Design of Efficient Ambipolar Host Materials for Organic Blue Electrophosphorescence: Theoretical Characterization of Hosts Based on Carbazole Derivatives

Density functional theory calculations were carried out to investigate the electronic structures of representative ambipolar hosts for blue electroluminescence, based on two carbazole end groups and meta-terphenyl (mTP)-like bridges. The bridge molecular segments include mTP, 2,6-bisphenylpyridine, 3,5-bisphenylpyridine, and 2,6-bisphenylpyrimidine. While the ionization potentials and electron affinities of these molecules are mainly determined by their hole- and electron-transport subunits, respectively, each subunit impacts the electronic properties of the other upon their binding, mainly in an inductive way. Importantly, the lowest triplet state of the hosts is determined to be confined into the mTP-like bridges since these are the subunits with lowest individual triplet energy. Extension of the phenyl-based π-conjugated system via meta linkages is found to be effective in modulating the electron affinity value while maintaining a high triplet energy.

本研究采用密度泛函理论（Density functional theory）计算，对以双咔唑（carbazole）端基与间三联苯（meta-terphenyl, mTP）类桥联结构为骨架的典型蓝色电致发光双极性主体材料的电子结构展开探究。该类桥联分子片段涵盖mTP、2,6-二苯基吡啶、3,5-二苯基吡啶以及2,6-二苯基嘧啶。尽管此类分子的电离势（ionization potential）与电子亲和势（electron affinity）分别主要由其空穴传输亚基与电子传输亚基决定，但两个亚基在结合后会主要通过诱导效应对彼此的电子性质产生影响。值得注意的是，该类主体材料的最低三重态（triplet state）被限定于mTP类桥联结构内，原因在于此类亚基本身具备最低的三重态能量。通过间位连接拓展苯基类π共轭体系（π-conjugated system），可在维持较高三重态能量的同时，有效调控电子亲和势数值。