Supramolecular nesting of cyclic polymers

Advances in template-directed synthesis make it possible to create artificial molecules with protein-like dimensions, directly from simple components. These synthetic macromolecules have a proclivity for self-organization that is reminiscent of biopolymers. Here, we report the synthesis of monodisperse cyclic porphyrin polymers, with diameters of up to 21 nm (750 C–C bonds). The ratio of the intrinsic viscosities for cyclic and linear topologies is 0.72, indicating that these polymers behave as almost ideal flexible chains in solution. When deposited on ​gold surfaces, the cyclic polymers display a new mode of two-dimensional supramolecular organization, combining encapsulation and nesting; one nanoring adopts a near-circular conformation, thus allowing a second nanoring to be captured within its perimeter, in a tightly folded conformation. Scanning tunnelling microscopy reveals that nesting occurs in combination with stacking when nanorings are deposited under vacuum, whereas when they are deposited directly from solution under ambient conditions there is stacking or nesting, but not a combination of both

Evidence confirms an anthropic origin of Amazonian Dark Earths

Archaeology of the America

Ultrafast delocalization of excitation in synthetic light-harvesting nanorings

Rings of chlorophyll molecules harvest sunlight remarkably efficiently during photosynthesis in purple bacteria. The key to their efficiency lies in their highly delocalized excited states that allow for ultrafast energy migration. Here we show that a family of synthetic nanorings mimic the ultrafast energy transfer and delocalization observed in nature. π-Conjugated nanorings with diameters of up to 10 nm, consisting of up to 24 porphyrin units, are found to exhibit excitation delocalization within the first 200 fs of light absorption. Transitions from the first singlet excited state of the circular nanorings are dipole-forbidden as a result of symmetry constraints, but these selection rules can be lifted through static and dynamic distortions of the rings. The increase in the radiative emission rate in the larger nanorings correlates with an increase in static disorder expected from Monte Carlo simulations. For highly symmetric rings, the radiative rate is found to increase with increasing temperature. Although this type of thermally activated superradiance has been theoretically predicted in circular chromophore arrays, it has not previously been observed in any natural or synthetic systems. As expected, the activation energy for emission increases when a nanoring is fixed in a circular conformation by coordination to a radial template. These nanorings offer extended chromophores with high excitation delocalization that is remarkably stable against thermally induced disorder. Such findings open new opportunities for exploring coherence effects in nanometer molecular rings and for implementing these biomimetic light-harvesters in man-made devices. This journal i

Linking rock age and soil cover across four islands on the Galápagos archipelago

The Galápagos Islands have been a place of preeminent scientific discoveries. The archipelago has formed on a tectonic plate moving over a volcanic hotspot, which has generated a chain of volcanic islands of different ages. This tectonic setting has favored the evolution of a unique flora and fauna on the islands; however, little is known on how it has influenced the formation and evolution of soils and belowground ecosystems. In order to enable the investigation of soil and ecosystem evolution, the aim of this study was to establish a soil chronosequence covering the full age range from the younger islands in the west to the older islands in the east of the archipelago. Six sites in the humid zones of the islands as well as two drier comparison sites were selected for this study. We characterized the geochemistry and mineralogy of the soil parent materials and constrained their ages using 14C (<10 ka) and 40Ar/39Ar dating. The parent materials are scoriaceous with bulk densities ≤1.6 g/cm3. Their major element composition is basaltic, including both alkali basalts and tholeiites. The scorias show amorphous contents between 20 and 85%, and their mineralogical composition is dominated by plagioclase and pyroxene. The numerical ages of the soil parent materials at the humid sites were: 1.45 ± 0.06 ka and 4.29 ± 0.09 ka on Isabela Island, 26 ± 7 ka on Floreana Island, 165.5 ± 11.6 ka on Santa Cruz Island, 825.6 ± 11.2 ka and 1070 ± 10 ka on San Cristóbal Island. Hence, the ages are distributed logarithmically covering a time span of approximately one million years. Along this chronosequence, we observed a non-linear increase in solum thickness, rubification degree and clay content with rock age. Changes were rapid in the initial phase of pedogenesis and more gradual in the later stages. Compared to the humid sites of the chronosequence, the pedogenic development at the dry comparison sites was much less advanced. The established chronosequence shall provide a solid basis for future studies on soil and ecosystem evolution in the unique environment of the Galápagos Islands