Electronic Structure of Carbazole-Based Phosphine Oxides as Ambipolar Host Materials for Deep Blue Electrophosphorescence: A Density Functional Theory Study

Abstract

We report the results of Density Functional Theory calculations on a series of carbazole-based phosphine oxides that experimental data have shown to be promising ambipolar host molecules for deep blue electrophosphorescence. The hosts under investigation contain either 1, 2, or 3 carbazole subunits attached to the phenyl rings of a triphenylphosphoryl group, with the carbazoles acting as hole transporters/acceptors and the triphenylphosphoryl groups as electron transporters/acceptors. The results underline that, in addition to the strong inductive effect of the phosphoryl groups, the LUMO of these hosts is further stabilized by the molecular orbital interactions among the phenyl rings of the triphenylphosphoryl group, which is modulated by the electron-withdrawing inductive effects of the carbazole subunits. The lowest triplet state of the hosts correspond to localized transitions within the carbazole units, which leads to a high triplet energy on the order of 3 eV. We describe the important buffer role of the phenyl rings in preventing the phosphoryl moiety from negatively affecting the hole-accepting characteristics and high triplet energies of the carbazole units.

Review of organic light-emitting diodes with thermally activated delayed fluorescence emitters for energy-efficient sustainable light sources and displays

“…In a way, they are similar to so-called ambipolar host molecules and often even contain similar building blocks, which are either arranged in a different way or used in different combinations in order to realize the large bandgap, which is needed to accommodate dopant molecules. [50][51][52][53][54][55] In both hosts and TADF emitters, donor moieties are electron-rich functional units that have a rather deep HOMO, which allow them to be oxidized easily and partake in the transport of holes. Contrarily, acceptor moieties are electron-deficient functional units with a deep LUMO.…”

The DFT study on non‐conjugated polymer host materials based on styrene derivatives for phosphorescent polymer light‐emitting diodes

“…Research shows that substituent group could effectively influence the electronic structure and E T values of the related molecules. [17,49] The values of lowest E T of the related molecules are listed in Table 1. 3 and Ir(piq) 3 , the higher values of E T of all host molecules are achieved.…”

“…[16] As we all know, a large conjugation length of host material has a small band gap, which normally means small triplet energy. [17,18] For example, polyfluorene and derivatives, it is frequently used as host material, and the E T is 2.15-2.3 eV, [19,20] which is lower than guest materials for green and blue emitters. So designing the non-conjugated polymer would be a good idea to reduce the triplet exciton delocalization and improve triplet energy.…”

Identifying efficient blue-phosphorescent polymer light-emitting diode host materials based on carbazole derivatives with C/Si-centered substituents using density functional theory

“…These can be qualitatively analyzed by investigating the ionization potentials (IPs), electron affinities (EAs), and reorganization energies (λ) of the relevant molecules [11], and the relevant parameters are collected in Table 5.…”

“…In general, a host material should fulfill the following requirements: (1) its lowest triplet energy (E T ) should be higher than that of the phosphorescent guest molecule to prevent reverse energy transfer from the guest to the host [9,10]; (2) there should be facile and balanced charge injection from the hole transport layer (HTL) and the electron transport layer (ETL) to the emitter; (3) the charge-and exciton-transfer rates should be rapid in order to reduce the device's driving voltage. Compared with the host materials of red and green PLEDs, the development of high-performance blue-phosphorescent host materials is still a challenge [11,12]. The host material should have a higher E T than Iridium (III) bis[[4-6-difluorophenyl]-pyridinato-N,C 2′ ] picolinate (FIrpic), which is commonly used as the guest Electronic supplementary material The online version of this article (doi:10.1007/s00894-015-2725-y) contains supplementary material, which is available to authorized users.…”