Uncovering the Mechanism of Thermally Activated Delayed Fluorescence in Coplanar Emitters Using Potential Energy Surface Analysis

Ln3+ Induced Thermally Activated Delayed Fluorescence of Chiral Heterometallic Clusters Ln2Ag28

A series of TADF-active compounds: 0D chiral Ln-Ag(I) clusters L-/D-Ln2Ag28-0D (Ln=Eu/Gd) and 2D chiral Ln-Ag(I) cluster-based frameworks L-/D-Ln2Ag28-2D (Ln=Gd) has been synthesized. Atomic-level structural analysis showed that the chiral Ag(I) cluster units {Ag14S12} in L-/D-Ln2Ag28-0D and L-/D-Ln2Ag28-2D exhibited similar configurations, linked by varying numbers of [Ln(H2O)x]3+ (x=6 for 0D, x=3 for 2D) to form the final target compounds. Temperature-dependent emission spectra and decay lifetimes measurement demonstrated the presence of TADF in L-Ln2Ag28-0D (Ln=Eu/Gd) and L-Gd2Ag28-2D. Experimentally, the remarkable TADF properties primarily originated from {Ag14S12} moieties in these compounds. Notably, {Ag14S12} in L-Eu2Ag28-0D and L-Gd2Ag28-2D displayed higher promote fluorescence rate and shorter TADF decay times than L-Gd2Ag28-0D. Combined with theoretical calculations, it was determined that the TADF behaviors of {Ag14S12} cluster units were induced by 4 f perturbation of Ln3+ ions. Specially, while maintaining ΔE(S1-T1) small enough, it can significantly increase k(S1→S0) and reduce TADF decay time by adjusting the type or number of Ln3+ ions, thus achieving the purpose of improving TADF for cluster-based luminescent materials.

Conformational Control of Donor-Acceptor Molecules Using Non-covalent Interactions

Controlling the architecture of organic molecules is an important aspect in tuning the functional properties of components in organic electronics. For purely organic thermally activated delayed fluorescence (TADF) molecules, design is focused upon orthogonality orientated donor and acceptor units. In these systems, the rotational dynamics around the donor and acceptor bond has been shown to be critical for activating TADF; however, too much conformational freedom can increase the non-radiative rate, leading to a large energy dispersion of the emitting states and conformers, which do not exhibit TADF. To date, control of the motion around the D-A bond has focused upon steric hindrance. In this work, we computationally investigate eight proposed donor-acceptor molecules, exhibiting a B-N bond between the donor and acceptor. We compare the effect of steric hindrance and noncovalent interactions, achieved using oxygen (sulfur) boron heteroatom interactions, in exerting fine conformational control of the excited state dynamics. This work reveals the potential for judiciously chosen noncovalent interactions to strongly influence the functional properties of TADF emitters, including the accessible conformers and the energy dispersion associated with the charge transfer states.

Triplet dynamics reveal loss pathways in multi-resonance thermally activated delayed fluorescence emitters

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are of interest for light-emitting applications due to their narrow emission bandwidths and high photoluminescence quantum yields. Whilst there have been numerous examples of multi-resonance molecules exhibiting efficient TADF, the photophysics and mechanism of TADF in multi-resonance emitters have not been investigated to the same extent as the more conventional spatially separated donor-acceptor TADF materials, limiting the development of MR-TADF devices. Here we study the photophysics of a multi-resonance TADF material, OQAO(mes)2, using transient absorption spectroscopy to spectrally resolve the triplet population(s). We identify multiple triplet populations with distinct spectral contributions, and resolve the dynamics between them. Unlike conventional donor-acceptor TADF materials that have previously been studied, we find these triplet states are not formed in equilibrium, instead exhibiting a slow evolution from a high-energy triplet to a low-energy triplet. Delayed fluorescence predominantly reflects the lifetime of the high-energy triplet state, indicating that the formation of the low-energy triplet is a loss pathway for TADF. We also find that greater amounts of the low-energy triplet are formed in a higher dielectric environment, which leads to less delayed fluorescence. These triplet dynamics have significant implications for TADF in devices, as depending on the identity of the triplet formed by electrical excitation, there will either be a significant barrier to TADF, or a competing nonradiative decay pathway.

Investigating Hydrogen Bonding in Quinoxaline-Based Thermally Activated Delayed Fluorescent Materials

In recent years, hydrogen bonding (H bonding) as an intramolecular locking strategy has been proposed to enhance photoluminescence, color purity, and photostability in thermally activated delayed fluorescence (TADF) materials. Rigidification as a design strategy is particularly relevant when using electron-deficient N-heterocycles as electron acceptors, because these materials often suffer from poor performance as orange to near-infrared emitters as a result of the energy gap law. To critically evaluate the presence of H bonding in such materials, two TADF-active donor-acceptor dyads, ACR-DQ and ACR-PQ, were synthesized. Despite their potential sites for intramolecular H bonding and emissions spanning yellow to deep red, computational analyses (including frequency, natural bond orbital, non-covalent interaction, and potential energy surface assessments) and crystal structure examinations collectively suggest the absence of H bonding in these materials. Our results indicate that invoking intramolecular H bonding should be done with caution in the design of rigidified TADF materials.

Intramolecular Hydrogen Bonding Rigidifying Flexible Bridge for Solution- and Vacuum-Processed TSCT-TADF Emitters

To realize strong donor-acceptor face-to-face stacking for efficient through-space charge transfer-type thermally activated delayed fluorescence, a conceptually new design strategy is proposed to couple flexible bridges with adequate rigidity via built-in intramolecular hydrogen bonds (IHBs). The resulting emitter ACE-CN has a planarized benzyl methyl ether bridge self-anchored by the C-H···O IHB and shows a high photoluminescence quantum efficiency of 93%. The solution- and vacuum-processed devices exhibited high external quantum efficiencies of 11.8% and 24.7%, respectively.

Enhancing Light-Emitting Efficiency of Blue Through-Space Charge Transfer Emitters via Fixing Configuration Induced by Intramolecular Hydrogen Bonding

Closely aligned configuration of the donor (D) and acceptor (A) is crucial for the light-emitting efficiency of thermally activated delayed fluorescence (TADF) materials with through-space charge transfer (TSCT) characteristics. However, precisely controlling the D-A distance of blue TSCT-TADF emitters is still challenging. Herein, an extra donor (D*) located on the side of the primary donor (D) is introduced to construct the hydrogen bonding with A and thus modulate the distance of D and A units to prepare high-efficiency blue TSCT emitters. The obtained "V"-shaped TSCT emitter presents a minimal D-A distance of 2.890 Å with a highly parallel D-A configuration. As a result, a high rate of radiative decay (>107 s-1) and photoluminescence quantum yield (nearly 90%) are achieved. The corresponding blue organic light-emitting diodes show maximum external quantum efficiencies (EQEmax) of 27.9% with a Commission Internationale de L'Eclairage (CIE) coordinate of (0.16, 0.21), which is the highest device efficiency of fluorene-based blue TSCT-TADF emitters. In addition, the TSCT-TADF emitter-sensitized OLEDs also achieve a high EQEmax of 29.3% with a CIE coordinate of (0.12, 0.16) and a narrow emission.

A molecular descriptor of a shallow potential energy surface for the ground state to achieve narrowband thermally activated delayed fluorescence emission

Narrowband thermally activated delayed fluorescence (TADF) molecules have extensive applications in optoelectronics, biomedicine, and energy. The full-width at half-maximum (FWHM) holds significant importance in assessing the luminescence efficiency and color purity of TADF molecules. The goal is to achieve efficient and stable TADF emissions by regulating and optimizing the FWHM. However, a bridge from the basic physical parameters (such as geometric structure and reorganization energy) to the macroscopic properties (delayed fluorescence, efficiency, and color purity) is needed and it is highly necessary and urgent to explore the internal mechanisms that influence FWHM. Herein, first-principles calculations coupled with the thermal vibration correlation function (TVCF) theory were performed to study the energy consumption processes of the excited states for the three TADF molecules (2,3-POA, 2,3-DPA, and 2,3-CZ) with different donors; inner physical parameters affecting the FWHM were detected. By analyzing the basic geometric and electronic structures as well as the transition properties and reorganization energies, three main findings in modulating FWHM were obtained, namely a large local excitation (LE) proportion in the first singlet excited state is advantageous in reducing FWHM, a donor group with weak electron-donating ability is beneficial for achieving narrowband emission, and small reorganization energies for the ground state are favorable for reducing FWHM. Thus, wise molecular design strategies to achieve efficient narrowband TADF emission are theoretically proven and proposed. We hope that these results will promote an in-depth understanding of FWHM and accelerate the development of high color purity TADF emitters.

Realizing High-Efficiency Orange-Red Thermally Activated Delayed Fluorescence Materials through the Construction of Intramolecular Noncovalent Interactions

The development of highly efficient orange and red thermally activated delayed fluorescence (TADF) materials for constructing full-color and white organic light-emitting diodes (OLEDs) remains insufficient because of the formidable challenges in molecular design, such as the severe radiationless decay and the intrinsic trade-off between the efficiencies of radiative decay and reverse intersystem crossing (RISC). Herein, we design two high-efficiency orange and orange-red TADF molecules by constructing intermolecular noncovalent interactions. This strategy could not only ensure high emission efficiency via suppression of the nonradiative relaxation and enhancement of the radiative transition but also create intermediate triplet excited states to ensure the RISC process. Both emitters exhibit typical TADF characteristics, with a fast radiative rate and a low nonradiative rate. Photoluminescence quantum yields (PLQYs) of the orange (TPA-PT) and orange-red (DMAC-PT) materials reach up to 94 and 87%, respectively. Benefiting from the excellent photophysical properties and stability, OLEDs based on these TADF emitters realize orange to orange-red electroluminescence with high external quantum efficiencies reaching 26.2%. The current study demonstrates that the introduction of intermolecular noncovalent interactions is a feasible strategy for designing highly efficient orange to red TADF materials.