Forschungsprojekte

The singlet-triplet energy gap (ΔE_ST) plays a crucial role in chromophores for OLEDs. Through appropriate molecular design, this gap and thus the optical and reactivity properties can be finely adjusted (“tuned”). In TADF chromophores, such as twisted donor-acceptor or multiresonance systems, the ΔE_ST gap is small due to the spatial separation of the electron and hole densities. The project aims to fine-tune triazine and triangulene luminophores electronically in order to specifically influence ΔE_ST and clarify fundamental questions about low energy gaps.

This project will be performed under the supervision of Prof. Dr. Thomas J. J. Müller (Organic Chemistry). 

Thermally activated delayed fluorescence (TADF) requires two energetically close excited states of different spin multiplicity, which can be mutually coupled due to various mechanisms. Three classes of molecular emitters have proven particularly successful for the targeted design of such TADF emitters: 

1) Push-pull systems with twisted donor and acceptor groups, which are kept apart by an aromatic spacer to increase the dipole moment and lead to charge transfer (CT)-based broad-band emission,

2) Rigid planar heterocyclic aromatic systems with orthogonal electron and hole charge densities known as multiresonant (MR) TADF emitters,

3) Organometallic complexes exploiting the coinage transition metals Cu(I), Ag(I), or Au(I) as central atoms with low-energetic metal-ligand charge transfer (MLCT) states. 

In all three cases, an understanding of reverse intersystem crossing (rISC) from the excited triplet state to the excited singlet state is decisive in the action of TADF. There are various structural and electronic design strategies to make this concept beneficially work for OLED technology. Most of these emitters tend to react sensitively to external stimuli such as temperature or pressure. In turn, this liability could offer a neat way to use the luminescence of a TADF emitter for remote temperature or pressure sensing. It is the goal of this project to find fundamental guiding principles and calibration strategies by means of (time-resolved) spectroscopy how to tailor TADF emitters towards this particular application and make them multifunctional materials for both display and sensing technology.

This project will be performed under the supervision of Jun.-Prof. Dr. Markus Suta (Inorganic Photoactive Materials). 

This project aims to establish a theoretical framework for understanding and optimizing chromophores with small singlet–triplet energy gaps (ΔE_ST)—a key factor for efficient optoelectronic and photocatalytic systems. Small ΔE_ST values enable mechanisms such as thermally activated delayed fluorescence (TADF), inverse intersystem crossing from higher states (HIGHrISC), and the recently proposed singlet–triplet inversion (INVEST), which can improve light emission and control photo-redox reactivity. The relevant energetic and kinetic quantities are usually obtained from (time-resolved) spectroscopy. Spectroscopic measurements are almost always interpreted in conjunction with quantum chemical calculations. Such calculations can also be used to identify molecular target structures. 

This project will be performed under the supervision of Prof. Dr. Shirin Faraji (Theoretical and Computational Chemistry). 

The HIGHrISC or hot exciton process involves reverse intercombination (rISC) from a higher triplet state. This process can be used to convert triplet excitons into light in organic light-emitting diodes (OLEDs) or to detect triplet excitations in biological contexts. In contrast to other approaches such as room-temperature phosphorescence and thermally activated delayed fluorescence (TADF), the HIGHrISC approach allows the use of narrow-band emitting chromophores with high radiative rate constants. Since HIGHrISC behavior contradicts the rules (not laws!) of photophysics, its occurrence is surprising. It was discovered by chance in the Gilch research group while investigating certain aromatic carbonyls. Sporadic references to HIGHrISC behavior (though not under this name) can also be found in the literature for other types of chromophores. This project aims to systematically search for HIGHrISC chromophores for the first time. As mentioned above, HIGHrISC emitters can also serve as probes for triplet excitations in biological contexts. This project will therefore also search for applications of new HIGHrISC emitters in biomedical diagnostics.

This project will be performed under the supervision of Prof. Dr. Peter Gilch (Femtosecond Spectroscopy). 

For the synthesis of complex molecular structures, the underlying carbon skeleton is usually built up by consecutive C-C bond formation. Cascade reactions, in which several C-C bonds are formed rapidly one after the other starting from an initial reaction, reduce the synthesis effort and facilitate purification. Radical transformations that can be induced photocatalytically by visible light are particularly suitable for this purpose. Ideally, with a clever choice of starting material and reaction conditions, either a complex multicyclic structure can be obtained directly as the sole product, or the reaction can be deliberately controlled so that several products are formed and separated, as is the aim in drug development. However, both variants require an in-depth mechanistic understanding of the individual steps and their individual speeds. Based on a recently developed cascade transformation, the importance of the photocatalyst, intersystem crossing, and electronic modification of the substrate framework will be investigated. The focus will be on both maximum product selectivity and reaction control with lower selectivity but maximum structural diversity.

This project will be performed under the supervision of Prof. Dr. Constantin Czekelius (Asymmetric Synthesis). 

Photoredox catalysis utilizes the reduction or oxidation potential of electronically excited states to carry out chemical reactions. Triplet states play a key role in this process, as their longer lifetime allows bimolecular processes to occur, enabling efficient electron transfer between the photocatalyst and the substrate. In this project, methods for automated reaction discovery will be modified to include intersystem crossing. Analysis of the resulting chemical reaction network will elucidate main reaction pathways of photo-redox catalysis and allow the improvement of reaction routes and catalytic processes.

This project will be performed under the supervision of Jun.-Prof. Dr. Jan Meisner (Theory and Simulation of Complex Systems).