Membraneless organelles: Phasing in and out.

news & views librational modes) and thus changing how the sub-surface vibration couples with the surroundings. Common methods of optimizing the water-photolysis reaction conditions involve altering both the underlying semiconductor (through doping) and the electrolyte composition. This new vibrational mode could help to better understand the process on the molecular level, rather than relying on combinatorial methods and exploring an ever expanding parameter space. The observed vibrational mode is rather unique and truly interfacial in that it is localized in the sub-surface and couples to both bulk environments that it is in contact with: the hard semiconductor and the soft aqueous layer. While Fano resonances have been observed before for vibrations of molecules that are bound to a surface and coupled to the electronic states of a substrate, this is the first observation of a vibrational-mode coupling to the quasi-continuum of vibrational or librational modes of a solvent. The identification of such a distinct interfacial sub-surface vibration is quite remarkable. Not only is it remarkable, it could also be of great importance because monitoring chemical processes at interfaces is generally difficult. The interfacial region represents a very small fraction of any macroscopic sample, which makes singling out the interfacial molecules from the bulk experimentally challenging. Nonlinear optical experiments in the form of sum frequency generation spectroscopy can specifically probe surface vibrational modes while rejecting the large background of bulk vibrations due to the break of inversion symmetry at the surface 8. However, such surface-specific experiments impose further restrictions on the vibrational modes and the kind of samples that can be studied. In contrast, the vibrational mode discovered by Cuk and co-workers is inherently interfacial: critically it exists only at the interface with no bulk equivalent. Accordingly, the vibration and associated dynamics can be followed with time-resolved reflection spectroscopy without having to turn to nonlinear optical experiments. Because the oxyl-radical vibration couples to both the semiconductor electronic states responsible for driving the photocatalysis and the interfacial water librational modes directly involved in the water-splitting reaction, the new vibrational mode offers a window into the mechanism of and driving force for water photolysis. Cuk and co-workers observe that the vibration occurs within picoseconds of UV excitation and decays on the nanosecond time scale (subject to the reaction conditions). More generally, the unique vibrational mode could prove to be a new way of tracking the different steps of catalytic water-splitting: the exciton formation …