Shedding Light on Photocatalysis: Kerzig Group’s Ultrafast Research 

Prof. Dr. Christoph Kerzig is a Professor of Inorganic Chemistry and Photochemistry at Johannes Gutenberg University of Mainz (JGU Mainz), where he leads the Photochemistry Kerzig Group in the Department of Chemistry. Driven by the belief that photochemistry holds the key to solving many of the 21st century’s challenges, the group focuses exclusively on photochemical research.  

Their research vision is to develop and understand novel light-driven mechanisms and photoactive molecules with unmatched efficiency following a spectroscopy-guided approach. The group pursues two main research directions:  

• Designing more efficient mechanisms driven by one visible photon to improve the light-to-chemical energy conversion efficiencies. 

• Exploring two-photon mechanisms that have the potential to enable milder reaction conditions. 

Central to their strategy is a deep mechanistic understanding, achieved through a combination of numerous time-resolved spectroscopy techniques, lab-scale irradiation experiments, and theory. As part of their “make and measure” approach, the team synthesizes many of their own photoactive molecules and materials, encouraging students and researchers to master both synthesis and spectroscopy. 

THE ROLE OF SPECTROSCOPY IN THE KERZIG LAB 

At JGU Mainz, the Kerzig Group operates two laser-based spectroscopy laboratories, with flash photolysis as their core research method. This technique is used to characterize photoactive molecules and investigate both bimolecular photoreactions – light-induced interactions between two molecules – and complex reaction sequences on nanosecond to millisecond timescales. 

Their current setup includes a sensitive nanosecond transient absorption (TA) system equipped with a CCD camera, a photomultiplier tube (PMT) for TA detection, and two nanosecond Nd:YAG lasers for excitation. In addition to several instruments for steady-state optical spectroscopy, they also use a time-correlated single-photon counting (TCSPC) setup to monitor emission signals with lifetimes ranging from ~100 picoseconds to several microseconds.  

One of the group’s key research interests is the preparation and investigation of tailored bichromophoric photocatalysts that combine the beneficial properties of individual chromophores. These molecules contain two light-absorbing units (chromophores) that capture photons and work together to enhance light absorption or improve the efficiency of photochemical reactions. However, the intramolecular electron and energy transfer processes in such systems often occur on timescales too fast for their nanosecond TA setup to capture.  

“Previously, we had to rely on collaboration partners for ultrafast TA measurements, as a sophisticated femtosecond TA setup is out of reach for our group. Such systems usually require a permanently employed laser spectroscopist,” explains Prof. Kerzig.  

This changed with the introduction of HARPIA-LIGHT by Light Conversion – a compact, user-friendly, tabletop transient absorption spectroscopy system. Classified as a Class 1 laser product, HARPIA-LIGHT combines accessibility, versatility, and unparalleled performance in a compact, single-box design.  

 “With the user-friendly HARPIA-LIGHT setup, our group members could monitor the ultrafast processes in bichromophores or dyads directly after their preparation,” shares Prof. Kerzig.  

BRINGING ULTRAFAST MEASUREMENTS IN-HOUSE 

“Being able to record ultrafast TA data sets in our group significantly speeds up our research activities. On one hand, we no longer need to send all compounds of interest to collaboration partners, on the other, we can now study as many samples as we like under numerous conditions,” says Prof. Kerzig.  

“The system was easy to use for all my co-workers. Even without a strong spectroscopy background, I believe anyone could quickly learn how to operate it.” 

Upon receiving the HARPIA-LIGHT system, the group revisited a previously studied Coulombic dyad – a molecular system composed of two charged units interacting via electrostatic forces to influence the photochemical behavior. Earlier, this dyad was studied using a complex femtosecond TA system through a collaborative partner. With HARPIA-LIGHT, the team successfully replicated the key findings, including the rate constant and spectroscopic signatures for energy transfer between an osmium complex and a perylene counter anion.  

“Most importantly, we achieved this result within minutes, without any time-consuming beam adjustment and optimization,” shares Prof. Kerzig. 

NEW INSIGHTS AND FIRST PUBLICATION 

The team also has used HARPIA-LIGHT to investigate the intramolecular electron transfer dynamics in a bichromophoric photocatalyst comprising a ruthenium complex unit (to harvest visible photons) and a redox-active triarylamine TAA unit. The system enables the group to observe vibrational cooling, charge separation, and charge recombination, with the TAA radical cation being the most dominant spectroscopic signature at about 750 nm (Figure 1). 

These findings not only provided the group with a clearer picture of the system’s underlying photophysical and photochemical processes but also helped to understand the superior reactivity in photocatalysis. They also led to a scientific publication – the first using the HARPIA-LIGHT system. This marks a major milestone for both the Kerzig Group and Light Conversion, highlighting how modern technology can empower researchers to push the boundaries of ultrafast spectroscopy.