Research

Molecular Biophysical Chemistry and Energy Conversion Dynamics

- Ultrafast photoinduced charge separation and migration in thin films for photovoltaics

- Photoinduced charge carrier dynamics of solar cell dyes on semiconductor thin flms

- Photoinduced dynamics of chiral molecular systems 

- Ultrafast photoinduced processes of carotenoids

- Femtosecond solvation dynamics in ionic liquids

- Time-resolved photoelectron spectroscopy of anions

- Photochemistry of molecular switches

- Energy transfer of highly excited molecules

Our Mission:

Research in our group deals with the spectroscopy, kinetics and dynamics of physicochemically and biophysically relevant processes in the condensed phase, in the gas phase, and at their interfaces. We also investigate clusters and nanostructures. We employ various laser-based spectroscopic techniques with high temporal (50 femtoseconds) or spectral (0.1 wavenumbers) resolution. Interpretation of the experimental results is supported by global kinetic modelling, classical trajectory calculations, MD simulations and quantum-chemical methods.

1) Ultrafast photoinduced charge separation and migration in thin films for photovoltaics (DFG project: PIs K. Oum and T. Lenzer)

Organic-inorganic hybrid perovskites, such as methylammonium lead iodide, are particularly promising for applications in solar light harvesting and optoelectronics. We investigate the carrier dynamics of such materials using ultrafast pump - supercontinuum probe spectroscopy in the 260-1600 nm range. The perovskite systems are investigated over a wide range of carrier densities. Slow carrier recombination processes, "phonon bottlenecks" during carrier cooling and confinement effects in low-dimensional perovskite structures are identified. We investigate electron injection processes from the perovskite into the mesoporous titania scaffold and estimate the relative contributions of electron transport pathways in the perovskite and titanium dioxide. We also study the hole transfer from the perovskite into triarylamine-based hole transport materials (HTMs).

2) Interactions of ionic liquids with embedded probe molecules (project in the framework of the SPP 1191 of the DFG) (PIs K. Oum and T. Lenzer)

Ionic liquids (ILs) are unique fluids consisting exclusively of ions and represent one of the current challenges for experimental investigations and theoretical modeling in chemistry. Knowledge of the microscopic interactions in ILs is eventually required to predict their properties and design task-specific ILs for applications in diverse fields, such as synthesis, (bio)catalysis and electrochemistry. Surprisingly, attempts to characterize interactions between ILs and solute molecules by powerful techniques based on ultrafast spectroscopy have been fairly sparse. Such information is however crucial, because it is the local surrounding or "cybotactic region" of a molecule in the IL which crucially determines its dynamics and chemical reactivity. We therefore apply ultrafast PSCP spectroscopy to elucidate interactions of ILs with embedded probe molecules, such as specific types of carotenoids.

3) Ultrafast dynamics of biophysically relevant systems

Carotenoids belong to the most abundant pigments in nature. They have important functions, such as light-harvesting and photoprotection. Short-chain derivatives, for example, retinal, the aldehyde of vitamin A, play a key role as chromophores in rhodopsin photoreceptor proteins and fultil diverse functions, such as energy conversion, cellular signaling, vision and photopigment regeneration. We investigate the ultrafast excited state dynamics of carotenoids and retinoids in organic solvents, supercritical fluids and ionic liquids of different polarity and polarizability using UV-Vis-NIR broadband transient absorption spectroscopy in the range 260-1600 nm. We establish comprehensive kinetic mechanisms including vibrational cooling (the so-called S* state), which provide a unified picture of the complex excited-state dynamics of such polyene systems. 

4) Femtosecond dynamics and spectroscopy of size-selected clusters

In this project, the reaction dynamics of anion clusters are investigated experimentally by means of femtosecond photoelectron spectroscopy. These negatively charged clusters represent prototypical systems for the solvation of charged ions in the condensed phase. The anion species of interest is prepared in a mass-selective fashion and subsequently excited by a femtosecond laser pulse. The dynamics of the systems are then followed in real-time using a second femtosecond laser pulse. In the center of our interest are the photodissociation and recombination dynamics of a variety of negative anions, their vibrational spectroscopy and reactions with molecules. The experiments are carried out in an environment with a well-defined number of solvent molecules (typically 0-100) and therefore establish the connection to investigations in the condensed phase. In addition, the results are interpreted using quantum-chemical and semi-classical calculations as well as molecular dynamics simulations.

5) Energy transfer of highly vibrationally excited molecules

In many chemical processes, from the condensed to the gas phase, energy transfer processes play a key role. Collisions stabilize "hot" products or heat up "cold" reactants. For the investigation of such collision processes we have employed the KCSI method and developed very recently the KCSF method. In these time-resolved pump-probe experiments we employ REMPI and fluorescence techniques using nanosecond lasers. This enables us for the first time to directly determine the central quantity for the characterization of collisional energy transfer, the collisional transition probability P(E',E). Detailed results have been obtained by us so far for toluene, azulene, pyrazine and very recently also for trans-stilbene. Accompanying trajectory simulations provide further information on the temperature dependence and the mechanism of energy transfer.