Electronic structure and molecular orbital study of hole-transport material triphenylamine derivatives

https://doi.org/10.1016/j.jlumin.2005.11.016Get rights and content

Abstract

Recently, triphenylamine (TPA), 4,4′-bis(phenyl-m-tolylamino)biphenyl (TPD), 4,4′-bis(1-naphthylphenylamino)biphenyl (NPB) and their derivatives are widely used in the organic light-emitting diode (OLED) devices as a hole-transporting material (HTM) layer. We have optimized twenty different structures of HTM materials by using density functional theory (DFT), B3LYP/6-31G method. All these different structures contain mono-amine and diamine TPA derivatives. The energies of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) along with molecular orbitals for these HTMs are also determined. We have found that the central amine nitrogen atom and the phenyl ring, which is next to the central amine nitrogen atom, show significant contribution to the HOMO and LUMO, respectively. The sum of the calculated bond angles (α+β+γ) of the central amine nitrogen atom has been applied to describe the bonding and the energy difference for HOMO and LUMO in these TPA derivatives. Electronic structure calculations have been performed for these TPA derivatives. Again, the LCAO-MO patterns of HOMO and LUMO levels of these derivatives are used to investigate their electron density. A series of electron-transporting steps are predicted for these compounds employing these calculated results.

Introduction

During the past decade, the development of organic light-emitting devices (OLED) has become one of the foremost topics in chemistry and applied physics [1], [2], [3], [4], [5], [6], [7]. Since a large number of organic materials exhibit high-fluorescence quantum efficiencies in the visible region, the OLED should be used for the next generation of flat-panel display systems [8], [9], [10]. Unfortunately, these organic materials have low stability, insufficient luminescence efficiency and high driving voltage—the drawbacks that still remain to be overcome.

In order to improve the durability of the electroluminescence (EL) devices, the multilayered EL devices have been proposed, which contain hole-transporting materials (HTMs), electron-transporting materials (ETMs) and the emitting layer [1]. Almost twenty years ago, Kodak group proposed HTM1 and HTM2 (Fig. 1) for the HTM materials and investigated the hole-transporting process by means of the potential discharge techniques [7]. Then, chemist also developed a series of prototypical HTM materials, which include 4,4′-bis(phenyl-m-tolylamino)biphenyl (TPD) and 4,4′-bis(1-naphthylphenylamino)biphenyl (NPB) (Fig. 2) [8], [9], [10], [11], [12], [13], [14]. All of these HTMs contain the triphenylamine (TPA) moiety, which has a central amine nitrogen atom connected to three phenyl rings (Fig. 3). Several studies have been carried out to investigate the TPA compound theoretically [15], [16], [17]. Pacansky et al. [15] used ab initio Hartree–Fock (HF) method with the 3-21G basis set to optimize geometric structures of TPA for both neutral and radical cation molecules, and then the hopping process between a neutral and an ionized TPA was predicted. They also concluded from an ab initio HF calculations that the unpaired electron is 59% localized on the central amine nitrogen atom of TPA [15]. The calculations showed that the central amine nitrogen atom significantly contributes to the highest occupied molecular orbital (HOMO). Then, Sakanoue et al. [18] reported a molecular orbital (MO) study of TPD, which is a diamine compound. They concluded that the amine nitrogen atom and the phenyl ring of TPD are the main contributors to the HOMO and the lowest unoccupied molecular orbital (LUMO), respectively [18]. According to their theoretical calculation, the hopping process between a neutral and an ionized TPD has also been predicted [18]. NPB is another diamine HTM that has been applied in HTM widely; the electronic structure of NPB with various extra charges has been studied by means of the semiempirical PM3 and ab initio MO theories, as well as by the DFT and TDOS methods by Zhang et al. [19] In particular, they have used the projection TDOS to investigate the electronic structures of individual atoms in NPB. The calculation results reveal that the central amine nitrogen atom in NPB has a significant contribution to HOMO. Such calculation is also found to be the same as that for the TPA compound [19]. Very recently, Lin et al. [16] generated the ionization potential, electron affinity and reorganization energies for TPA, NPB and TPD by using DFT B3LYP/6-31G method.

In general, most of HTM materials applied in electronic devices contain the TPA moiety. In this study, we have considered twenty TPA derivatives of the TPA moiety, which are related to the HTM materials. First, we have generated the optimized geometric structure and the electronic structure of TPA by means of density functional theory (DFT). Then, the DFT method was applied to investigate the optimized structures and electronic structures for TPA derivatives (for mono-amine and diamine derivatives) systematically. The energies of MO levels for HOMO and LUMO are obtained by means of the DFT B3LYP/6-31G calculation; these MOs are used to describe the electron density and to predict the possible charge-hopping process between the neutral and the radical cations of TPA compounds. Relations between the energy level of HOMO and LUMO and the optimized structures are also determined in these HTMs. For the HTM, it has been observed that electron-accepting molecules with strong affinity serve as the hopping sites in electron conductors, while the donor molecules with low ionization potential serve as the hopping sites in hole conductors. Thus, the molecular design for HTMs may consider the ionization potential, which is related to the energy level of HOMO and the optimized structure for this compound. In this study, we would like to provide more information concerning the energies and the optimized structures for TPA derivatives, which can be useful to improve and design new HTM materials.

Section snippets

Computations

The DFT B3LYP method with the 6-31G* basis set has been used to generate the optimized structure for the ground-state TPA and its derivatives. The calculated MOs, HOMO and LUMO, were used to determine the electron density in these derivatives. The electron density could be employed to investigate the charge-flowing process in these compounds. In the present study, twenty different compounds have been studied which contain mono-amine and diamine TPA derivatives. All the calculations have been

Geometrical and electronic structures of TPA

The TPA molecule is composed of three phenyl rings with a central amine nitrogen atom; it could be visualized as a bladed propeller with C3 symmetry. Fig. 3 shows the side and the top views of the optimized structure of TPA. The optimized geometric parameters of TPA are presented in Table 1. In particular, the central NC3 fragment is an equivalent triangle and the C3 axis goes through the central amine nitrogen atom perpendicular to the NC3 plane. The three dihedral angles between the NC3 plane

Conclusion

According to the results described above, the central amine nitrogen atom and the neighboring phenyl ring in the TPA derivatives exhibit significant contributions to HOMO and LUMO, respectively. Our calculated results also indicated that the HOMO energy for the TPA derivatives is related to the sum of the calculated bond angles (α+β+γ) and the non-bonding electron of the central amine nitrogen atom. For the diamine HTMs, we conclude that the HOMO energy decreases with increase in the number of

Acknowledgment

We thank Prof. Alexander Mebel for reading the manuscript and the National Science Council of Taiwan, China, for supporting this research.

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