Benchmarking Charge-Transfer Excited States in TADF Emitters: ΔDFT Outperforms TD-DFT for Emission Energies

Charge-transfer (CT) excited states are crucial to organic light-emitting diodes (OLEDs), particularly to those based on thermally activated delayed fluorescence (TADF). However, accurately modeling CT states remains challenging, even with modern implementations of (time-dependent) density functional theory [(TD-)DFT], especially in a dielectric environment. To identify shortcomings and improve the methodology, we previously established the STGABS27 benchmark set with highly accurate experimental references for the adiabatic energy gap between the lowest singlet and triplet excited states (ΔEST). Here, we diversify this set to the STGABS27-EMS benchmark by including experimental emission energies (Eem) and use this new set to (re)-evaluate various DFT-based approaches. Surprisingly, these tests demonstrate that a state-specific (un)restricted open-shell Kohn-Sham (U/ROKS) DFT coupled with a polarizable continuum model for perturbative state-specific nonequilibrium solvation (ptSS-PCM) provides exceptional accuracy for predicting Eem over a wide range of density functionals. In contrast, the main workhorse of the field, Tamm-Dancoff-approximated TD-DFT (TDA-DFT) paired with the same ptSS-PCM, is distinctly less accurate and strongly functional-dependent. More importantly, while TDA-DFT requires the choice of two very different density functionals for good performance on either ΔEST or Eem, the time-independent U/ROKS/PCM approaches deliver excellent accuracy for both quantities with a wide variety of functionals.

Molecular Engineering of Push-Pull Diphenylsulfone Derivatives towards Aggregation-Induced Narrowband Deep Blue Thermally Activated Delayed Fluorescence (TADF) Emitters

Narrowband deep blue thermally activated delayed fluorescent (TADF) materials have attracted significant attention. Herein, four asymmetrical structured TADF emitters based on diphenylsulfone (DPS) acceptor and 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor with progressive performances were developed. The tert-butyloxy auxiliary electron-donor was adopted to restrict the intramolecular rotations and provide efficient steric hindrance. Regioisomerization by altering the substitution position of DMAC on DPS unit further enhanced the intra- and inter-molecular interactions. The accompanying effects yielded increased energy level, minimized reorganization energy, and inhibited non-radiative transitions in the crystals of tBuO-SOmAD, which achieved narrowband deep-blue emission peaking at 424 nm (FWHM=64 nm, Φ_F =33.6 %) through aggregation-induced, blue-shifted emission (AIBSE). In addition, deep-blue organic light emitting diodes (OLEDs) based on tBuO-SOmAD realized the electroluminescence (EL) spectrum peaking located at 435 nm and CIE coordination of (0.12, 0.09).

A Calix[3]acridan-Based Host-Guest Cocrystal Exhibiting Efficient Thermally Activated Delayed Fluorescence

A supramolecular strategy to construct thermally activated delayed fluorescence (TADF) materials through host-guest charge transfer interactions was proposed. Consequently, a new class of macrocycle namely calix[3]acridan was conveniently synthesized in 90 % yield. The host-guest cocrystal formed by calix[3]acridan and 1,2-dicyanobenzene exhibited efficient TADF properties due to intense intermolecular charge transfer interactions. Moreover, the spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital resulted in a very small singlet-triplet energy gap of 0.014 eV and hence guaranteed an efficient reverse intersystem crossing for TADF. Especially, a high photoluminescence quantum yield of 70 % was achieved, and it represents the highest value among the reported intermolecular donor-acceptor TADF materials.

PCM-ROKS for the Description of Charge-Transfer States in Solution: Singlet-Triplet Gaps with Chemical Accuracy from Open-Shell Kohn-Sham Reaction-Field Calculations

The adiabatic energy gap between the lowest singlet and triplet excited states ΔE_ST is a central property of thermally activated delayed fluorescence (TADF) emitters. Since these states are dominated by a charge-transfer character, causing strong orbital-relaxation and environmental effects, an accurate prediction of ΔE_ST is very challenging, even with modern quantum-chemical excited-state methods. Addressing this major challenge, we present an approach that combines spin-unrestricted (UKS) and restricted open-shell Kohn-Sham (ROKS) self-consistent field calculations with a polarizable-continuum model and range-separated hybrid functionals. Tests on a new representative benchmark set of 27 TADF emitters with accurately known ΔE_ST values termed STGABS27 reveal a robust and unprecedented performance with a mean absolute deviation of only 0.025 eV (∼0.5 kcal/mol) and few deviations greater than 0.05 eV (∼1 kcal/mol), even in electronically challenging cases. Requiring only two geometry optimizations per molecule at the ROKS/UKS level in a compact double-ζ basis, the approach is computationally efficient and can routinely be applied to molecules with more than 100 atoms.

Efficient Intramolecular Charge-Transfer Fluorophores Based on Substituted Triphenylphosphine Donors

Triphenylphosphine (TPP)-based luminescent compounds are rarely investigated because of the low photoluminescence quantum yield (PLQY). Here, we demonstrate that introducing steric hindrance groups to the TPP moiety and separating the orbitals involved in the transition can drastically suppress the non-radiative decay induced by structural distortion of TPP in the excited state. High PLQY up to 0.89 as well as thermally activated delayed fluorescence are observed from the intramolecular charge-transfer (ICT) molecules with substituted TPP donors (sTPPs) in doped films. The red organic light-emitting diodes employing these emitters achieve comparable external quantum efficiencies to the control device containing a classical phosphorescent dye, revealing the great potential of the ICT emitters based on electrochemically stable sTPPs.

Face-to-Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence

Intramolecular through-space charge-transfer (TSCT) excited states have been exploited for developing thermally activated delayed fluorescence (TADF) emitters, but the tuning of excited state dynamics by conformational engineering remains sparse. Designed here is a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π-π interactions, the radiative decay rate of the lowest singlet excited state (S₁ ) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields of up to 0.99 in doped thin films. A high-efficiency electroluminescence device, with a maximum external quantum efficiency (EQE) of 23.96 %, was achieved and maintains >20 % at a brightness of 1000 cd m^-2 . This work sheds light on the importance of conformation control for achieving high-efficiency intramolecular exciplex emitters.

Tuning Optical Properties by Controlled Aggregation: Electroluminescence Assisted by Thermally-Activated Delayed Fluorescence from Thin Films of Crystalline Chromophores

Several photophysical properties of chromophores depend crucially on intermolecular interactions. Thermally-activated delayed fluorescence (TADF) is often influenced by close packing of the chromophore assembly. In this context, the metal-organic framework (MOF) approach has several advantages: it can be used to steer aggregation such that the orientation within aggregated structures can be predicted using rational approaches. We demonstrate this design concept for a DPA-TPE (diphenylamine-tetraphenylethylene) chromophore, which is non-emissive in its solvated state due to vibrational quenching. Turning this DPA-TPE into a ditopic linker makes it possible to grow oriented MOF thin films exhibiting pronounced green electroluminescence with low onset voltages. Measurements at different temperatures clearly demonstrate the presence of TADF. Finally, this work reports that the layer-by-layer process used for MOF thin film deposition allows the integration of the TADF-DPA-TPE in a functioning LED device.

Dual-Phase Thermally Activated Delayed Fluorescence Luminogens: A Material for Time-Resolved Imaging Independent of Probe Pretreatment and Probe Concentration

Developing luminescent probes with long lifetime and high emission efficiency is essential for time-resolved imaging. However, the practical applications usually suffer from emission quenching of traditional luminogens in aggregated states, or from weak emission of aggregation-induced emission type luminogens in monomeric states. Herein, we overcome this dilemma by a rigid-and-flexible alternation design in donor-acceptor-donor skeletons, to achieve a thermally activated delayed fluorescence luminogen with high emission efficiency both in the monomeric state (quantum yield up to 35.3 %) and in the aggregated state (quantum yield up to 30.8 %). Such a dual-phase strong and long-lived emission allows a time-resolved luminescence imaging, with an efficiency independent of probe pretreatment and probe concentration. The findings open opportunities for developing luminescent probes with a usage in larger temporal and spatial scales.

Proton-Activated Amorphous Room-Temperature Phosphorescence for Humidity Sensing and High-Level Data Encryption

Supramolecular co-assembling terpyridine-derivatives with nanoclay (LP) are exploited to acquire efficient amorphous room-temperature phosphorescence (RTP). Experimental and theoretical investigations reveal that this co-assembly not only brings about a configuration transformation from the trans-trans (a) to the cis-trans (a'') form via the protonating process, significantly narrowing the singlet-triplet energy gap, thereby effectively facilitating the single-triplet ISC processes, but also well protects the triplet state and suppresses the nonradiative transitions via restricting molecular rotation and vibration by the hydrogen-bond interactions between them. Additionally, the flexible and transparent films, through co-assembling 1@LP (or 2@LP) with polyvinyl alcohol (PVA), also display excellent phosphorescence performance. Owing to their distinctive RTP performances, the RH sensing and high-level data encryption are achieved.

Selective Expression of Chromophores in a Single Molecule: Soft Organic Crystals Exhibiting Full-Colour Tunability and Dynamic Triplet-Exciton Behaviours

Soft luminescent materials are attractive for optoelectronic applications, however, switching dominant chromophores for property enrichment remains a challenge. Herein, we report the first case of a soft organic molecule (DOS) featuring selective expression of chromophores. In response to various external stimuli, different chromophores of DOS can take turns working through conformation changes, exhibiting full-colour emissions peaking from 469 nm to 583 nm from ten individual single crystals. Dynamic triplet-exciton behaviours including thermally activated delayed fluorescence (TADF), room-temperature phosphorescence (RTP), mechanoluminescence (ML), and distinct mechano-responsive luminescence (MRL) can all be realized. This novel designed DOS molecule provides a multifunctional platform for detection of volatile organic compounds (VOCs), multicolour dynamic displays, sensing, anticounterfeiting, and hopefully many others.