Photophysics of TADF Guest–Host Systems: Introducing the Idea of Hosting Potential

Abstract

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.

Hot Vibrational States in a High-Performance Multiple Resonance Emitter and the Effect of Excimer Quenching on Organic Light-Emitting Diodes

“…Given that we observe the emission onset and peak positions from the 0.1 wt % DPEPO film are very similar to those observed in a 1 wt % UGH film, this shows that the excimer state is not affected by host dielectric or polarizability but instead is more dependent on (different) host packing forces. 29 As in solution, the DF from films always comes from the exciton state not the excimer, Figure S14 . In all hosts, DF is highly suppressed at 80 K in line with solution measurements and again indicating that a different rISC mechanism is at work in ν-DABNA compared to intramolecular D-A TADF molecules.…”

The Many Facets of Molecular Orientation in Organic Optoelectronics

“…Note that even if a molecule does not exhibit a sizeable PDM in the ground state, it can do so in the excited state, and directions of the respective PDM vectors can be different as well. This is particularly pronounced if the excited state has charge-transfer character, as it is the case for thermally activated delayed fluorescence (TADF) emitters, and can induce remarkable photophysical effects that are the subject of dedicated studies [38,39] but will not be discussed here.The focus of this review are polar molecules that show a non-vanishing net alignment of their PDMs in thin films, so-called spontaneous orientation polarization (SOP). [30] As indicated in Figure 2d, this leads to a macroscopic dielectric polarization P 0 which is related to the presence of interfacial charge densities σ of opposite sign at the interfaces to neighboring layers, if those are non-polar, and is described by an order parameter…”

Vibrationally Assisted Direct Intersystem Crossing between the Same Charge-Transfer States for Thermally Activated Delayed Fluorescence: Analysis by Marcus–Hush Theory Including Reorganization Energy

“…In another recent investigation, the behavior of DMAC-TRZ in solid solutions was explained by the inhomogeneity of its geometry caused by rotational isomerism and distribution of CT states. 9 Another recent theoretical model assumed that both spin–orbit coupling (SOC, T 2 /T 1 → S 1 ) and the so-called direct SOC (T 1 → S 1 ) play important roles in rISC. 10 However, the direct T 1 → S 1 transition was predicted to be efficient when the nature of T 1 and S 1 states is different: the presented calculations predicted 92% CT nature for the S 1 state and 95% LE nature for the T 1 state.…”