Molecular structural and environmental control of the excited state behaviour for efficient zinc(II) based low-energy TADF emitters in quantum IT applications

• Applicants: Professorin Dr. Christel M. Marian; Professor Dr. Jens Pflaum; Professor Dr. Andreas Steffen
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 565562605

The investigation of fast and efficient molecular emitters that can harvest triplet excitons is highly relevant for high-performance OLEDs, but even more so for the development of future quantum information technologies. In such application scenarios, the molecular photon sources need to generate light in the deep red to near-IR region of the electromagnetic spectrum to allow for coupling into commercial fiber optic networks at the 850 nm band. However, low-energy triplet excited states are prone to non-radiative decay (knr) as a consequence of the energy-gap law, and current molecular triplet emitters based on precious 5d metals that exhibit strong spin-orbit coupling for phosphorescence are inefficient in the 650-900 nm wavelength range. Due to this limitation, currently no electrically driven molecular photon sources in the NIR for IT applications exist. However, the thermally activated delayed fluorescence (TADF) mechanism, by which the spin-forbidden phosphorescence is bypassed due to reverse intersystem-crossing from the triplet state to the singlet excited state at room temperature with subsequent spin-allowed emission, bears great potential to overcome the low luminescence efficiency in the desired energy regime. Within the previous funding phase of the SPP 2102, the Marian and Steffen groups developed Zn(II)-based TADF emitters, of which the radiative rate constants in the visible outcompete those of the most efficient commercial phosphorescent transition metal complexes. Based on these findings, the applicants now propose to design Zn(II)-based red to near-IR TADF emitters for quantum IT applications. To this end, control and optimization of the photophysical parameters of the TADF process is mandatory and requires a thorough understanding of the structural and environmental effects on the ensemble and at thesingle molecule level, which has been established only for very few emitter classes and is completely unknown for Zn(II) complexes. Specifically, we will investigate i) the molecular structural optimization of the TADF process including excitonic emitter coupling, ii) local host-guest effects on the emitters and their photodynamics and iii) coupling of the molecular emitters to plasmonic nanostructures of suitable resonance energy. Having achieved knowledge how the local host environment and plasmonic nanostructures affect the TADF emitter properties, we will eventually fabricate prototypical devices of our best Zn(II) complexes and determine their opto-electronic characteristics in OLEDs and as electrically driven single-photon sources. To reach these ambitious goals, a highly interdisciplinary approach is mandatory and justifies the combination of the complementary expertises of the three applicants within this collaborative project.

• DFG Programme: Research Grants