Entropy-driven charge-transfer complexation yields thermally activated delayed fluorescence and highly efficient OLEDs

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Entropy-driven charge-transfer complexation yields thermally activated delayed fluorescence and highly efficient OLEDs
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Introduction of exciplex and its Development predicament

The exciplex-forming systems with thermally activated delayed fluorescence (TADF) are widely used for fabricating organic light-emitting diodes (OLEDs). The formation of exciplex originates from donor (D) and acceptor (A) blends, where D and A belong to two individual molecules. Upon homogeneous mixing via either vapour codeposition or spin-coating, complexation between D and A molecules occurs, and their relative frontier orbital (the HOMO and LUMO) are mainly populated in D and A molecules, respectively, leading to through-space charge transfer and hence an exciplex-like configuration. The separation of the HOMO and LUMO in space greatly reduces the electron correlation and hence the electron exchange energy. Energy difference between singlet and triplet state (ΔEST) is thus close to zero for most exciplexes, rendering exciplex emission with a TADF property.1,2

The simple synthetic route and high emission colour tunability make exciplex-type TADF popular in the field of OLEDs. However, the lack of structure information and thermodynamic parameters of the exciplex state hampers its further development. Despite our recent work that presented the use of transient IR and grazing incident X-ray diffraction (GIXD) to study the structure of TADF exciplexes3, direct structural information is still pending. To probe the structure of exciplex, affording D/A cocrystals is essential. Nevertheless, our efforts to grow exciplex cocrystal in recent years are in vain. This is because the weak interaction between D/A in exciplex-forming systems result in negligible driving force of cocrystal formation, making it difficult to grow exciplex cocrystal. Until early 2022, Zhou et al.4 reported a calix [3] acridan‐based host–guest cocrystal exhibiting TADF. Although relevant thermodynamic parameters and corresponding OLED devices are lacking, this work demonstrates the power of using supramolecular complexes to grow exciplex cocrystal, and even the possibility of fabricating OLED devices.

Using supramolecular approach to resolve the exciplex structure and thermodynamics

Enlightened by Zhou’s pioneering work, we strategically designed and synthesized an 1,3,5-triazine based cage-like supramolecular host Trz-cage as acceptor (A, Fig. 1). The crystal structure of Trz-cage contains three dichloromethane molecules, indicating the cavity of Trz-cage is large enough to encapsulate various donor guests (D).

Fig. 1. Cage-like acceptor host Trz-cage. (a) synthetic route. (b) absorption and emission spectra. (c) crystal structure.

Next, we chose a triazatruxene-based molecule TrMe as donor guest to form donor-acceptor inclusion complex (D@A) with Trz-cage, namely TrMe@Trz-cage. The TrMe@Trz-cage complex structure is fully characterized by X-ray crystallography (Fig. 2a), and the emission spectra of TrMe@Trz-cage measured in DCM show an obvious exciplex emission around 500 nm to 700 nm (Fig. 2d). The various absorption titration experiments (Fig. 2b-c) indicate the TrMe@Trz-cage formation is an entropy-driven process, which could be rationalized by the desolvation of solvent molecules from the Trz-cage cavity that can provide a favourable entropic change. This entropy-driven process plays a crucial role in generating TrMe@Trz-cage exciplex emission. 

Fig. 2. Inclusion complex TrMe@Trz-cage. (a) crystal structure. (b) complexation between Trz-cage and TrMe. (c) thermodynamics parameters. (d) absorption and emission spectra.

TADF properties of TrMe@Trz-cage and the Corresponding OLED device

Then, time-correlated single-photon counting (TCSPC) technique is utilized to probe the TADF properties of TrMe@Trz-cage in toluene (Fig. 3a), which show obvious prompt and delay component that is ascribed to TADF. Also, the corresponding rate constants of TrMe@Trz-cage in toluene were extracted (Fig. 3b). The fast reverse intersystem crossing rate (krisc) of ~107 s-1 and high photoluminescence quantum yield (PLQY) of 63% in toluene motivated us to investigate the OLED device performances of TrMe@Trz-cage. As depicted in Fig. 3c-d, TrMe@Trz-cage is doped into mCP host to be fabricated into OLED device, and the maximum quantum efficiency (EQEmax) can reach as high as 15.2%.  

Fig. 3. TADF properties and the OLED device. (a) TCSPC spectrum. (b) various rate constants. (c) OLED device configuration. (d) electroluminescence spectra.

Emission colour tunability of exciplex inclusion complex

Last, we prepared the other three indolocarbazole-based donor guests with various donor strength to afford a series of new inclusion complexes, ICzMeCN@Trz-cage, ICzMe@Trz-cage and ICzMeOMe@Trz-cage, for which the associated crystal structures were also solved (Fig. 4a-c). Importantly, the exciplex emission of the three cocrystals can be fine-tuned to 441, 491, and 575 nm, respectively, as compared to that of TrMe@Trz-cage at 542 nm, generating a broad spectrum of exciplex by using the cage-like Trz-cage unit (Fig.4d). In addition, except for ICzMeCN@Trz-cage, the exciplex emission of all the complexes exhibit intense TADF properties.

Fig. 4. Emission color tuning of the  exciplex inclusion complexes. (a) various donor guests. (b) energy diagram. (c) crystal and chemical structures. (d) emission spectra.

Summary and Outlook

In this study, we present the use of supramolecular approach to develop inclusion complex with TADF exciplex emission. The exciplex structure, thermodynamics and photophysics are resolved, and the corresponding OLED device are fabricated. We believe the future development on using supramolecular approach to design TADF materials and even the other optoelectronic materials will be far-reaching.

References

  1. Goushi, K., Yoshida, K., Sato, K. & Adachi, C. Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion. Nat. Photonics 6, 253-258 (2012).
  2. Liu, X. K. et al. Prediction and design of efficient exciplex emitters for high‐efficiency, thermally activated delayed‐fluorescence organic light‐emitting diodes. Adv. Mater. 27, 2378-2383 (2015).
  3. Lin, T.-C. et al. Probe exciplex structure of highly efficient thermally activated delayed fluorescence organic light emitting diodes. Nat. Commun. 9, 1-8 (2018).
  4. Zhou, H. Y., Zhang, D. W., Li, M. & Chen, C. F. A Calix [3] acridan‐Based Host–Guest Cocrystal Exhibiting Efficient Thermally Activated Delayed Fluorescence. Angew. Chem. Int. Ed. 134, e202117872 (2022).

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