Theoretical Studies On The Intramolecular Charge-transfer Luminescent Materials And Related Molecular Design

The excited state is defined as an electron state, in which the electrons are excitedto a higher level but not be ionized. According to quantum mechanics, the excitedstate of a system is the quantum state, which shows the higher energy than that of anyground states. Generally speaking, the molecules in the excited state are unstable,which just can maintain a short period of time. So the excited states ususlly attenuateenergies through a variety of ways. Charger transfer state (CT) is a special form of theexcited states, in which the hole and electron localized at the different molecules orthe different moieties of the molecule, which correspond to the intermolecular chargertransfer state and the intramolecular charger transfer state (ICT). The intermolecularcharger transfer state has been widely used in organic solar cell, and the ICT state hasbeen extensively applied in OLEDs. However, the fluorescent efficiency of ICT stateis usually very low due to the small transition dipole moment. This can be ascribed tothe small mesomeric interaction and the weak orbital overlap between donor and acceptormoieties. In recent years, a series of ICT compounds with intramolecularDonor-Acceptor structure have been used in organic electroluminescent field with highlyefficient luminescence, such as the triphenylamine substituted polycyclic aromatichydrocarbons system. However, for such molecular system, the formation process ofthe ICT state is still unclear, as well as the essencial mechanism responsible for thehighly efficient fluorescence. which has seriously hampered the improvement of suchmaterials. Therefore, the relevant investigation on the light-emitting mechanism of ICT state and the development of ICT state light-emitting materials is particularlyimportant.According to the updating of the computer hardware and software, thecomputational chemistry has also been developed. The speed, accuracy and precisionof the theoretical calculation are constantly improved. At present, quantum chemistryhas become an important means to study material structure and performance, whichcan offer the microscopic property that can not be observed in the experiments. In thisthesis, we adopt the quantum chemical calculation to describe the properties of theground state and the excited state in terms of microscopic molecular geometricstructure and electronic structure. Combining with the experimental phenomenon, wedescribed the high efficient light-emitting principle of ICT state, the influence of thedonor and acceptor intensity on the properties of ICT state and the molecular designof the high-performance ICT state electroluminescent materials.For triphenylamine substituted anthracene (TPA-AN), the experimental dataindicate that TPA-AN shows the typical ICT state properties, and emission spectrashows markedly red shifts and width with growing solvent polarity. And theexperimental results show that TPA-AN shows the high solution luminous efficiency(62%). We investigated the molecular structure and electronic structure from theperspective of theoretical calculations in order to discuss the principle of the ICT stateachieving high luminous efficiency. The optimized structure of ground state showsthat there is a larger twist angle between the TPA and AN, and the electronic transitionof the ICT is forbidden due to the weak molecular orbital overlap. Indeed, theabsorption indicates the locally-excited character with the transition of AN→ANexperimentally and theoretically. The potential energy surface of TPA-AN is very flat,different geometric conformation in the range of60-120°is easy to arrived as a resultof the small twist potential. The localized excited state LE and intramolecular chargetransfer ICT state mixed with each other with the relaxation of the geometry,TPA-AN exhibites the mixed excited state properties. The ICT state energy isgradually reduced under the conditions of the polar solvent, the geometry of excitedstate is toward complanation. In this situation, the electronic transition of ICT state is changed from forbidden to be allowed. The allowed transition of ICT state as aneffective complement of local excited state is the essential mechanism for the highefficiency of TPA-AN.On the basis of high efficient principle of ICT state, we discussed the influence ofthe donor and acceptor intensity on the ICT state properties and the radiationefficiency. We design three model compound (TPA-AN, TPA-AC and TPA-PN) withdifferent acceptor according to introduce the N to AN moiety. We studiedsystematically the geometry of the ground state and excited state, the electronicstructure, the absorption and emission character and the solvent effect based on suchthree model compounds adopting the density functional theory (DFT) andtime-dependent density functional theory (TDDFT). The calculation results indicatethat the geometry structure of such three compounds in ground state shows the largertwist angle and mixed excited state properties. Which ascribe to the flat potentialenergy surface of the three compounds and the small twist potential in differentgeometry conformation. Thus, mixed excited state properties of compounds don't relyon the donor and acceptor intensity, but on the twist structure of the compound. Andthe component of ICT state in the mixed state increased along with the increasedstrength of the acceptor. And at the same time, the dipole moment of the excited statealso increased, the absorption and emission spectra show markedly red shiftscharacter. On the other hand, the calculated electronic transition moment increasedalong with the increased intensity of the acceptor, which shows the increasedluminescent efficiency.With the shortage of ICT state luminescent materials, we construct a series of ICTstate luminescent materials with the HOMO-LUMO spatial separating character,which can achieve the high-performance light-emitting in the field of organicelectroluminescence. In this part, we choose the triphenylamine as the donor, thedifferent aromatic groups as acceptor, which is connected with a single bond. As aresult of the larger twist angle between the donor and acceptor, the conjugationbetween such two parts is interrupted. And the HOMO and LUMO are localized at thedonor and acceptor moiety. So it is easy to build the particular molecular structure with the separated injection of the charge, the separated transport moiety and theseparated oxidation and reduction sites. Specifically, we design the red, green andblue light-emitting materials by selecting the different acceptor. Some of materials hasbeen validated and published in experiment, which both demonstrate ICT stateproperties, high luminous efficiency and high stability. Furthermore, the blue materialTPA-AN was choosen as the model compound, we changed the compositional ratio ofthe hole transport moiety (TPA) and the electron transport moiety (AN) due to thespecial structure of TPA to achieve the adjustment of the charge transport property.Under the premise of high hole transport property, we achieved the improvement ofthe electron mobility. And based on this strategy, we design the n-type and theambipolar transport materials. As a result of the separated oxidation and reductionsites, the energy is decreased in the process of gains and losses of electrons toimprove the stability of electronic devices. The experimental result indicated that suchmaterials exhibited the longer device lifetime and high luminous efficiency. Thismaterials have broad application prospects in electroluminescent, and they show greatbreakthrough in high stability and high efficiency.