We characterise the structural properties of the quasi-liquid layer (QLL) at two low-index ice surfaces in the presence of sodium chloride (Na$^+/$Cl$^-$) ions by molecular dynamics simulations. We find that...

We investigate the effect of molecular geometry and conformational flexibility on electronic coupling and charge transfer interactions within propeller-shaped perylene diimide (PDI) tetramer arrays differing by the number of covalent linkages to a central spirobifluorene core. Electronic spectra of tetramers with one (“floppy”) or two (“rigid”) bay covalent linkages display evidence of charge transfer character in either ground or excited states. Floppy tetramers exhibit marked red-shifted and broadened absorption features that we assign as overlapping inter-PDI charge transfer and PDI-centered π–π* transitions, whereas rigid tetramers retain features similar to single PDI molecules, albeit with broader line widths. Interestingly, both tetramers exhibit charge transfer character in their fluorescence emission, but this is most prominent in the rigid tetramer, which displays dominant long-lived excimer behavior in addition to a minority component resembling single PDI-like emission. We then use single-molecule spectroscopy and imaging to understand how conformational-dependent charge transfer properties influence tetramer photophysics. Over 90% of single rigid tetramers display telegraphic (i.e., two-level) blinking behavior with relatively short “on” times compared to ∼60% of single floppy tetramer transients, which tend to exhibit emission from multiple levels. Electronic structure simulations were next performed to aid in the assignment of electronic transitions and photophysical behavior. Floppy tetramer canonical and natural transition orbitals reveal remarkable similarities with significant charge transfer character in the lowest energy excited states involving transverse PDI units and appreciable spirobifluorene contributions in the ground electronic state. Rigid tetramers exhibit greater electronic delocalization, and calculated absorption transition energies show good agreement with experiment, although excited-state interactions are less straightforward to discern from simulations. Raman spectroscopy and polarization-dependent single-molecule spectroscopy were also performed, supporting assignments based on theoretical predictions and electronic spectroscopy results. Overall, we demonstrate the importance of molecular geometry and conformational flexibility of multichromophore arrays in determining the nature of electronic interactions in ground and excited states, which can eventually be harnessed to improve performance attributes at the materials level.

Outstanding questions about the RNA world hypothesis for the emergence of life on Earth concern the stability and self-replication of prebiotic aqueous RNA. Recent experimental work has suggested that solid substrates and low temperatures could help resolve these issues. Here, we use classical molecular dynamics simulations to explore the possibility that the substrate is ice itself. We find that at -20 C, a quasi-liquid layer at the air/ice interface solvates a short (8nucleotide) RNA strand such that phosphate groups tend to anchor to specific points of the underlying crystal lattice, lengthening the strand. Hydrophobic bases, meanwhile, tend to migrate to the air/ice interface. Further, contacts between solvent water and ribose 2-OH' groups are found to occur less frequently for RNA on ice than for aqueous RNA at the same temperature; this reduces the likelihood of deprotonation of the 2-OH' and its subsequent nucleophilic attack on the phosphate diester. The implied enhanced resistance to hydrolysis, in turn, could increase opportunities for polymerization and self-copying. These findings thus offer the possibility of a role for an ancient RNA world on ice distinct from that considered in extant elaborations of the RNA world hypothesis. This work is, to the best of our knowledge, the first molecular dynamics study of RNA on ice.

We characterise the structural properties of the quasi-liquid layer (QLL) at two low-index ice surfaces in the presence of sodium chloride (Na + /Cl − ) ions by molecular dynamics simulations. We find that the presence of a high concentration of Na + /Cl − pairs changes the surface melting behaviour from step-wise to gradual melting. The ions lead to an overall increase of the thickness and the disorder of the QLL, and to enhanced surface roughness. The local structure of the QLL is similar to that of liquid water, and the differences between the basal and primary prismatic surface are attenuated by the presence of Na + /Cl − pairs. These changes modify the crystal growth rates of different facets and the solvation environment at the surface of sea-water ice with a potential impact on light scattering and environmental chemical reactions.