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.

本研究通过密度泛函理论（Density functional theory）计算，对以双咔唑（carbazole）端基与间三联苯（meta-terphenyl, mTP）类桥连单元为构建骨架的代表性蓝色电致发光双极性主体材料的电子结构进行了系统探究。该类桥连分子片段包含间三联苯、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与电子亲和能分别主要由其空穴传输亚基和电子传输亚基决定，但两个亚基结合后，主要通过诱导效应相互影响对方的电子性质。尤为关键的是，主体材料的最低三重态被限定在间三联苯类桥连单元中，因为这类亚基自身的三重态能量最低。研究发现，通过间位连接拓展苯基类π共轭体系，可在维持较高三重态能量的同时，有效调控电子亲和能的数值。