chain alkyloxy group on the molecule motion has also been researched to understand the in-depth reason why the polycrystal MP2 and MP3 will turn into the amorphous EHOP2 and EHOP3.The mechanism of glass transition of EHOP2 and EHOP3 are also carefully studied.And the results showed that there are great differences for glass transition between small molecule and polymer semiconductors.After subsititution of non-conjugated pyrene,the glass transition temperature of EHOP2 and EHOP3 was lower than that of the materials with tolyl substitution.It is also related with the ?-? hyperconjugation effect.When the second-order phase transition is studied firstly in organic semiconductors,it is thought to be also detrimental to device performance.And in order to avoid the second-order phase transition in the OLED,the suitable materials ought to have Tg of over 200?.This result means that new material systems should be re-designed and re-established to take full attention of second-order phase transition.5.In order to study the influence of evolution of molecular structure and the corresponding aggregation structure on its optoelectronic properties,we use the Suzuki coupling reaction to prepare two pyrene derivative MP3 and MP4(the only structrual difference is that there is one subistituent difference on the C9 of the fluorene moiety).Then the organic nano-structures were prepared through reprecipitation of MP3 and MP4 from the solvent.By studying the morphology of the two compounds,we found that the structure of symmetrical molecules is more prone to aggregation.And because of asymmetrical substitutions of pyrenyl and phenyl groups in MP3,the device of MP3 is better than that of MP4.6.Electric spectroscopy is important research method to the basic parameter of organic semiconductors such as carrier dynamics,which is the base of organic electronics.The hole-only device(ITO / NPB / Ag)was prepared to test the admittance spectrum.Then the admittance model and particle swarm algorithm(PSO)were used to fitting the experimental data to achieve the carrier mobility and disperse parameters of NPB.The results showed that hole transporting in NPB is non-disperse transporting of space charge limited current.The mobility of NPB is also determined by negative differential susceptance,impedance imaginary part,equivalent current model-transmission line model,and the results were compared and all matched to the Poole-Frenkel model.Through the investigation on the relationship between the mobility of the material and the thickness of the film,it is found that the thickness ratio of the interface layer to the total film thickness is an important factor which affects the mobility.On this basis,the concept of the interface trap free energy is proposed.7.Up to now,the defect state in organic semiconductors has still not been thoroughly studied,especially for the synthesized materials.2,7-Dipyrenyl-9-phenyl-9-pyrenyl fluorene(MP3)was a wonderful synthesized luminescent material in organic electronics.The admittance spectroscopy method was used to study the defect state of MP3,and the Walter and Mott-Schottky models were applied to extract the defect distribution of the material.And in addition,the Gaussian model was used to describe the width distribution of defect state.The result showed that,in the MP3,the total defect density was about 6×1016 cm-3,with the distribution range of 0.32 eV-0.70 eV.