Theoretical Investigations On Electronic Structures And Photophysical Properties Of The Branch-Shaped Organic Light-Emitting Materials

Organic light-emitting diodes (OLEDs) materials have attracted much attentions in the past several years due to their outstanding advantages, such as simple structure, high yield, low cost, a self-luminous, fast response and high-resolution, etc. Recently, the job to find multifunctional, efficient and high performance OLEDs materials is still continuing.In this paper, from the point of view of the novel branch-shaped OLED materials design, we systematically investigated two types of branch-shaped OLED materials with DFT, TDDFT and CIS quantum chemistry methods, including that two series of bridged triphenylamine derivatives and six series of triazatruxene/heterofluorene branching co-polymers. Their ground and the lowest excited state conformations, frontier molecular orbital, ionization potential (IP), electronic affinities (EA), reorganization energies (λ), absorption and emission spectra were investigated systematically. The theoretical studies show that it can help to understand the microcosmic electroluminescent mechanism and contribute to orientate the synthesis and design of the novel OLED materials by exploring the structure-property relations. The following is the main relations:1. Two series of bridged triphenylamine derivatives with different substituent groups are investigated. Calculated results show that the electronic structures and properties of the bridged TPA derivatives can be tuned by different substituent groups. The compounds with electron-donating groups have lower–EHOMO and IP values than the compounds with electron-withdrawing groups. The compounds with electron-donating, electron-withdrawing and bulky groups all cause red shift of the maximum absorption peaks. Compounds A₂, A₃, A₆, A₇, B₂, B₃, B₆ and B₇ can be used as hole-injection/transporting materials, while A1, A3, B1 and B3 have a good hole and electron transporting balance, implying they can be used as nice ambipolar materials. Compounds A1-A4, A6-A8, B1, B4 and B6-B8 have larger energy gaps and triplet energies than Ir(ppy)₃, indicating that they can be used as host in the device of ITO/MoO₃/NPB/mCP/host:Ir(ppy) ₃/TAZ/LiF/Al. Considering B8 is used as host, the efficiency of energy transfer is higher, hole and electron can be trapped on the Ir(ppy)3 guest directly, and exciton formation is more efficient. Therefore compound B8 will be a promising host material. These studied results indicate that the bridged TPA derivatives are good candidates for optoelectronic application.2. The electronic structures and optical properties of six series of triazatruxene/heterofluorene branching co-polymers are investigated. It is found that different hetero atoms can tune various properties of co-polymers and improve performance. Our calculated results show that the introduction of different bridge atoms in branching fluorenes almost has no effect on planar structure of the central TAT. The chain conjugation of each branch of the studied co-oligomers is enhanced with the increased unit number. TNF has a better hole injection/transport ability than other series. On the other hand, TBF has has a better electron injection/transport ability than other series. Noteworthily, the differences betweenλhole andλelectron of TCF, TSiF, TNF and TBF are very small, indicating that they have a good balanced charge-transport rate, and can be used as nice ambipolar OLED emitting materials. With the increased units, the conjugation in the backbone enhances, resulting in the narrowed energy gaps and gradually red-shifted absorption and emission spectra. TBF has red light-emitting feature and can be used as red light-emitting materials, other five series of co-oligomers have deep blue light-emitting features and can be used as blue light-emitting materials. These studied results indicate that the branch-shaped triazatruxene/heterofluorene co-polymers have many interesting properties and are good candidates for optoelectronic application.