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.

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.

The molecular photophysics and thermally activated delayed fluorescence (TADF) in spiro compounds are distinct because of the rigid orthogonal C–C bridging bond between donor and acceptor. The photophysics is found to be highly complex, with unprecedented multiple anti-Kasha emissions from three different singlet states, two of which are one-photon forbidden. The TADF mechanism is critically controlled by local acceptor nπ* states; the singlet nπ* state undergoes rapid intersystem crossing populating an energetically close acceptor ππ* triplet state. The acceptor triplet nπ* state couples nonadiabatically to a CT triplet state mediating reverse intersystem crossing. When the nπ* and CT states are energetically close, TADF is greatly enhanced with rISC rate reaching 10 7 s –1 . We observe neither DF from the singlet nπ* state nor electron transfer (ET) to form the 1 CT because there is no ET driving force; however, ET from the higher-energy donor singlet ππ* state readily occurs along with donor emission.

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.

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.