The origin of the apparent negative charge at hydrophobic-water interfaces has fueled one of the biggest debates in physical chemistry for several decades. The most common interpretation given to explain this observation is that negatively charged hydroxide ions (OH-) bind strongly to the interfaces. Here, using first principles calculations of the air-water and oil-water interfaces consisting of thousands of atoms, we unravel a mechanism that does not require the presence of OH-. We show that small amounts of charge transfer along hydrogen bonds and asymmetries in the hydrogen bond network, associated with local topological defects can lead to the accumulation of negative surface charge at both interfaces. For water near oil, we show that there is also some spillage of electron density into the oil leaving it negatively charged. The surface charge densities at both interfaces is computed to be approximately −0.015 e/nm 2 in agreement with electrophoretic experiments. We also show, using an energy decomposition analysis, that the electronic origin of this phenomena is rooted in a collective polarization and charge transfer effect.

Fluorescence in biological systems is usually associated with the presence of aromatic groups. Here, by employing a combined experimental and computational approach, we show that specific hydrogen bond networks can significantly affect fluorescence. In particular, we reveal that the single amino acid L-glutamine, by undergoing a chemical transformation leading to the formation of a short hydrogen bond, displays optical properties that are significantly enhanced compared with L-glutamine itself. Ab initio molecular dynamics simulations highlight that these short hydrogen bonds prevent the appearance of a conical intersection between the excited and the ground states and thereby significantly decrease nonradiative transition probabilities. Our findings open the door to the design of new photoactive materials with biophotonic applications.

Isorhodopsin is the visual pigment analogue of rhodopsin. It shares the same opsin environment but it embeds 9-cis retinal instead of 11-cis. Its photoisomerization is three times slower and less effective. The mechanistic rationale behind this observation is revealed by combining high-level quantum-mechanical/molecular-mechanical simulations with ultrafast optical spectroscopy with sub-20 fs time resolution and spectral coverage extended to the near-infrared. Whereas in rhodopsin the photoexcited wavepacket has ballistic motion through a single conical intersection seam region between the ground and excited states, in isorhodopsin it branches into two competitive deactivation pathways involving distinct conical intersection funnels. One is rapidly accessed but unreactive. The other is slower, as it features extended steric interactions with the environment, but it is productive as it follows forward bicycle pedal motion.

We report a linear-scaling density functional theory (DFT) study of the structure, wall-polarization absolute band-alignment and optical absorption of several, recently synthesized, open-ended imogolite (Imo) nanotubes (NTs), namely single-walled (SW) aluminosilicate (AlSi), SW aluminogermanate (AlGe), SW methylated aluminosilicate (AlSi-Me), and double-walled (DW) AlGe NTs. Simulations with three different semi-local and dispersion-corrected DFT-functionals reveal that the NT wall-polarization can be increased by nearly a factor of four going from SW-AlSi-Me to DW-AlGe. Absolute vacuum alignment of the NT electronic bands and comparison with those of rutile and anatase TiO2 suggest that the NTs may exhibit marked propensity to both photo-reduction and hole-scavenging. Characterization of the NTs' band-separation and optical properties reveal the occurrence of (near-)UV inside-outside charge-transfer excitations, which may be effective for electron-hole separation and enhanced photocatalytic activity. Finally, the effects of the NTs' wall-polarization on the absolute alignment of electron and hole acceptor states of interacting water (H2O) molecules are quantified and discussed.