The thermally activated delayed fluorescence (TADF) donor–acceptor (D–A) molecule, DMAC–TRZ, is used as a TADF emitter “probe” to distinguish the environmental effects of a range of solid-state host materials in guest–host systems. Using the guest’s photophysical behavior in solution as a benchmark, a comprehensive study using a variety of typical TADF organic light-emitting diode hosts with different characteristics provides a clearer understanding of guest–host interactions and what affects emitter performance in solid state. We investigate which are the key host characteristics that directly affect charge-transfer (CT) state energy and singlet triplet energy gaps. Using time-resolved photoluminescence measurements, we use the CT state energy distribution obtained from the full width at half-maximum (fwhm) of the emission band and correlate this with other photophysical properties such as the apparent dynamic red shift of CT emission on-set to estimate the disorder-induced heterogeneity of D–A dihedral angles and singlet triplet gaps. Further, the delayed emission stabilization energy value and time-dependent CT band fwhm are shown to be related to a combination of host’s rigidity, emitter molecule packing, and the energy difference between guest and host lowest energy triplet states. Concentration dependence studies show that emitter dimerization/aggregation can improve as well as reduce emission efficiency depending on the characteristics of the host. Two similar host materials, mCPCN and mCBPCN, with optimum host characteristics show completely different behaviors, and their hosting potential is extensively explored. We demonstrate that type I and type III TADF emitters behave differently in the same host and that the materials with intrinsic small Δ E ST have the smallest disorder-induced CT energy and reverse intersystem crossing rate dispersion. We also present an optimized method to define the actual triplet energy of a guest–host system, a crucial parameter in understanding the overall mechanism of the TADF efficiency of the system.

The photophysics of multiple resonance thermally activated delayed fluorescence molecule ν-DABNA is described. We show coupling of a 285 cm –1 stretching/scissoring vibrational mode of peripheral phenyl rings to the S 1 state, which dictates the ultimate emission full-width at half maximum. However, a separate high amplitude mode, 945 cm –1 of the N -biphenyl units, mediates the reverse intersystem crossing (rISC) mechanism. Concentration-dependent studies in solution and solid state reveal a second emission band that increases nonlinearly with concentration, independent of the environment assigned to excimer emission. Even at concentrations well below those used in devices, the excimer contribution affects performance. Using different solvents and solid hosts, rISC rates between 3–6 × 10 5 s –1 are calculated, which show negligible dependence on environmental polarity or host packing. At 20 K over the first 10 ns, we observe a broad Gaussian excimer emission band with energy on-set above the S 1 exciton band. An optical singlet-triplet gap (ΔE ST ) of 70 meV is measured, agreeing with previous thermal estimates; however, the triplet energy is also found to be temperature-dependent. A monotonic increase of the exciton emission band full-width at half maximum with temperature indicates the role of hot transitions in forming vibrational excited states at room temperature (RT), and combined with an observed temperature dependency of ΔE ST , we deduce that the rISC mechanism is that of thermally activated reverse internal conversion of T 1 to T N ( n ≥ 2) followed by rapid rISC of T N to S 1 . Organic light-emitting diodes with ν-DABNA as a hyperfluorescent emitter (0.5 wt % and 1 wt %) exhibit an increase of maximum external quantum efficiency, reaching 27.5% for the lower ν-DABNA concentration. On the contrary, a Förster radius analysis indicated that the energy transfer ratio is smaller because of higher donor–acceptor separation (>2.4 nm) with weak sensitizer emission observed in the electroluminescence. This indicates excimer quenching in 1 wt % devices.

We present a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter paradigm, demonstrating that the structure need not require the presence of acceptor atoms.

We present a detailed and comprehensive picture of the photophysics of thermally activated delayed fluorescence (TADF). The approach relies on a few-state model, parametrized ab initio on a prototypical TADF dye, that explicitly accounts for the nonadiabatic coupling between electrons and vibrational and conformational motion, crucial to properly address (reverse) intersystem crossing rates. The Onsager model is exploited to account for the medium polarity and polarizability, with careful consideration of the different time scales of relevant degrees of freedom. TADF photophysics is then quantitatively addressed in a coherent and exhaustive approach that accurately reproduces the complex temporal evolution of emission spectra in liquid solvents as well as in solid organic matrices. The different rigidity of the two environments is responsible for the appearance in matrices of important inhomogeneous broadening phenomena that are ascribed to the intertwined contribution from (quasi)­static conformational and dielectric disorder.

Commonly, thermally activated delayed fluorescence (TADF) emitters present a twisted donor–acceptor structure. Here, electronic communication is mediated through-bond via π-conjugation between donor and acceptor groups. A second class of TADF emitters are those where electronic communication between donor and acceptor moieties is mediated through-space. In these through-space charge-transfer (TSCT) architectures, the donor and acceptor groups are disposed in a pseudocofacial orientation and linked via a bridging group that is typically an arene (or heteroarene). In most of these systems, there is no direct evidence that the TSCT is the dominant contributor to the communication between the donor and acceptor. Herein we investigate the interplay between through-bond localized excited (LE) and charge-transfer (CT) states and the TSCT in a rationally designed emitter, TPA-ace-TRZ , and a family of model compounds. From our photophysical studies, TSCT TADF in TPA-ace-TRZ is unambiguously confirmed and supported by theoretical modeling.

Developing high‐efficiency purely organic blue organic light‐emitting diodes (OLEDs) that meet the stringent industry standards is a major current research challenge. Hyperfluorescent device approaches achieve in large measure the desired high performance by combining the advantages of a high‐efficiency thermally activated delayed fluorescence (TADF) assistant dopant with a narrowband deep‐blue multi‐resonant TADF (MR‐TADF) terminal emitter. However, this approach requires suitable spectral overlap to support Förster resonance energy transfer (FRET) between the two. Here, a color tuning of a recently reported MR‐TADF B,N‐heptacene core through control of the boron substituents is demonstrated. While there is little impact on the intrinsic TADF properties—as both singlet and triplet energies decrease in tandem—this approach improves the emission color coordinate as well as the spectral overlap for blue hyperfluorescence OLEDs (HF OLEDs). Crucially, the red‐shifted and more intense absorption allows the new MR‐TADF emitter to pair with a high‐performance TADF assistant dopant and achieve maximum external quantum efficiency (EQEmax) of 15% at color coordinates of (0.15 and 0.10). The efficiency values recorded for the device at a practical luminance of 100 cd m–2 are among the highest reported for HF TADF OLEDs with CIEy ≤ 0.1.

Hidden photophysics is elucidated in the very well‐known thermally activated delayed fluorescence (TADF) emitter, DMAC‐TRZ. A molecule that, based on its structure, is considered not to have more than one structural conformation. However, based on experimental and computational studies, two conformers, a quasi‐axial (QA) and a quasi‐equatorial (QE) are found, and the effect of their co‐existence on both optical and electrical excitation isexplored. The relative small population of the QA conformer has a disproportionate effect because of its strong local excited state character. The energy transfer efficiency from the QA to the QE conformer is high, even at low concentrations, dependent on the host environment. The current accepted triplet energy of DMAC‐TRZ is shown to originate from the QA conformer, completely changing the understanding of DMAC‐TRZ. The contribution of the QA conformer in devices helps to explain the good performance of the material in non‐doped organic light‐emitting diodes (OLEDs). Moreover, hyperfluorescence (HF) devices, using v‐DABNA emitter show direct energy transfer from the QA conformer to v‐DABNA, explaining the relatively improved Förster resonance energy transfer efficiency compared to similar HF systems. Highly efficient OLEDs where green light (TADF‐only devices) is converted to blue light (HF devices) with the maximum external quantum efficiency remaining close to 30% are demonstrated.

Donor–acceptor (D–A) thermally activated delayed fluorescence (TADF) molecules are exquisitely sensitive to D–A dihedral angle. Although commonly simplified to an average value, these D–A angles nonetheless exist as distributions across the individual molecules embedded in films. The presence of these angle distributions translates to distributions in the rates of reverse intersystem crossing ( k rISC ), observed as time dependent spectral shifts and multiexponential components in the emission decay, which are difficult to directly quantify. Here we apply inverse Laplace transform fitting of delayed fluorescence to directly reveal these distributions. Rather than a single average value, the crucial k rISC rate is instead extracted as a density of rates. The modes and widths of these distributions vary with temperature, host environment, and intrinsic D–A torsional rigidity of different TADF molecules. This method gives new insights and deeper understanding of TADF host–guest interactions, as well as verifies future design strategies that target D–A bond rigidity.