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Abstract:

Melanins are ubiquitous natural pigments that color human skin, overripe bananas, cephalopod ink, and iridescent peacock feathers to name just a few examples. While sunscreening is the best-known function of melanin in skin, melanins also bind metal ions and scavenge radicals. Property emergence in melanins is of great interest not only for understanding their biological function but also for creating bioinspired, multi-functional materials that exploit melanin鈥檚 intrinsic electronic and ionic conductivity, redox activity, ability to stabilize free radicals, and broadband optical absorption. These properties are attractive for applications in bioelectronics, catalysis, energy conversion and storage. Remarkably, the atomistic structures present in melanins are obscure despite more than a century of effort, impeding understanding of their structure-function relationships. To gain insight into the chromophores of melanin, femtosecond transient absorption experiments have been carried out on synthetic melanin polymers. Transient spectral holes centered about the laser excitation wavelength are detected at room temperature, providing evidence of absorbers with a broad distribution of transition energies. The observed bleach recovery dynamics provide valuable insights into couplings among melanin鈥檚 chromophores. By combining femtosecond time-resolved infrared (TRIR) spectroscopy with the ability to select chromophore subensembles with a tunable UV-vis excitation pulse, a vibrational fingerprinting technique is demonstrated that correlates electronic and vibrational properties of melanin鈥檚 chromophores. These top-down studies are augmented by a bottom-up research program to construct a melanin mimic from structurally well-defined subunits. This work has led to intriguing stabilized quinones that reproduce several of melanin鈥檚 hallmark properties.

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Bio:

Bern Kohler is Professor and Ohio Eminent Scholar in the Department of Chemistry and Biochemistry at The Ohio State University. He and his co-workers study biopolymer photophysics and photochemistry using ultrafast laser spectroscopy. Further research projects include the study of fundamental, light-driven events in organic and inorganic nanomaterials of interest for solar energy conversion and photocatalysis. Prof. Kohler received a B.S. degree in chemistry from Stanford University (1985), a Ph.D. in physical chemistry from MIT (1990), and he completed postdoctoral research at the Swiss Federal Institute of Technology in Zurich, Switzerland and at the University of California, San Diego. Prof. Kohler is a fellow of the AAAS and the winner of the 2017 Inter-American Photochemical Society Award in Photochemistry. He is a Past-President of the Telluride Science Research Center and has served on the Executive Committee of the ACS Physical Division.