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, DFT）计算，对以双咔唑端基和间三联苯（meta-terphenyl, mTP）类桥连结构为骨架的代表性蓝色电致发光双极性主体材料的电子结构展开了系统研究。所用桥连分子片段包括间三联苯、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势分别主要由其空穴传输亚单元和电子传输亚单元决定，但两个亚单元在结合后会以诱导效应为主要方式，相互影响对方的电子性质。值得注意的是，由于间三联苯类桥连亚单元具备最低的单体三重态能量，因此该类主体材料的最低三重激发态被确定为局限于该桥连结构中。研究发现，通过间位连接拓展基于苯基的π共轭体系，可在维持较高三重态能量的同时，有效调控电子亲和势的数值。