A time-resolved vibrational spectroscopy glimpse into the oxygen-evolving complex of photosynthesis

In memoriam of Professor Bridgette Barry (March 1, 1957-January 20, 2021)

We remember our colleague and friend, Bridgette Barry. Bridgette's numerous seminal contributions to photosynthesis research, beginning with her discovery that two functionally differentiated, redox active tyrosines are involved with the action of the oxygen-evolving complex, continue to impact studies of Photosystem II in laboratories around the world. Dr. Barry was widely recognized for employing clever adaptations of spectroscopic tools to probe unique experimental systems. Taken as a whole, she developed an in depth understanding of how biological proton coupled electron transfer reactions are subject to exquisite control over direction and kinetics. Bridgette dedicated significant time and energy in service to the scientific community. She was committed to her lab members and cared deeply that they each achieve their goals. She was an important role model and promoted women in her department and beyond. Bridgette will be deeply missed by the international scientific community and by those of us who had the honor to know her.

Mechanism of photocatalytic water splitting in TiO2. Reaction of water with photoholes, importance of charge carrier dynamics, and evidence for four-hole chemistry

We show for the first time that the photogenerated hole lifetime in TiO 2 is a strong determinant of the ability of TiO 2 to split water. Hole lifetimes were measured using transient absorption spectroscopy over a range of excitation intensities. The lifetimes of the holes were modulated by the use of exogenous scavengers and were also found to vary systematically with the excitation intensity. In all cases the quantum yield of oxygen production is found to be linked to the light intensity used, ranging from below 1 sun equivalent to nearly 1 sun equivalent. We also provide evidence that oxygen production requires four photons for each molecule of oxygen, which is reminiscent of the natural photosynthetic water-splitting mechanism. This in turn suggests a mechanism for oxygen production which requires four-hole chemistry, presumably via three, as yet unidentified intermediates. It is also shown that at excitation densities on the order of 1 sun, nongeminate electron-hole recombination limits the quantum yield significantly.