Theoretical Investigations Of Electroluminescent Property Of The Organic Star-Shaped And Macrocyclic Molecules As Light-Emitting Materials

For low drive voltage, wide viewing angle, low cost, high brightness and full-color displays, organic light-emitting technology is considered as a new flat-panel display technology. To achieve full-color display, the three primary colors, i.e., blue, green, and red light-emitting mate-rials are required. However, comparing with the green and blue light-emitting materials, the red ones with high brightness and efficiency are scarce. Moreover, the existing red light-emitting materials are mostly hole injection type materials, their electron injection/transport ability and charge transfer balance are poor. So, design and development the red light-emitting materials with good electron injection/transport ability is particular urgent.In this paper, we systematically investigated two series of novel OLED materials with DFT, TDDFT and CIS quantum chemistry methods, including a set of 2,1,3-benzothiadiazole-based star-shaped molecules and two series of macrocyclic organic molecules based on pyrene fused porphyrins and tetrathiaannulene. Their ground and the lowest excited state conformations, fron-tier molecular orbital, ionization potential (IP), electronic affinities (EA), reorganization energies (λ), absorption and emission spectra were investigated systematically. The photophysical proper-ties of the synthetic molecules, especially their electron injection and transport ability were im-proved by introducing effective functional groups. The theoretical study is intended to establish the structure-photophysical property relationship for designing efficient light-emitting materials for OLEDs. The following is the main results:1. A series of 2,1,3-benzothiadiazole-based star-shaped molecules was investigated. Calculated results show that the properties of the p-conjugated bridge and the end-group significantly affect the carrier injection and transport characteristics of these molecules, especially for S-BTDP and S-EBTD. Among these molecules, S-BTDP exhibits better electron injection ability due to the introduction of 2-(pyridin-2-yl) pyridine as the end-group. However, S-EBTD, with ethylene as p-conjugated bridge, has excellent hole injection and carrier transport behaviors. We also calcu-lated the singlet-to-triplet exciton-formation cross-section ratio (σ_S/σ_T), the exciton formation fractions (χ_S), and the absorption and emission spectra of these molecules. We calculated thatσS/σT ranges from 1.78 to 2.76 and thatχS is ca. 0.37–0.48. These molecules have two absorption bands in the range of 340–410 nm and 500–613 nm, respectively. The calculated emission spectra range from 619 to 706 nm. It can be deduced that the studied 2,1,3-benzothiadiazole-based star-shaped molecules can serve as efficient red light-emitting electroluminescent materials.2. Two series of macrocyclic organic molecules based on pyrene fused porphyrins and tetra-thiaannulene were investigated. The calculated ionization potentials (IPs), electron affinities (EAs), and reorganization energies (λ) show that introducing pyrene fragment and electron withdrawing groups into the twoβ-position of core can effectively improve the electron injection and transport ability. In addition, the hole and the electron injection ability of the molecules in Series T are sig-nificantly better than that of Series P. In other words, the molecules in Series T show more com-petitive in the injection of carriers. Moreover, the calculated exciton binding energy (E_b) of the molecules in Series T (0.12-0.31 eV) are much smaller than that of Series P (0.27-0.58 eV), thus, we can speculate that the molecules in Series T will hold larger exciton formation rate. Their visi-ble absorption wavelengths calculated by TD-B3LYP show two relative weak Q bands absorption peaks and one intense B bands absorption peak, and the latter is in good agreement with corre-sponding experimental values. The calculated emission spectra range from 533-597nm and 618-724nm for Series P and Series T, respectively. The radiative lifetimes of these molecules are reaching tens to hundreds of nanoseconds due to the introduction of pyrene fragment with excep-tionally long fluorescence lifetime.