welcome to my website at the Chair for BioMolecular Optics of the LMU Munich.
The desire to utilize the energy from the sunlight for chemical processes has attracted a deep scientific interest all over the world. In contrast to photovoltaics in photocatalysis the incident photons from the sun are absorbed by a photocatalytic chromophore and are directly used to drive a chemical processes to form the desired product. My scientific work combines the fields of time resolved spectroscopy, quantum chemistry and organic chemistry to provide insights on the molecular level into the field of photocatalysis. Fundamental knowledge of intermolecular and intramolecular charge transfer processes is inevitable for a fundamental understanding of the unverlying intermediate steps of a photocatalytic process.
Below you can find a brief overview of on-going and completed projects:
Charge Transfer in Donor-Bridge-Acceptor Molecules
Light induced charge separation is one of the most important processes when light serves as driving force for catalytic processes. However, fast recombination of the photo-induced charge separated state often inhibits photocatalysis with a high quantum yield. Especially when the catalytic reaction is limited by diffusion, the life time of the catalytically active charge separated state is often too short for efficient electron transfer reactions between photocatalyst and substrate. Hence it is essential to establish strategies to achieve long-lived light-induced charge separated states. One approach is the photo-induced transfer of an electron from the electron donating chromophore to the catalytic active centre of the molecule, which is some distance away. This charge separated state is long-lived due to the slow charge recombination of electron and hole over a large distance. To control and optimize the photo-induced charge separation, the consolidated knowledge of the physics of the molecular charge transfer process is indispensable. A 3,5-dimethyl-4-(9-anthracenyl)-julolidine chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN)n bridges serves as donor-bridge-acceptor-system (DBA) for the investigation of the charge transfer processes through the fluorenone bridge molecules.
As expected for different bridge lengths (n=1-3) the rate of the electron transfer decreases with increasing distance between donor and acceptor. Marcus theory predicts an exponential dependence on the distance which can described with an attenuation factor beta . In the literature, a high beta value is assigned to a coherent tunneling mechanism through the nearly insulating bridge (superexchange), whereas a low beta value is assigned to an incoherent hopping electron transfer mechanism through the well conducting bridge. Although we observed a relatively high beta value of beta = 0.34/Å in the present DBA-system, we were able to observe incoherent hopping of the electron through the bridge. Usually for a beta-value of beta = 0.34/Å one would expect a coherent superexchange mechanism of the electron through the bridge molecules. 
 A. B. Ricks, K. E Brown, M. Wenninger, S. D. Karlen, Y. A. Berlin, D. T. Co, M. R. Wasielewski, Journal of the American Chemical Society 2012, 134, 4581−4588
Light-Driven Enantioselective Organocatalysis
Chirality is one of the most fascinating properties of matter. The quest to catalyze the synthesis of chiral molecules enantioselectively and the desire to understand the underlying reaction mechanism belong to the intellectually most challenging tasks of chemistry . Processes in which light energy serves as driving force for enantioselective bond formation require the design of chiral catalysts to harvest light and allow sensitization of the substrate by energy or electron transfer . We use chiral xanthone with a hydrogen-bonding motif as light-driven organocatalyst to foster the intramolecular [2+2] photocycloaddition of a substituted quinolone and to achieve a significant rate acceleration and a high enantioselectivity (94 % ee).
The triplet energy levels of xanthone and quinolone and the ultrafast intersystem crossing of xanthone within 1.5 ps suggest a triplet-triplet energy transfer mechanism to be the catalytic process as proposed in . However, ultrafast transient absorption spectroscopy provides a completely different picture: Excitation at λ = 355 nm, where only the xanthone catalyst absorbs, leads to an instantaneous transient signature of excited quinolone. Ab-initio calculations at the RI-CC2/TZVP level of theory show an excitonic coupling between catalyst and substrate which are only 0.3 nm apart from each other. The corresponding electronic transition leads to an excited singlet state delocalized over the xanthone and quinolone moieties which undergoes ultrafast ISC and ends in a delocalized triplet state with strong charge transfer character. This leaves the quinolone in the radical cation state as the initial step of the [2+2] photocycloaddition.
 C. Müller, A. Bauer, T. Bach, Angewandte Chemie International Edition 2009, 48, 6640-6642
 C. Müller, A. Bauer, M. M. Maturi, M. C. Cuquerella, M. A. Miranda, T. Bach, Journal of the American Chemical Society 2011, 133, 16689–16697
In photoredox catalysis photon absorption of the catalyst leads to an intermolecular electron transfer resulting in a charge transfer (CT) excited state. Establishing a long-lived CT-state is one of the basic principles of efficient solar energy conversion. If substantial spin-spin interaction between catalyst and substrate is still present, CT-states can be assigned pure singlet or triplet character. This configuration has significant influence on the CT lifetime because charge recombination in the triplet manifold is spin-forbidden. Hence charge separation between catalyst and substrate in the triplet manifold is a desired process for efficient photocatalysis. 
One of the most commonly used cofactors in nature is flavin where it acts as redox switch in many metabolic cycles. It shows a strong optical absorption in the visible and therefore it can be used as photocatalyst. After photo-excitation riboflavin tetraacetate, the flavin compound used by us as catalyst, undergoes intersystem crossing on the nanosecond timescale.
Using transient absorption spectroscopy from femto- to microseconds we positively identify the triplet state of flavin as the key intermediate in flavin-catalyzed photo-oxidation of methoxybenzyl alcohol (MBA). The electron transfer from MBA to the excited singlet state of flavin within some picoseconds is a competing process: it is followed by rapid back transfer within 50 ps as seen from femtosecond spectroscopy in pure MBA. In acetonitrile/water solutions with low concentrations of MBA, the S1 quenching is governed by diffusion and proceeds on the low nanosecond timescale leaving enough time for intersystem crossing.
Based on the observed rates of each individual step we determine the related quantum yields. So we were able to quantitatively model the concentration dependence of the product quantum yield of the photocatalytic reaction. This prediction for best possible conversion of light energy into chemical energy is indeed confirmed by direct reaction quantum yield measurements. 
 Jan W. Verhoeven, Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2006, 7, 40-60
 U. Megerle, M. Wenninger, R. J. Kutta, R. Lechner, B. König, B. Dick, E. Riedle, Physical Chemistry Chemical Physics 2011, 13, 8869-8880
Charge Transfer in DNA
Charge transfer processes through DNA have been intensively investigated over the last years using a variety of methods and model systems. In order to learn more about the dynamics and life time dependent charge injection into DNA, we studied the flavin triggered photooxidation of DNA hairpins by ultrafast transient absorption spectroscopy. We were also interested to investigate how these electron transfer (ET) processes could be harnessed to create novel systems with long-lived charge separated states that might lead to new principles for the conversion of light into chemical energy. Several oligonucleotides containing a covalent bound flavin molecule were synthesized. Depending on the base sequence we observed different quenching dynamics of the excited flavin. 
 M. Wenninger, D. Fazio, U. Megerle, C. Trindler, S. Schiesser, E. Riedle, T. Carell, ChemBioChem 2011, 12, 703-706
DFG-Graduiertenkolleg Chemische Photokatalyse