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Molecular Organization in Materials for Molecular Electronics Application

Highly efficient organic electronic devices with excellent external quantum yields can be achieved following the concept of thermally activated delayed fluorescens, where the designed small gap between the energy of the signlet and the triplet excited state allows for a thermally activated back transfer of the non-radiative triplet excitons to the fluorecent singlet state. Most the developements of new materials and designs of TADF organic LED devices were aiming for the ultimat external quantum efficiency (EQE). Dispite the TADF design of the novell materials, however, the novell materials have to provide a long term stable and well balanced charge transfer in order to ensure a stable and efficient functionallity of the devices. In many cases, small paracitic molecules like water or oxigen can reside in the materials and act locally as electron or hole traps, respectively, spoiling the initially balanced charge transfer essential for the homogeneous and efficient operation of the TADF layer in the OLED device.

Solid state NMR measurements can provide some insight into the local molecular packing arrangement of the new TADF molecules, even if the materials cannot be crystallized from solution as required for x-ray diffraction studies.

Chemical structures and solid-state NMR spectra of a series of carbazole substituted triazine based TADF materials. The addition of tert-butyl groups at the carbazole units improves the solution processibility and the crystallisation of the material, but is not beneficial the performance of electronic devices.

Chemical structures and solid-state NMR spectra of a series of carbazole substituted triazine based TADF materials. The addition of tert-butyl groups at the carbazole units improves the solution processibility and the crystallisation of the material, but is not beneficial the performance of electronic devices.

Remarkably, the electron chemical characteristics of the TADF materials shown above are very similar, while highly efficient OLED devices were obtained only with the three-fold carbazole substituted material. This indicates that the local molecular packin is crucial for the device performance and not only synthetically adjusted HOMO and LUMO levels. Unfortunately, the spectral resolution even in the ¹⁹F MAS-NMR spectrum is relatively poor, due to the strong signal broadening influence of pooly aligned carbazole rings in close spatial proximity to the ¹⁹F sites, which makes an elucidation of the local molecular packing based on NMR alone impossible. Structural analysis of the material via single crystal x-ray diffracton methods is hamplered by the incorporation of sovent molecules in the solution grown crystalls, while TEM crystallographic approches provide numerous different crystall structures with variing cell sizes and symmetries for different areas of the vapor deposited material.

Here ¹⁹F double-quantum (DQ) NMR methods further insight, as the spatial distribution of the dipolar coupled ¹⁹F sites can be obtained from the determined crytsall structures, and the computed DQ excitation profile can be compared with experimental NMR data of the bulk material.

19F double quantum NMR results TADF sample. — ¹⁹F double quantum NMR results TADF sample. The DQ correlation spectrum at the left indicates that there is no significant spatial proximity (d_ij > 0.5nm) between ¹⁹F sites close to the donor units of the material while the DQ excitation behavior at the right confirms the crystall structures from TEM studies and provides a frequency distribution of the different lattice structures.

¹⁹F double quantum NMR results TADF sample. The DQ correlation spectrum at the left indicates that there is no significant spatial proximity (d_ij > 0.5nm) between ¹⁹F sites close to the donor units of the material while the DQ excitation behavior at the right confirms the crystall structures from TEM studies and provides a frequency distribution of the different lattice structures.

Literature and References

Oskar Sachnik, Xiao Tan, Dehai Dou, Constantin Haease, Naomi Kinaret, Kun-Han Lin, Denis Andrienko, Martin Baumgarten, Robert Graf, Gert-Jan A. H. Wetzelaer, Jasper J. Michels, and Paul W. M. Blom, "Elimination of charge-carrier trapping by molecular design," Nature Materials 22, 1114-1120 (2023).

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