No abstract

This work reports the incorporation of a triphenylamine‐based macrocyclic donor to design new donor‐π‐acceptor‐configured blue thermally activated delayed fluorescence (TADF) emitters. The X‐ray structure analyses manifest the degree of twisted conformations that can be modulated by methyl substituents of the π‐bridge and macrocyclic donor, leading to well‐separated highest occupied natural transition orbital and lowest unoccupied natural transition orbital frontier orbitals, thus sufficiently small singlet–triplet energy difference (ΔEST) for TADF. The theoretical analyses elucidate the structure–property relationship and reveal the beneficial effect of macrocyclic donor on increasing reverse intersystem crossing (RISC) process that can contribute to improved triplet‐upconversion efficiency. The blue device employing c‐NN‐TRZ as emitter gave a maximum external quantum efficiecny (EQEmax) of 26.3% as compared to that (19.1%) of the device using the model compound DPA‐MeTRZ without the macrocyclic donor, suggesting the contribution of macrocyclic donor to enhance device performance. Benefiting from the combined advantages of macrocyclic donor and methyl substituents, the device incorporating c‐NN‐MeTRZ as emitter achieves an outstanding EQEmax of 32.2%, which is attributed to the more horizontally oriented emission dipoles as well as the significantly accelerated RISC rate constant (kRISC) resulting from reduced ΔEST. This work represents a new strategy of designing twisted TADF emitter incorporating macrocyclic donor to achieve highly efficient blue device.

Designing in‐plane‐oriented light‐emitting dipoles is known as a critical method to develop high‐efficiency organic light‐emitting diodes (OLEDs) by enhancing light extraction. However, in‐plane‐oriented light‐emitting dipoles must demonstrate sufficient polarization memory extended into light emission lifetime window, generating extended anisotropy dynamics shown as the necessary condition to increase light extraction toward developing high‐efficiency OLEDs. This paper reports experimental studies on anisotropy dynamics of light‐emitting dipoles in both time and energy domains by using time‐resolved and steady‐state photoluminescence anisotropy measurements based on the in‐plane oriented exciplex‐heterostructured [BCzPh:CN‐T2T] host dispersed with phosphorescent molecules. It is found that, when host–guest Coulomb scattering is suppressed by parallel placing of the in‐plane‐configured phosphorescent Ir(ppy)2(acac) molecules into the in‐plane‐oriented exciplex‐heterostructured [BCzPh:CN‐T2T] host, the anisotropy dynamics of light‐emitting dipoles can be extended into microseconds time window comparable with its phosphorescence lifetime, satisfying the necessary condition in time domain to increase light out‐coupling efficiency toward developing high external quantum efficiencies (EQEs) in Ir(ppy)2(acac):exciplex system. More importantly, by suppressing host–guest Coulomb scattering, the high‐energy transition dipoles can still maintain extended anisotropy dynamics in the energy domain in Ir(ppy)2(acac):exciplex system while hot electrons are relaxing toward lowest unoccupied molecular orbital (LUMO). Consequently, the extended anisotropy dynamics of light‐emitting dipoles demonstrate a high EQE of 34.01% in the Ir(ppy)2(acac):exciplex OLED.

were behind the development of novel highly active research areas in organic electronics and photonics, namely: thermally activated delayed fluorescence (TADF) [1] and organic long-lived luminescence, that includes organic roomtemperature phosphorescence (RTP) [2,3] and organic long-persistent luminescence (LPL). [4] Each of these distinct luminescence phenomena originates from complex emission mechanisms enabled by the crossover between various types of excited states with different electron spin multiplicities. [5][6][7][8][9][10][11][12] Despite the manipulation of excited states energy levels is a difficult task, suitable emitting compounds have been engineered for each luminescence subtype, [13][14][15][16][17][18][19][20][21] as well as for co-existing emissions (e.g., simultaneous RTP and TADF). [22][23][24] To date, the best performance materials in each class exhibit emission lifetimes that are lower than 1 µs for TADF molecules, [25] and that can last up, after ceasing the excitation, to a few tens of seconds for RTP materials [26] and to an hour for LPL systems. [4] In terms of materials engineering, most of the suitable emitting molecules combine electron-donor (D) and Controlling and predicting the long-lived room-temperature phosphorescence (RTP) from organic materials are the next challenges to address for the realization of new efficient organic RTP systems. Here, a new approach is developed to reach these objectives by considering host-guest doped crystals, as well-suited model systems in that they allow the comprehensive understanding of synergetic structural interactions between crystalline host matrices and emitting guest molecules, one of the key parameters to understand the correlation between the solid-state organization and crystal RTP performances. Two series of σ-conjugated donor/acceptor (D-σ-A) carbazolebased matrices and isomeric 1H-benzo[f]indole-based dopants are designed, capable of exploring a wide variety of conformations thanks to large rotational degrees of freedom provided by the σ-conjugation. By correlating the results of single-crystal X-ray diffraction analysis and photoluminescence properties, a necessary and sufficient condition for RTP is established that paves the way for the development of new long-lived RTP host-guest doped systems.