Abstract
The exploration of heavy atom effect in organic semiconductors for organic light-emitting diode (OLED) applications has attracted much attention recently. While such effect has been extensively investigated in those incorporated with selenium, copper, silver and gold, there are only few studies on the role of germanium (Ge) on the luminescence and structural properties of emitters. Herein, we reveal the importance of the incorporation of Ge in multi-resonance thermally activated delayed fluorescence emitters that show improved luminescence properties than its carbon and silicon counterparts. We present the distinct single crystal structures of the two conformers of the organogermane emitter that co-exist in the solid state. We describe their conformational changes from open to folded geometries upon thermal stimulation under vacuum, as supported by variable-temperature single crystal diffraction analysis and theoretical calculations. From molecular dynamics simulations, we show that the folded form prevents a close proximity to the sensitizer in solid-state packing, thereby reducing Dexter energy transfer and facilitating efficient Förster energy transfer. Together with the spin-vibronic coupling and heavy atom effect, organogermane emitter shows an accelerated spin-flip process than its carbon and silicon counterparts. Based on the Ge emitter, we achieve a blue emission peaking at 479 nm with a narrow spectral full-width-at-half-maximum of 25 nm and a maximum external quantum efficiency of 38.4%. More importantly, we report the LT90 (90% of the initial luminance at 1000 cd m-2) of 2.2 h for Ge-based OLEDs, unlocking the full potential of organogermane emitters for operationally stable OLEDs. We anticipate our study provides insights into the design of organogermane compounds for optoelectronics applications.