Ashok Keerthi

Molecular transport through capillaries made with atomic-scale precision

Nanometre-scale pores and capillaries have long been studied because of their importance in many natural phenomena and their use in numerous applications. A more recent development is the ability to fabricate artificial capillaries with nanometre dimensions, which has enabled new research on molecular transport and led to the emergence of nanofluidics. But surface roughness in particular makes it challenging to produce capillaries with precisely controlled dimensions at this spatial scale. Here we report the fabrication of narrow and smooth capillaries through van der Waals assembly, with atomically flat sheets at the top and bottom separated by spacers made of two-dimensional crystals with a precisely controlled number of layers. We use graphene and its multilayers as archetypal two-dimensional materials to demonstrate this technology, which produces structures that can be viewed as if individual atomic planes had been removed from a bulk crystal to leave behind flat voids of a height chosen with atomic-scale precision. Water transport through the channels, ranging in height from one to several dozen atomic planes, is characterized by unexpectedly fast flow (up to 1 metre per second) that we attribute to high capillary pressures (about 1,000 bar) and large slip lengths. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement that we associate with an increased structural order in nanoconfined water. Our work opens up an avenue to making capillaries and cavities with sizes tunable to ångström precision, and with permeation properties further controlled through a wide choice of atomically flat materials available for channel walls.

Layered Electron Acceptors by Dimerization of Acenes End- Capped with 1,2,5-Thiadiazoles.

Layered electron acceptors D1-4 equipped with terminal 1,2,5-thiadiazole groups have been constructed using a one-pot protocol of acene dimerization. Their molecular structures are determined using single-crystal X-ray diffraction analysis. Photophysical and electrochemical properties of these molecules present a marked dependence on conjugation length and molecular geometry. An aggregation-induced emission peak and an intramolecular excimer emission (IEE) band were observed for D2 and D4, respectively. This work paves the way for the efficient synthesis of layered heteroacenes.

Cyclization of Pyrene Oligomers: Cyclohexa-1,3-pyrenylene

First synthesis of the macrocycle cyclohexa(1,3-pyrenylene) is achieved in six steps starting with pyrene, leading to a non-aggregating highly twisted blue-light-emitting material. The cyclodehydrogenation of the macrocycle offers a promising synthesis route to holey-nanographene.

Dithieno[2,3- d ;2′,3′- d ]benzo[2,1- b ;3,4- b ‘]dithiophene: a novel building-block for a planar copolymer

A planar heteroacene building block, dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;3,4-b′]dithiophene (DTmBDT), is reported via a facile synthetic procedure. Single-crystal X-ray diffraction of Br2-DTmBDT reveals that dodecyl chains interdigitate, still enabling close π-stacking of 3.42 A. A very high molecular weight quasi-planar copolymer PDTmBDT-DPP exhibited a high hole mobility of 0.36 cm2 V−1 s−1 in preliminary studies of organic field-effect transistors.

Magnetic edge states and coherent manipulation of graphene nanoribbons

Graphene, a single-layer network of carbon atoms, has outstanding electrical and mechanical properties1. Graphene ribbons with nanometre-scale widths2,3 (nanoribbons) should exhibit half-metallicity4 and quantum confinement. Magnetic edges in graphene nanoribbons5,6 have been studied extensively from a theoretical standpoint because their coherent manipulation would be a milestone for spintronic7 and quantum computing devices8. However, experimental investigations have been hampered because nanoribbon edges cannot be produced with atomic precision and the graphene terminations that have been proposed are chemically unstable9. Here we address both of these problems, by using molecular graphene nanoribbons functionalized with stable spin-bearing radical groups. We observe the predicted delocalized magnetic edge states and test theoretical models of the spin dynamics and spin–environment interactions. Comparison with a non-graphitized reference material enables us to clearly identify the characteristic behaviour of the radical-functionalized graphene nanoribbons. We quantify the parameters of spin–orbit coupling, define the interaction patterns and determine the spin decoherence channels. Even without any optimization, the spin coherence time is in the range of microseconds at room temperature, and we perform quantum inversion operations between edge and radical spins. Our approach provides a way of testing the theory of magnetism in graphene nanoribbons experimentally. The coherence times that we observe open up encouraging prospects for the use of magnetic nanoribbons in quantum spintronic devices.By functionalizing molecular graphene nanoribbons with stable spin-bearing nitronyl nitroxide radical groups, delocalized magnetic edge states are observed, with microsecond-scale spin coherence times.

Edge Functionalization of Structurally Defined Graphene Nanoribbons for Modulating the Self-Assembled Structures

Edge functionalization of bottom-up synthesized graphene nanoribbons (GNRs) with anthraquinone and naphthalene/perylene monoimide units has been achieved through a Suzuki coupling of polyphenylene precursors bearing bromo groups, prior to the intramolecular oxidative cyclo-dehydrogenation. High efficiency of the substitution has been validated by MALDI-TOF MS analysis of the functionalized precursors and FT-IR, Raman, and XPS analyses of the resulting GNRs. Moreover, AFM measurements demonstrated the modulation of the self-assembling behavior of the edge-functionalized GNRs, revealing that GNR-PMI formed an intriguing rectangular network. This result suggests the possibility of programming the supramolecular architecture of GNRs by tuning the functional units.

The Design of Radical Stacks: Nitronyl-Nitroxide-Substituted Heteropentacenes

Abstract The first alkyl chain‐anchored heteropentacene, dithieno[2,3‐d;2′,3′‐d′]benzo‐[1,2‐b;3,4‐b′]dithiophene (DTmBDT), mono‐ or disubstituted with a nitronyl nitroxide group has been prepared through a cross‐coupling synthetic procedure of the corresponding dibromo‐derivative (Br2‐DTmBDT) with a nitronyl nitroxide‐2‐ide gold(I) complex. The synthesized nitroxides possess high kinetic stability, which allowed us to investigate their structure and thermal, optical, electrochemical, and magnetic properties. Single‐crystal X‐ray diffraction of both mono‐ and diradicals revealed that the nitronyl nitroxide group lies almost in the same plane as the nearest side thiophene ring. Such arrangement favors formation of edge‐to‐edge dimers, which then form close π‐stacks surrounded by interdigitating alkyl chains. Before melting, these nitronyl nitroxide radical substituted molecules undergo at least two different phase transitions (PTs): for the monoradical, PTs are reversible, accompanied by hysteresis, and occur near 13 and 83 °C; the diradical upon heating shows a reversible PT with hysteresis in the temperature range 2–11 °C and an irreversible PT near 135 °C. PTs of this type are absent in Br2‐DTmBDT. Therefore, the step‐by‐step substitution of bromine atoms by nitronyl nitroxide groups changes the structural organization of DTmBDT and induces the emergence of PTs. This knowledge may facilitate crystal engineering of π‐stacked paramagnets and related molecular spin devices.

Publisher Correction: Magnetic edge states and coherent manipulation of graphene nanoribbons

In Fig. 1 of this Letter, there should have been two nitrogen (N) atoms at the 1,3-positions of all the blue chemical structures (next to the oxygen atoms), rather than one at the 2-position. The figure has been corrected online, and the original incorrect figure is shown as Supplementary Information to the accompanying Amendment.