The Kohler group investigates the photophysical and photochemical dynamics of excited electronic states in the condensed phase. Molecular excited states are prepared ‘instantly’ with femtosecond laser pulses and interrogated as a function of time using electronic and vibrational spectroscopy. In analogy to microscopy, which provides access to miniscule spatial dimensions, femtosecond laser spectroscopy reveals the temporal evolution of the microscopic world. Current research projects seek to understand how DNA is damaged by UV light, and to develop photochemical approaches for harnessing energy from sunlight.
DNA Photophysics and Photochemistry
DNA is constantly damaged by solar UV light, triggering immunosuppression, cell death, and skin cancer. The Kohler group has pioneered a bottom-up approach for understanding excited electronic states created in DNA by UV light that began over a decade ago with the first accurate measurements of the fluorescence lifetimes of single DNA and RNA nucleosides. The ability to detect and observe DNA excited states has spawned a very active and interdisciplinary field that has rapidly advanced understanding of DNA’s electronic structure. Although most excitations in single DNA bases decay nonradiatively in hundreds of femtoseconds, much longer-lived excited states are observed in single- and double-stranded DNA.
DNA is an exquisitely customizable platform for exerting microscopic control over the spatial organization of interacting chromophores. This control and the self-assembling character of DNA are driving forces behind the fields of DNA photonics and DNA nanotechnology. At present, the ability to create novel structures exceeds a priori understanding of light-induced charge and energy transport in these materials. To fill this knowledge gap, work in the Kohler Group is focused on understanding how non-covalent interactions in DNA strands control the relaxation of excited electronic states.
Knowledge of deactivation pathways for excited electronic states in DNA is fundamental for understanding how nucleic acids are damaged by UV light, but the impact of this work extends beyond photobiology. Experimental work in DNA photophysics has created a deep symbiosis between experimental spectroscopists and electronic structure theorists that has rapidly advanced understanding and led to innovative computational approaches. Efforts to understand nonradiative decay in nucleobases have also led to fundamentally new insights into photochemical dynamics that are applicable to broad classes of molecular excited states. The combination of a tailorable multichromophoric molecule that is highly amenable to study by time-resolved spectroscopy and high-level quantum chemistry has put the field of DNA photophysics at the crossroads of frontier topics in chemical dynamics such as photoinduced PCET, nonadiabatic surface dynamics, and charge and exciton migration.
Mid-infrared transient absorption experiments performed in the Kohler Group have recently demonstrated that UV radiation efficiently triggers electron transfer between stacked nucleobases. These charge transfer states are similar to ones found in solar cells constructed of organic polymers, raising the intriguing possibility that understanding the fate of excited states in nucleic acids could reveal new strategies for directing energy or charge in multichromophoric systems made from organic building blocks, while minimizing undesired photochemical pathways that compromise photostability.
Some goals of our current research are to: 1) understand how base-stacking and base-pairing structure mediate electronic energy relaxation in single- and double-stranded DNAs; 2) determine the factors responsible for the significant site-to-site variation in DNA photoproduct yields in biologically relevant model systems, including DNA-protein complexes, and 3) systematically study photoinduced proton-coupled electron transfer (PCET) in double-stranded DNA model compounds by femtosecond time-resolved infrared spectroscopy.
Chen, J.; Zhang, Y.; Kohler, B. “Excited States in DNA Strands Investigated by Ultrafast Laser Spectroscopy,” Top. Curr. Chem. 2015, 356, 39 – 88.
Chen, J.; Kohler, B. “Base Stacking in Adenosine Dimers Revealed by Femtosecond Transient Absorption Spectroscopy,” J. Am. Chem. Soc. 2014, 136, 6362-6372.
Zhang, Y.; Dood, J.; Beckstead, A.; Li, X.-B.; Nguyen, K. V.; Burrows, C. J.; Improta, R. “Efficient UV-induced charge separation and recombination in an 8-oxoguanine-containing dinucleotide,” Proc. Natl. Acad. Sci. USA 2014, 111, 11612-11617.
Schreier, W. J.; Schrader, T. E.; Koller, F. O.; Gilch, P.; Crespo-Hernández, C. E.; Swaminathan, V. N.; Carell, T.; Zinth, W.; Kohler, B. "Thymine Dimerization in DNA is an Ultrafast Photoreaction," Science 2007, 315, 625-629.
Crespo-Hernández, C. E.; Cohen, B.; Kohler, B. “Base stacking controls excited-state dynamics in A-T DNA,” Nature 2005, 436, 1141-1144.
Excited-State Dynamics of Metal Complexes Introduction
Our research targets improved understanding of the photophysical and photochemical events that take place following excitation of metal-containing complexes and nanomaterials. The dynamics of charge separation and charge recombination are fundamental events that contribute to the photocatalytic splitting of water into hydrogen and oxygen, and which can be observed using femtosecond spectroscopy. New researchers can contribute to several projects.
Interligand Electron Transfer in Transition Metal Photosensitizers
Ruthenium(II) polypyridyl complexes are attractive building blocks for capturing solar energy on account of their intense metal-to-ligand charge transfer (MLCT) absorption and the redox activity of their long-lived triplet metal-to-ligand charge transfer (3MLCT) excited states. The MLCT excited states of ruthenium(II) polypyridyl complexes are best described as states in which a d electron from the metal ion is transferred to a π* orbital of a single ligand. The excited electron appears to be localized on one ligand at a time. When two or more types of ligands are present the distribution of 3MLCT excited states depends on the propensity of the different ligands to accept an electron. If the barriers separating the 3MLCT states localized on different ligands are sufficiently low, then the excited electron can transfer or hop from one ligand to another in a process known as interligand electron-transfer (ILET). Researchers in the Kohler Group use the transient absorption and time-resolved absorption anisotropy techniques to study the dynamics of ILET. Understanding ILET (or exciton hopping) is important for designing photosensitizers that optimally direct a photoexcited electron to a desired ligand.
Photophysics and Photochemistry of Metal Oxide Nanomaterials
Our work is currently focused on the high-valent metals cerium and iron. In spite of being a rare earth element, cerium is more abundant than cobalt, lead, and tin and is roughly as abundant as copper. Earth-abundant metals like cerium and iron are needed to make solar energy nanomaterials cost effective. Cerium(IV) ammonium nitrate (CAN) is widely used as a sacrificial oxidant in research into water oxidation catalysts and as a one-electron oxidant in organic synthesis. Meanwhile, the stable oxide of cerium (CeO2) is attracting a great deal of interest, particularly in nanocrystalline form. CeO2 is a wide bandgap semiconductor like better-known TiO2, but cerium ions can be easily switched between the 3+ and 4+ oxidation states by light. CeO2 is an important component in solid oxide fuel cells and as a mimic of superoxide dismutase that inhibits cellular damage by reactive oxygen species in vivo. Cerium(IV) hydrolyzes extensively in aqueous solution. We have discovered that cerium oxide nanocrystals form readily and spontaneously simply by dissolving CAN in water. Femtosecond transient absorption experiments are performed to understand the separation and recombination dynamics of charge carriers in CeO2. Iron(III) also hydrolyzes extensively in water leading to mono-, di-, and polynuclear complexes ligated by water molecules and hydroxo groups. UV excitation of iron(III) aquo species generates hydroxyl radicals that can be used to degrade organic pollutants. Deep UV femtosecond transient absorption spectroscopy is used to study the primary photoprocesses that occur in this photo-Fenton chemistry following oxygen-to-metal charge transfer.
Photoinduced Electron Transfer in Metallopolymers
Multimetal complexes containing ruthenium(II) and cobalt(III) are of interest as light-sensitized catalysts that are easily synthesized via “click” chemistry. The focus of our recent work is on understanding how the triazole linkers present in these materials mediate electron transfer between metal centers.