Conformation Control of Iminodibenzyl-Based Thermally Activated Delayed Fluorescence Material by Tilted Face-to-Face Alignment With Optimal Distance (tFFO) Design.

Yu Kusakabe,Hiromichi Nakagawa,Hironori Kaji,Yoshimasa Wada,Katsuyuki Shizu

doi:10.3389/fchem.2020.00530

Yu Kusakabe, Hiromichi Nakagawa + Show 3 more

Open Access

https://doi.org/10.3389/fchem.2020.00530

Copy DOI

Journal: Frontiers in Chemistry	Publication Date: Aug 14, 2020
Citations: 8	License type: CC BY 4.0

Affiliation: Kyoto University

Abstract

In organic light-emitting diodes (OLEDs), all triplet excitons can be harvested as light via reverse intersystem crossing (RISC) based on thermally activated delayed fluorescence (TADF) emitters. To realize efficient TADF, RISC should be fast. Thus, to accomplish rapid RISC, in the present study, a novel TADF emitter, namely, TpIBT-tFFO, was reported. TpIBT-tFFO was compared with IB-TRZ, which contains the same electron donor and acceptor segments, specifically iminodibenzyl and triazine moieties. TpIBT-tFFO is based on a recently proposed molecular design strategy called tilted face-to-face alignment with optimal distance (tFFO), whereas IB-TRZ is a conventional through-bond type molecule. According to quantum chemical calculations, a very large RISC rate constant, kRISC, was expected for TpIBT-tFFO because not only the lowest triplet state but also the second lowest triplet state were close to the lowest excited singlet state, as designed in the tFFO strategy. IB-TRZ has two different conformers, leading to dual emission. Conversely, owing to excellent packing, the conformation was fixed to one in the tFFO system, resulting in single-peaked emission for TpIBT-tFFO. TpIBT-tFFO displayed TADF type behavior and afforded higher photoluminescence quantum yield (PLQY) compared to IB-TRZ. The kRISC of TpIBT-tFFO was determined at 6.9 × 106 s−1, which is one of the highest values among molecules composed of only H, C, and N atoms. The external quantum efficiency of the TpIBT-tFFO-based OLED was much higher than that of the IB-TRZ-based one. The present study confirms the effectiveness of the tFFO design to realize rapid RISC. The tFFO-based emitters were found to exhibit an additional feature, enabling the control of the molecular conformations of the donor and/or acceptor segments.

Highlights

Emitting materials for organic light-emitting diodes (OLEDs) have, in recent years, been actively investigated to improve the utilization of triplet excitons
The details concerning the synthesis and characterization of both TpIBT-tFFO and IB-TRZ are provided in the Supplementary Material
The donor of IB-TRZ-A, IB, exhibits a butterfly shape bended in the middle (Figure 2), while TpIBT-tFFO has only one geometry, corresponding to an equatorial conformation

Summary

INTRODUCTION

Emitting materials for organic light-emitting diodes (OLEDs) have, in recent years, been actively investigated to improve the utilization of triplet excitons. Effective RISC is expected by minimizing the energy difference between S1 and T1, namely, EST This can be realized by splitting the distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), in most cases resulting in charge transfer (CT) type S1 and T1 (1CT and 3CT, respectively; Endo et al, 2011). Many of the reported TADF materials are composed of donor and acceptor moieties, i.e., D-A type molecules (Yang et al, 2017). In the tFFO molecular design, the degeneration of the three energy levels, i.e., 1CT, 3CT, and 3LE, can be realized with a significant spin–orbit coupling matrix element value (SOCMEV) between the singlet and triplet states, resulting in efficient RISC. A large kRISC of 6.9 × 106 s−1 was obtained for TpIBT-tFFO, which is one of the highest kRISC values for molecules composed of only H, C, and N atoms (The highest reported value is for a tFFO-based molecule; Wada et al, 2020)

RESULTS AND DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT