Realization of Pyreno[4,5-d]imidazole-based HLCT emissions through tuning the energy level of the charge-transfer state

Abstract

Organic light-emitting diodes (OLEDs) have lately been used in tablet displays, for instance in mobile phones and televisions, and are considered to be the next generation of displays and solid-state lighting due to their light weight, high flexibility, and energy efficiency. Hybridized local and charge-transfer (HLCT) excited state fluorophores, capable of making full use of excitons by reverse intersystem crossing from high-lying triplet states to singlet states, have drawn increasing attention in OLED applications. The HLCT state can be efficiently achieved by constructing D-π-A type molecules, whose characteristics of their excited state are widely influenced by the difference of torsion angles between donor (D) and acceptor (A). Herein, pyreno[4,5-d]imidazole and triphenylamine were selected as the electron acceptor and electron donor, respectively. Based on this molecular design, two D-π-A type molecules, pTPA-PyI and mTPA-PyI were designed and synthesized with different torsion angles between D and A. In addition, the introduction of fluorine atoms in molecules could provide more and stronger charge transfer (CT) state components into the excited states of the molecule, which could facilitate the formation of HLCT states with improving exciton utilization of the molecule. As a result, pTPA-PyI with linear molecular geometry possessed HLCT properties, which could achieve a large yield of singlet exciton formation. The non-doped pTPA-PyI-based OLED demonstrated steady blue emission with a peak at 460 nm and CIE coordinates of (0.15, 0.20). The highest current efficiency was 7.7 cd A−1, the maximum brightness was 48637 cd m−2, and the external quantum efficiency (EQE) was 6.38%. When the brightness was 1000 cd m−2, the efficiency roll-off was incredibly low as 0.6%, yet the EQE of 6.34% was still achievable. Additionally, it possesses a high exciton-utilization efficiency of up to 97%. However, the locally excited (LE) and CT states of mTPA-PyI could not sufficiently hybridize due to its larger torsion angle between D and A with the smaller dipole moment and the interrupted conjugation structure of the molecule which led to high energy level of CT state. As a result, by altering the torsion angle between D and A, it is feasible to change the constituents and levels of the LE and CT states in order to actualize the HLCT feature.