Research Topics & Research Projects

Within RT1 we will explore the concept of linking local photoswitching events to light-controlled structure formation and (transient) macroscopic structural changes in membranes and at interfaces particularly focusing on cooperativity and interactivity. Prime examples for light-controlled structure formation are the Langmuir-Blodgett films and poled glass surfaces. In the former, we will utilize amphiphiles, which populate an intramolecular charge-transfer state upon photoexcitation. Continuous irradiation during the Langmuir synthesis will lead to a photostationary state and, hence, to altered intermolecular interactions. During glass formation, similar reactions can be induced by making use of the significantly more sluggish relaxation kinetics. Reactions, which are not purely thermal, such as ion implantation, electro-thermal poling or light-driven relaxation widen the range of exploitable states, which we shall investigate, e.g., by second harmonic generation. Considering light-controlled (transient) structural modifications, PHINT explores local switching events in a multiscale perspective from sub-100 fs to ms and beyond – depending on the specific material system. The combination of spectroscopic experiments and theory, including will yield multi-time-scale insights into the local switching events and the derived structural changes. An example are the structural changes in biomimetic membranes. Illumination will change the steric demand of the integrated dyes or locally modulate the hydrophobicity and hence translate into (transient) structural changes. Both the formation and the structural modification of membranes and interfaces in response to light pose a challenge for theoretical understanding. Therefore, the multi-layer and multi-scale approach ONIOM will be further developed, as well as subsystem DFT and DFT embedding for the application to inorganic interfaces and glasses and NEO methods that explicitly account for the nuclear quantum effect of hydrogen atoms.

In RT2, we focus on how light-switching events in membranes influence lateral molecular mobility and transmembrane transport. With a variety of fluorophores and photoswitches, such as azobenzene and spironaphthoxazine, we explore how changes like cis-trans isomerization in lipid bilayers affect the lateral diffusion of membrane components using advanced fluorescence microscopy, including STED-FCS. We will also study the diffusion of small model fluorophores into MOF-based membranes, and how modifying the pore structure or polarization can alter mobility, using both fluorescence microscopy and dielectric relaxation spectroscopy. Additionally, we examine how light-induced structural changes can control transport properties, including altering K⁺ transport through biomimetic membranes and gas diffusion through carbon nanomembranes and MOF membranes. We combine material chemistry, time-resolved spectroscopy, and theoretical models to optimize photoinduced membrane modulations for improved responsivity and longevity, aiming to develop materials with novel, light-controllable transport features.

In RT3 we explore how light stimulation influences the functionality of membranes and interfaces, widening the perspective beyond their transient structural and transport changes to consider functional outcomes. This includes using changes in ion transport in biomimetic membranes for possible future applications e.g. in ionic circuits and artificial cells. The focus will be on how the duration and intensity of the photostimulation (cooperativity) affect the duration of the ionic currents (longevity). For carbon nanomembranes and MOF structures, research will explore how light-induced polarization changes impact gas permeability, striving towards concepts of gas separation membranes with enhanced transport control through dual-color photoswitching. We will also investigate light-modulated adsorption and proton permeation in poled glass surfaces for potential sensor applications. Light-driven charge transfers and redox reactions will be examined for their effects on membrane poling and interface adsorption, using experimental and theoretical approaches. Techniques such as time-resolved vibrational sum-frequency and second harmonic generation will assess transient electric fields and adsorption changes, considering the impact of electrolyte properties on switching dynamics and membrane stability.

Interfaces and membranes with properties controlled by light can be utilized in a wide range of applications. In addition, and from a more fundamental perspective, concepts to switch the macroscopic properties of materials by tailoring light-matter interactions will have an increasing impact of technological developments interfacing, e.g., materials’ chemistry with optical sensing, biophysics with photonics. Therefore, it seems indispensable to transfer knowledge on photochemistry and photophysics to the broad public via didactically substantiated approaches on school teaching and science communication. In this spirit RP12/Cartarius aims with research in physics education at connecting PHINT's topics with school curricula via a lab course for pupils of secondary schools and exhibits at Deutsches Optisches Museum accessible for the broad public.

Supervisor: Indra Schröder
Co-Supervisor: Stefanie Gräfe
Linked to: RT2, RT3, RT4

Light-induced changes in ion transport through cellular membranes are investigated in biophysical, biological, and medical research. For synthetic biology, membranes with optically controllable properties are an attractive building block. RP1 will create and characterize lipid membranes with tuneable electrical rectification.

Mutants of the viral potassium ion (K+) channel protein KcvNTS and photo switchable molecules, such as azobenzenes will be linked and reconstituted into artificial cellular membranes formed from purified lipids (planar lipid bilayers and vesicles). Light-triggered cis/trans isomerization of the azobenzene will allow light-controlled flux of K+ ions out of the “cell” (outward rectification) and into the “cell” (inward rectification). Simulations of simple models for how the conformational change of the photoswitch could induce changes in the protein structure will be performed by RP9.

Together with RP2, we will simultaneously measure trans-membrane currents and observe molecular characteristics in the membrane on a high-resolution fluorescence microscope. We will investigate the cooperativity and longevity of the light-induced changes in channel functionality. The responsivity of the bioinspired membranes and the associated molecular changes upon illumination will be characterized by time-resolved spectroscopy in collaboration with RP7 and RP8.

The functionalized membranes will be further characterized and optimized with possible downstream applications in mind, e.g., as building blocks for artificial cells. Examples of properties that we will explore are: 

• Orthogonally addressing channel mutants with different electrical properties by using different photo switches.
• The formation of microenvironments of altered local K+ concentrations around the channels.
• Possible influence of light-switching on the lateral diffusion of the channel proteins in the membrane.
• Transferring our functional membranes into a Langmuir-Blodgett setup (RP3) or transferring Langmuir-Blodgett membranes into our recording setup. This would open the way for future lab-on-chip applications.

Supervisor: Christian Eggeling
Co-Supervisor: Holger Cartarius
Linked to: RT1, RT2, RT3, RT4

The function of proteins depends decisively on their folding and geometric restrictions. For membrane proteins such as receptors or ion channels, these properties are highly dependent on environmental membrane characteristics such as lipid ordering. Modulation of the latter may therefore offer possibilities to influence protein functionality and thus signaling like electrical currents and membrane potentials in the case of ion channels. To this end, we are photo-switching the structure or function of membrane proteins via embedding amphiphilic and photo-switchable dyes into the membrane. Upon photo-switching, fluidity or lipid order changes can be induced very locally, fast and reversibly. We are identifying and validating various photo-switchable membrane dyes for improving responsivity of the photo-switching and longevity of the thereby light-modulated protein function. Here we employ advanced fluorescence microscopy and spectroscopy tools such as super-resolution STED microscopy, fluorescence correlation spectroscopy (e.g., STED-FCS), and spectral imaging in combination with environment-sensitive membrane dyes. In close collaboration with RP1, we will specifically investigate the potassium ion channel Kcv and its functional dependencies on photo-switchable environmental lipid ordering. The final goal will be to test modulation of functionality of this channel using black lipid bilayers and measuring electrical current over the bilayer.

Supervisor: Martin Presselt
Co-Supervisor: Eva von Domaros
Linked to: RT1, RT3, RT4

Efficient photo-induced charge separation is essential for many applications, such as solar cells and photocatalysis. This project explores the fundamental question how molecularly thin membranes made from dye molecules can become polarized when exposed to light, potentially enabling novel applications in membrane and surface technologies.
We focus on how the structural reorganization of these membranes is influenced by:

• The molecular structure and alignment of the dyes
• The degree of packing during membrane formation

To create and study these membranes, we use Langmuir-Blodgett and Langmuir-Schaefer techniques, which allow precise control over the assembly of amphiphilic molecules at the air–water interface. We combine in situ and ex situ characterization methods such as:

• Brewster-angle microscopy
• Polarized UV-vis spectroscopy
• Photothermal deflection spectroscopy (PDS)

Through collaborations with theoretical and experimental partners (RPs 7–9, 11), we study:

• The interaction between dye molecules in mixed membranes
• The mechanical and supramolecular stability of polarized films
• How membrane polarization evolves over time

This project contributes to the understanding of how molecular-level light responses can be translated into macroscopic, functional membrane properties, with potential impact in photonics, sensors, and materials science.

Supervisor: Anrey Turchanin
Co-Supervisor: Martin Presselt
Linked to: RT2, RT3

To reduce energy consumption in filtration technologies, there is a growing demand for novel ultrathin membranes. In this context, two-dimensional (2D) materials such as molecular carbon nanomembranes (CNMs), which are approximately 1 nm thick, show great promise. CNMs feature sub-nanometer porosity and, interestingly, exhibit at room temperature ~1000 times higher permeation rates for H₂O compared to He, despite the similar kinetic diameters of the two species.

The objective of this project is to develop and investigate CNMs with light-controlled permeation properties. For this purpose, CNMs will be functionalized with photoswitches to enable the tuning of their permeation behavior using light. Both the permeation characteristics and underlying mechanisms will be studied using highly sensitive mass spectrometry measurements. Additionally, the structural properties and chemical composition of the membranes will be analyzed at the nanoscale using state-of-the-art spectroscopy and microscopy methods, including photoelectron spectroscopy and various scanning probe techniques.

This research is part of a broader collaboration within the RTG “PhInt” and involves partnerships with leading institutions in Germany and across Europe.

Supervisor: Alexander Knebel
Co-Supervisor: Benjamin Dietzek-Ivanšić
Linked to: RT2, RT3

MOFs (metal-organic frameworks) are inorganic-organic hybrid materials consisting of metal-nodes and organic linker molecules assembling porous crystalline lattices, in which photo-switches can be grafted chemically or physically into the empty pore spaces. Photoswitchable MOFs can be used for instance, for the controlled release of molecules from the pores. The grafting of photoswitches to the MOF is done either physically, as guest molecules through physisorption, or chemically through pre- and post-synthetic procedures on the linkers’ backbones. The molecular response and changes to the soft-porous lattices of MOF membrane films will be investigated through combinations of optical and dielectric spectroscopy.

Photoswitchable MOF membranes are thin films which can be used to investigate switching related changes to mass transport in these films on the lengths and time scale. To elucidate the nature of switching effects, time resolved spectroscopy will be a versatile tool to us. Moreover, MOFs are soft-porous and highly polarizable insulators, allowing for lattice mobility characterization in temperature-dependent dielectric spectroscopy, detecting different fast liberations, large amplitude twists and slow librations of the linkers. In a combination with solid state nuclear magnetic resonance spectroscopy, we can paint a picture of the photophysical properties of the photoswitchable moieties, the nature of the lattice mobility, and investigate the mutual effects of both on the mass transport properties.

We aim to develop a physical method for the understanding of photoinduced isomerization processes in the confined pore space of soft-porous metal-organic frameworks (MOFs) and tracking of molecular motions to gather insights into previously undetectable responses, such as re-orientation of grafted photo switches and corresponding small and local lattice distortions. Using the response of the MOFs ions and changes of the dipolar moment of photo switches to an alternating current electric field, we expect to detect and track ionic, dipolar and molecular inside the porous frameworks.

Supervisor: Isabelle Staude
Co-Supervisor: Andrey Turchanin
Linked to: RT2, RT3

To study of the dynamics of light-induced charge transfer and trapping reactions in membranes and surfaces, it is necessary to monitor the reaction dynamics in a time-resolved fashion with a surface-sensitive technique. Time-resolved second harmonic generation (SHG) can meet both these requirements, making it a powerful method to study processes occurring at interfaces as well as in monolayer or few-layer semiconductors. In photo-polarizable interfaces and membranes photoexcitation leads to a shift of charge density within the materials. For example, in self-assembled molecular monolayers charge-transfer can shift electron density from one side of the membrane to the other side. Importantly, the second harmonic (SH) signal is expected to change markedly upon photoexcitation, since the nonlinear polarizability depends on the (time-dependent) charge density distribution.
In this project, we employ and further develop the technique of time-resolved SHG to trace transient changes in the charge-density distribution in photo-polarizable interfaces and membranes realized in other projects. These include functionalized carbon nanomembranes, heterostructures of transition metal dichalcogenides, rare earth and transition metal ion doped oxide glasses and molecularly thin Langmuir layers. 
Furthermore, we will implement time-resolved SHG momentum spectroscopy, aiming to unveil the distribution of orientation of supramolecular structures from the recorded spatial SHG emission patterns.

Supervisor: Benjamin Dietzek-Ivanšić
Co-Supervisor: Indra Schröder
Linked to: RT2, RT3 

Understanding molecular mechanisms underlying light-driven switching of membrane and surface properties requires an in-depth understanding of photochemical events occurring in complex environments. We employ ultrafast time-resolved spectroscopy to detail the photoinduced dynamics and vibrational sum-frequency generation (VSFG) to study the impact of (transient) charge density distributions on the interfacial structure of photo-polarizable membranes. Thereby, we will obtain a holistic view on the local light-induced switching events on timescales ranging from sub-100 fs to several ns.

Pump-intensity dependent transient spectroscopy will elucidate cooperative effects in photoswitching and the role of intermolecular interactions of dyes in the membranes. Additionally, UV/Vis pump-pump-probe spectroscopy allows us to follow multiple switching events, e.g., a cis-trans isomerization followed by a trans-cis isomerization in the same set of molecules.
Finally, the VSFG signal will reflect progress of photochemical process at interfaces and their impact on the interfacial structure.

The project will strongly benefit from interactions within the RTG. The hands-on-training in time-resolved ultrafast spectroscopy will be complemented by theory to mechanistically understand the experimental observables. Ample collaborations with the teams working on material synthesis and functional characterization will not only lead to spectroscopic mechanistic insights into the light-driven switching events in materials and membranes but also train the PhD researcher in interdisciplinary research and communication.

Supervisor: Stefanie Gräfe
Co-Supervisor: Lothar Wondraczek
Linked to: RT1, RT2, RT3

Most of the systems of interest within PHINT are inherently multi-scale, necessitating suchlike theoretical description. Thus, an accurate and reliable modelling of the microscopic quantum properties is needed to understand, e.g., responsivity. At the same time, also the macroscopic environment must be included, as photo-induced molecular-scale structural changes may lead to macroscopic rearrangements, interactivity. The lifetime of the light-switched changes in functionality, longevity, is determined not only by the quantum properties of the molecular switch but also influenced by its environment.
To theoretically describe these systems and effects, hybrid quantum-classical schemes have been developed and very success-fully applied, in particular, in the context of biological systems. These methods enable the calculation of larger systems by dividing them into layers which are described at a different level of theory. Within this project, we aim at further developing and applying multi-scale methods beyond current implementations. The doctoral researchers within RP9 will further develop advanced multiscale simulation schemes towards excited-state problems and apply these for various sample cases within the PHINT consortium. 

Applications include the investigation of photo-switchable ion channels in biological membranes which lead to macroscopic structural changes upon excitation (RP1), gas transport through carbon nanomembranes (RP4) or light-induced changes in Langmuir layers (RP3). The such calculated photo-induced (transient) structure, dynamics and functionality will be directly compared and evaluated in close collaboration with the corresponding spectroscopy projects (RP7 and RP8). In collaboration with RP10, we will compare methodological approaches on multi-scale approaches and compare for selected aspects our herein developed approach with their implementation of subsystem DFT.

Supervisor: Marek Sierka
Co-Supervisor: Alexander Knebel
Linked to: RT1, RT2, RT3, RT4

The subproject RP10 focuses on developing computationally efficient and accurate quantum chemical (QC) methods for modeling light-driven processes at inorganic surfaces and membranes. The work centers on advancing subsystem density functional theory (DFT) and DFT-based embedding to properly describe macroscopic polarization effects in extended systems—phenomena that are difficult to capture using conventional electronic structure approaches. Standard periodic boundary condition methods and slab models are inherently limited in handling macroscopic polarization and are computationally demanding for large or complex systems.

In subsystem DFT, the total electron density of a complex system is expressed as the sum of densities from smaller subsystems, enabling the division of the overall electronic structure problem into more manageable subproblems. This allows large-scale calculations without system-specific parameterization. In DFT-based embedding, the system is split into an active subsystem and an environment, allowing high-level wavefunction methods to be applied to the active part while describing the environment with DFT. In RP10, this approach will be extended to both molecular and periodic systems using Gaussian-type orbitals (GTOs), eliminating the need for artificial three-dimensional supercells required by plane-wave methods. The implementation will support both energy and gradient calculations, enabling structural optimizations and the study of photoinduced polarization arising from molecular conformational changes and charge density redistribution.

Initially, the NAKE-based subsystem DFT approach will be applied to weakly interacting systems such as two-dimensional molecular membranes (RP3, RP7). In a second phase, it will be extended with projection techniques to handle strongly interacting systems—such as carbon- and MOF-based membranes (RP4, RP5)—and to capture electronic transitions between subsystems, which local NAKE functionals cannot describe.

In parallel, established DFT and wavefunction-based methods will be combined with the embedding approach to investigate the mechanisms of light-driven processes, material responsivity, and the properties of photostationary states at inorganic (glass) surfaces (RP6) and inorganic–organic interfaces (RP4, RP5). Computational results will be linked with experimental studies (RP7, RP8), and used in RP9 and RP11 to develop machine learning models that predict macroscopic material properties from molecular-level descriptors.

RP10 thus provides both method development and application, offering doctoral researchers the opportunity to simulate real systems studied experimentally within the RTG. It contributes centrally to understanding and tailoring light-responsive materials, and also serves as an analytical tool for other subprojects, bridging theory and experiment within the research program.

Supervisor: Eva von Domaros
Co-Supervisor: Christian Eggeling
Linked to: RT1, RT3

When light interacts with matter, it elevates the electrons to excited states, which then lead to changes in molecular structure and properties. In state-of-the-art computational approaches, electrons are treated as quantum particles, whereas nuclei are approximated as classical point charges.
In this project, we aim at understanding how optical excitations alter the structure formation in molecularly thin membranes, which are synthesized and characterized in RP3. To obtain atomistic insights, we address the following research questions:

• How does the electronic structure (before and after excitation) affect nuclear geometry?
• How does the molecular structure affect electronic structure and hence, optical properties?
• How can the interaction between electrons as quantum particles and nuclei be described from theory?

To answer these questions, we combine established state-of-the-art computational tools with novel electronic structure methods. Therein, we treat relevant, light nuclei as quantum particles and calculate them on the same footing as the electrons in so-called nuclear electronic orbital (NEO) methods. In our group, these are being extended to be applicable to 2D periodic systems.

Supervisor: Holger Cartarius
Co-Supervisor: Isabelle Staude
Linked to: RT4

What does the public need to know about photoactive interfaces? Light-driven processes offer great opportunities to connect topics from school physics and chemistry such as solar cells or colour-dependent light sensors with modern science. In addition, physical and chemical concepts associated with light have a strong connection to our everyday life and are currently becoming increasingly important, especially in the contexts of sustainable energies or medical applications. Thus, PhInt strongly connects scientific progress with public interest.

In this spirit, RP12 extends PhInt’s scientific mission by elements of education and science communication. On the one hand, the scientific topics of PhInt are especially appealing for a didactical perspective due to the connections mentioned above. On the other hand, RP12 opens the opportunity to strengthen the collaborations of alle doctoral researchers in PhInt with their individual backgrounds due to a common goal in science education, for which RP12 is in contact with all doctoral researchers.

The aim of RP12 is to develop didactical concepts for two main target groups, viz. pupils of secondary schools and the general public. The research of the teaching concepts is done within an extended scheme of didactics reconstruction. Educational materials for pupils of secondary schools are developed and disseminated via a lab course in the university’s physics school student lab, teacher training workshops, and online accessible teaching resources. The focus of the lab course is on the clarification and demonstration of physical and chemical principles known from school curricula that are important in PhInt. In the final stage, it will include analogue experiments, some real experiments, and theory parts with worksheets and problems to be solved.

The general public is addressed via exhibits in Deutsches Optisches Museum (D.O.M.). To achieve this, didactical input is given to all research projects in PhInt for a dissemination of their work. An individual science communication approach to transfer their research to museum exhibits is developed and discussed with the researchers. In the museum the challenge is to address the highly diverse audience with different cultural and educational backgrounds. It is tackled by two levels of explanation that can be selected by the visitors themselves. 

Every didactical concept, independent of its target group, must be evaluated with respect of its teaching goals. Evaluation in RP12 is done via qualitative and quantitative methods in test runs at the physics school student lab. This ensures both high quality and ongoing optimization of the materials, and thus strengthens the project outcome in total.