Design, Synthesis And Excited State Property Of Twisting Donor-Acceptor Molecules

The organic light-emitting diodes （OLEDs） as one of the most importantapplications of organic luminescent molecules have been widely studied. In the pastdecades, the rapid development of OLEDs benefited mainly from the emergency oflayered nanoscale-thickness-film device structure and the application of highlyefficient phosphorescent materials. Nowadays, OLEDs have come to the stage ofapplication in industry production, but the cost of rare and expensive heavy metal inphosphorescent materials prevents it for further development in the future.The performance of OLEDs is ultimately decided by the theoretically attainableradiative excitons ratio for electroluminescence. In phosphorescent device the75%triplet excitions formed in electric condition by spin coupling statistics could beeffectively employed, in many cases, the proportion was nearly100%. If a pureorganic material can fully employ100%excitons like a metal-based phosphorescentmaterial, it is very significant for the development of OLEDs due to their advantage incost and resource. In current stage, some investigations based on delayed fluorescenceby transferring triplet excitons to singlet for employing more than25%excitonsthrough TTA or TADF methods have been reported. In extended pi-conjugatedmolecule or polymer with delocalized electronic state, a breakthrough in the singletexciton ratio was also observed, which is explained by larger singlet formingprobability in weakly binding state or triplet transferring to singlet in the high energyinter-chain CT state. This directs a concept for OLED material design to breakthrough the spin coupling statistics （singlet/triplet=1/3） in strongly bound excited stateby employing molecular exciton state with weak binding energy. Thus to findfluorescent molecules with properties of weakly bound exciton in the lowest excitedstate as well as high fluorescence at the same time would be a route towards thenext-generation highly efficient OLED material design. Charge-transfer （CT） exciton is a kind of weakly coupling interaction, in whichelectron and hole location sites are separated from and of a certain distance to eachother. The binding energy of CT exciton is supposed be a weak one in betweenWannier exciton （radius of¹00, or called free electron and hole） with bindingenergy of¹0meV and Frenkel exciton （radius of¹0, or call local excited exciton）with binding energy of1eV. But an unavoidable problem for the CT-statephotoluminescence is just its very low quantum efficiency, arising from the forbiddenelectron transition due to the little overlap and poor symmetry of their molecularorbital. Thus to find CT state molecules with high-efficiency fluorescence is of greatimportance currently for improving the highly efficient OLED.Twisting donor-acceptor molecule is an effective way to obtain the intramolecularCT state by breaking the conjugation between donor and acceptor to induce a CTtransition in between them. Through fixing the donor of triphenylamine andmodifying the acceptor of fluorescent molecular groups, a series of twistingdonor-acceptor molecules with high efficiency and whole-color-range emission canbe obtained. Frontier orbital and electrochemistry results show that, in the twistingdonor-acceptor molecules with twist angles of30-70degree between donor andacceptor, HOMO was localized mainly on triphenylamine and LUMO was localizedon fluorescent molecular groups, with some overlap between them, which isresponsible for the high efficiency of twisting donor-acceptor molecule. The twistconfiguration induces the hybrid of local excited state （LE） and CT state to form anew hybridized local and charge transfer （HLCT） state in the twisting donor-acceptormolecules. The bigger the twist angle is, the more the proportions of CT transitions inHLCT state are. When twist angle reach90degree, the HLCT become a pure CT statewith no light emission, that is, the hybrid of high-efficiency LE state into HLCT statemake it become a highly efficient CT state. HLCT acts as a LE state in the low polarsolvents, and a CT state in the high polar solvents. The single exponential lifetime ofHLCT state in different polar solvents confirm that it is a hybridized state, not amixture of LE and CT states. Theory calculation on the energy level of HLCT state indifferent polar solvents shows that HLCT displays a slow decrease in the low polarsolvents and a large one in the high polar solvents, which is consistent to that of solvatochromic effect of molecular spectra in the experiments. The nonlinear changeof HLCT state dipole is thought to be the root of the special solvatochromic effect oftwisting donor-acceptor molecules in different polar solvents.OLEDs based on twisting donor-acceptor molecules with HLCT state as emitterexhibit excellent properties, especially in deep-blue, deep-red and orange colordevices, among the best reported results with similar fluorescent color. Almost all thedevices show a breakthrough in the singlet exciton ratio of more than25%（90%inthe TPA-NZP case）. The decay of device emission shows nearly none delayed lightwith long lifetime. The breakthrough in the single exciton ratio in twistingdonor-acceptor molecular device is ascribe to the large singlet forming probability ofHLCT state with weak bonding energy, or the triplet transferring to singlet in thepossible inter-or intra-molecular CT state at the high energy level. Moreover, thesimultaneously stable oxidation and reduction in the twisting donor-acceptormolecules contribute much to the lifetime or stability enhancement of OLED device.Through the solution or PVT （physical vapor transport） methods, various singlecrystal structure and molecular stack mode are discovered for twisting donor-acceptormolecules. In the case of TPA-PY, the self-assembling separately of donor andacceptor is discovered in O crystal. Field effect transistors （FETs） based on slicecrystals of TPA-PE and TPA-PY （M crystal） by solution methods show large holetransporting mobilities. FETs based on TPA-PE crystal by PVT method exhibit higherhole transporting mobilities of0.02-0.1cm²V^-1s^-1and great device stability. Atomicforce microscopy measurements show that TPA-PE crystals display poor surfaces,indicating of possible higher mobility in TPA-PE crystal as well as the great potentialof twisting donor-acceptor molecular crystal in the FET area.