Uncovering Single-Molecule Photophysical Heterogeneity of Bright, Thermally Activated Delayed Fluorescence Emitters Dispersed in Glassy Hosts

Efficient red thermally activated delayed fluorescence emitters achieved through precise control of excited state energy levels

The variety of highly efficient red/near-infrared (NIR) materials with thermally activated delayed fluorescence (TADF) feature is extremely limited so far, and it is necessary to expand the candidate pool of excellent red/deep-red emitters. However, how to control the energy level alignment of the 1CT (singlet charge transfer) state and the 3LE (triplet local excitation) state to improve the emission efficiency of materials remains a challenge. Herein, based on our previously reported green fluorescent material 67dTPA-FQ, three new donor-acceptor type TADF materials (TQ-oMeOTPA, TsQ-oMeOTPA and SQ-oMeOTPA) were designed by introducing 4,4'-dimethoxy triphenylamine (MeOTPA) as the donor, and introduced S atoms on the acceptors to enhance the spin-orbit coupling (SOC) and CT effects. The theoretical calculations showed that the newly introduced MeOTPA and S atom successfully enhanced the CT effect of the materials, not only shifting the luminescence peak to the deep red region but also effectively adjusting the energy level alignment of the excited state, accelerating the reverse intersystem crossing process. Finally, the organic light-emitting diodes based on SQ-oMeOTPA exhibit an external quantum efficiency of 19.1%, with an emission peak at 619 nm. This work not only expands the candidate inventory of red TADF materials, but also proves the feasibility of designing emitters by adjusting the excited state energy levels, greatly broadening the diversity of TADF emitters in design, and providing a powerful means for rapidly screening efficient emitters in the future.

Shedding light on thermally-activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet-triplet gaps, sizable spin-orbit couplings, conformational flexibility and a balanced contribution of charge transfer and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially affected by the interaction of the TADF dye with its local environment. The concurrent optimization of the dye and of the embedding medium is therefore of paramount importance to boost practical applications of TADF. Towards this aim, refined theoretical and computational approaches must be cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we will address some of the most important issues in the field. Specifically, we will critically review theoretical and computational approaches to TADF rates, highlighting the limits of widespread approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices.

Theoretical Insights into the Optical and Excited State Properties of Donor-Phenyl Bridge-Acceptor Containing Through-Space Charge Transfer Molecules

A comparative new strategy to enhance thermally activated delayed fluorescence (TADF) of through-space charge transfer (CT) molecules in organic light-emitting diodes (OLEDs) is investigated. Generally, TADF molecules adopt a twisted donor and acceptor structure to get a sufficiently small ΔE_ST and a higher value of the spin-orbit coupling matrix element (SOCME). However, molecules containing donor-phenyl bridge-acceptor (D-p-A) units and featuring π-stacked architectures have intramolecular CT contribution through space and exhibit high TADF efficiency. We have explored the insights into the TADF mechanism in D-p-A molecules using the density functional theory (DFT) and time-dependent DFT methods. The calculated optical absorption and ΔE_ST values are found to be in good agreement with available experimental data. Interestingly, we found the origin of the SOCME to be the twisted orientation of the donor and bridge moieties. Also, we predicted similar molecules with enhanced OLED efficiency with different substitutions.

Temporal Dynamics of Solid-State Thermally Activated Delayed Fluorescence: Disorder or Ultraslow Solvation?

Time-resolved emission spectra of thermally activated delayed fluorescence (TADF) compounds in solid hosts demonstrate significant temporal shifts. To explain the shifts, two possible mechanisms were suggested, namely, slow solid-state solvation and conformational disorder. Here we employ solid hosts with controllable polarity for analysis of the temporal dynamics of TADF. We show that temporal fluorescence shifts are independent of the dielectric constant of the solid film; however, these shifts evidently depend on the structural parameters of both the host and the TADF dopant. A ≤50% smaller emission peak shift was observed in more rigid polymer host polystyrene than in poly(methyl methacrylate). The obtained results imply that both the host and the dopant should be as rigid as possible to minimize fluorescence instability.

QM/MM Study of Static and Dynamic Energetic Disorder in the Emission Layer of an Organic Light-Emitting Diode

Static and dynamic energetic disorder in emission layers of organic light-emitting diodes (OLEDs) is investigated through combined molecular dynamics and hybrid quantum mechanics/molecular mechanics (QM/MM) calculations. The analysis is based on a comparison of ensemble and time distributions of site energies of guest and host components in an emission layer. The law of total variance is applied to decompose the total disorder into its static and dynamic contributions. It is found that both contributions are of the same order of magnitude. While the dynamic disorder is not affected by intermolecular interactions, the static disorder for both guests and hosts is determined by the polarity of host molecules. The amount of static disorder affects charge-transport properties and exciton formation pathways, which consequently influence the overall efficiency of an OLED device. The simulations indicate that the amount of static disorder induced by the host should be considered for the optimization of the emission layer.

The theory of thermally activated delayed fluorescence for organic light emitting diodes

The recent progress in the theory of organic molecules exhibiting thermally activated delayed fluorescence (TADF) is discussed with a particular focus upon their application in highly efficient purely organic light emitting diodes (OLEDs).

Avoiding Energy Loss on TADF Emitters: Controlling the Dual Conformations of D-A Structure Molecules Based on the Pseudoplanar Segments

The recent introduction of thermally activated delayed fluorescence (TADF) emitters is regarded as an important breakthrough for the development of high efficiency organic light-emitting devices (OLEDs). The planar D and A groups are generally used to construct TADF emitters for their rigid structure and large steric hindrance. In this work, it is shown that many frequently used nonaromatic (noncontinuous conjugation or without satisfying Hückel's rule) planar segments, such as 9,9-dimethyl-9,10-dihydroacridine, are actually pseudoplanar segments and have two possible conformations-a planar form and a crooked form. Molecules constructed from pseudoplanar segments can thus have two corresponding conformations. Their existence can have significant impact on the performance of many TADF emitters. Two design strategies are presented for addressing the problem by either (1) increasing the rigidity of these groups to suppress its crooked form or (2) increasing the steric hindrance of the linked group to minimize energy of the emitters with the highly twisted form. Following these strategies, two new emitters are synthesized accordingly and successfully applied in OLEDs demonstrating high external quantum efficiencies (20.2% and 18.3%).

Resolving and Controlling Photoinduced Ultrafast Solvation in the Solid State

Solid-state solvation (SSS) is a solid-state analogue of solvent-solute interactions in the liquid state. Although it could enable exceptionally fine control over the energetic properties of solid-state devices, its molecular mechanisms have remained largely unexplored. We use ultrafast transient absorption and optical Kerr effect spectroscopies to independently track and correlate both the excited-state dynamics of an organic emitter and the polarization anisotropy relaxation of a small polar dopant embedded in an amorphous polystyrene matrix. The results demonstrate that the dopants are able to rotationally reorient on ultrafast time scales following light-induced changes in the electronic configuration of the emitter, minimizing the system energy. The solid-state dopant-emitter dynamics are intrinsically analogous to liquid-state solvent-solute interactions. In addition, tuning the dopant/polymer pore ratio offers control over solvation dynamics by exploiting molecular-scale confinement of the dopants by the polymer matrix. Our findings will enable refined strategies for tuning optoelectronic material properties using SSS and offer new strategies to investigate mobility and disorder in heterogeneous solid and glassy materials.