Organic light-emitting diode (OLED) technology has attracted extensive attention due to its increasing demand in the fields of full-color flat-panel display and lighting. In general, the primary colors, i.e., red, green, and blue, are required for full-color display or white OLED (including solid-state lighting). In particular, the material that can emit blue light plays an important role in color temperature management and the color rendering index. Therefore, many studies on new blue light materials have been conducted for higher efficiency and better performance. In recent years, a new class of imidazolyl materials, hydroxyl-substituted tetraphenylimidazole (HPI) and its analogs, have attracted more and more attention because they can be used not only as a blue light material but also as a probe to detect hydrogen sulfide, arsenite ions, and fluoride ions. In this paper, the electron donor substituents, 9,9-dimethyl-9,10-dihydroacridine (DMAC), triphenylamine (TPA), 10H-phenothiazine (PTZ), 10H-phenoxazine (PXZ), phenyl carbazole (PCz), and 5H-indole [3,2,1-de] phenazine (InPz) are used to disubstitute the 3,5 positions of N-phenyl of hydroxy-tetraphenylimidazole. As a result, six hydroxy-tetraphenylimidazole derivatives are obtained. After that, their electronic structures and photophysical properties are studied with density-functional theory (DFT) and time-dependent density-functional theory (TDDFT). In addition, as the compounds containing electron-rich groups, such as triphenylamine and carbazole, are often used as hole transport materials, we also investigate the possibility of the six derivatives as hole and electron transport materials.
The first step is to verify the accuracy of functionals. Specifically, B3LYP, PBE0, and CAM-B3LYP functionals combined with the 6-31G(d, p) basis set are used to optimize the ground-state and the first single-excited-state geometries of experimentally synthesized HPI-TPA and HPI-PCz. The absorption and emission spectra are calculated. For HPI-TPA and HPI-PCz, the calculated results obtained by B3LYP and CAM-B3LYP functionals are significantly different from the experimental values, while those obtained by PBE0 are close to the experimental values. The same trends are reflected in emission spectra. Therefore, the PBE0 functional is used in the subsequent calculations. Then, the PBE0 functional combined with the 6-31G(d, p) basis set is used to optimize the geometries of the six hydroxy-tetraphenylimidazole derivatives. Given the optimized geometry, the ionization potentials (IP), electron affinity potentials (EA), hole extraction potentials (HEP), electron extraction potentials (EEP), hole reorganization energy λhole, and electron reorganization energy λelectron can be calculated. Finally, the excited-state geometries of the six derivatives are optimized by the TD PBE0/6-31G(d, p) method, and the absorption and emission spectra of the studied molecules are calculated by the same method.
According to the Kohn-Sham frontier orbitals and the contribution of electron density for HPI-TPA, HPI-PTZ, HPI-PXZ, and HPI-InPz, the electron clouds of the highest occupied molecular orbits (HOMOs) are mainly located on the R-down group, and the HPI and R-up groups almost do not contribute to HOMOs. For HPI-PCz, the contribution of the HPI group to HOMOs is 99.3%, while the contributions of the R-up and R-down groups are almost zero. For HPI-DMAC, the contribution of the R-up group to HOMOs is 100.0%, while the contributions of the HPI and R-down groups are zero. In contrast, for HPI-TPA, HPI-PTZ, HPI-PXZ, and HPI-InPz, the contributions of the R-down group decrease from 98.8%, 100.0%, 99.9%, and 100.0% in HOMOs to 42.7%, 22.1%, 23.0%, and 23.4% in lowest unoccupied molecular orbits (LUMOs), respectively. For HPI-PCz, the contribution of HPI decreases from 99.3% in HOMO to 53.4% in LUMO. For HPI-DMAC, the contribution of the R-up group decreases from 100.0% in HOMO to 24.5% in LUMO. This shows that there is a lot of electron transfer from HOMO to LUMO. In addition, the results of HOMO energies, LUMO energies, HOMO-LUMO gaps, ionization potentials, and electron affinities demonstrate that HPI-TPA, HPI-DMAC, HPI-PTZ, HPI-PXZ, and HPI-InPz show good hole transport ability, and HPI-PTZ, HPI-PXZ, and HPI-InPz also show good electron injection ability. Moreover, it is found that the HOMO-LUMO energy gaps of the studied compounds gradually narrow along the order of TPA, PCz, DMAC, PTZ, PXZ, and InPz. This indicates that the electronic transition ability of hydroxy-tetraphenylimidazole derivatives can be regulated by substituents. The absorption and emission spectra show that for HPI-DMAC, HPI-PTZ, HPI-PXZ, and HPI-InPz, the S1→S0 electronic transition is forbidden because the oscillator strengths are almost zero. HPI-TPA and HPI-PCz can emit bright blue light.
In this paper, the photophysical properties of six disubstituted hydroxy-tetraphenylimidazole derivatives are studied by the TD PBE0/6-31G(d, p) method. The results show that there is a lot of electron transfer in all molecules during the transition from HOMO to LUMO. HPI-PTZ, HPI-PXZ, and HPI-InPZ have strong hole and electron injection abilities. The HOMO-LUMO energy gaps gradually narrow along the order of TPA, PCz, DMAC, PTZ, PXZ, and InPz. This indicates that the electronic transition capacity of hydroxy-tetraphenylimidazole derivatives can be regulated by substituents.
