Ayusman Sen

Cluster-Assembled Materials

Cluster-assembled materials offer the ability to tune component properties, lattice parameters, and thus coupling of physical properties through the careful selection and assembly of building blocks. Multi-atom clusters have been found to exhibit physical properties beyond those available from the standard elements in the periodic table; classification of the properties of such clusters effectively enables expansion of the periodic table to a third dimension. Using clusters as superatomic building blocks for hierarchically assembled materials allows these properties to be incorporated into designer materials with tailored properties. Cluster-assembled materials are currently being explored and methods developed to control their design and function. Here, we discuss examples of building block syntheses, assembly strategies, and property control achieved to date.

Catalytic motors for transport of colloidal cargo.

Autonomous micro- and nanomotors should, in principle, deliver materials in a site-directed fashion, powering the assembly of dynamic, nonequilibrium superstructures. Here we demonstrate that catalytic Pt-Au nanomotors can transport a prototypical cargo: polystyrene microspheres. In addition, motors with Ni segments can overcome both Brownian orientational fluctuations and biased rotation of the rod-sphere doublet to enable persistent steerable uniaxial motion in an external magnetic field. Assuming a cargo-independent motive force, the speeds are inversely proportional to the Stokes resistance, which we compute using a completed double-layer boundary integral equation. In addition, we demonstrate motors transporting cargo via chemotaxis toward a H2O2 fuel source.

Ortho -Phosphinobenzenesulfonate: A Superb Ligand for Palladium-Catalyzed Coordination–Insertion Copolymerization of Polar Vinyl Monomers

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.

Fantastic Voyage: Designing Self‐Powered Nanorobots

The use of swarms of nanobots to perform seemingly miraculous tasks is a common trope in the annals of science fiction.1 Although several of these remarkable feats are still very much in the realm of fiction, scientists have recently overcome many of the physical challenges associated with operating on the small scale and have generated the first generation of autonomous self-powered nanomotors and pumps. The motors can be directed by chemical and light gradients, pick up and deliver cargo, and exhibit collective behavior.

Electrochromic and optical waveguide studies of corona-poled electro-optic polymer films

Real-time monitoring of the corona-poling process that is used to create a bulk second-order nonlinear-optical susceptibility was accomplished by observing electrochromic shifts and intensity decreases of charge-transfer absorption bands in both dye-doped and covalently functionalized polymer films. By measuring small changes in the refractive-index anisotropy, the optical waveguiding technique was demonstrated to be a sensitive measure of the poling-induced order and its relaxation. The guest–host systems were formed from the dyes N,N-dimethylaminonitrostilbene, N,N-dimethylindoaniline (Phenol Blue), and 4-(N-(2-hydroxyethyl)-N-ethyl)-amino-4′-nitroazobenzene (Disperse Red 1), each dissolved in a poly(methyl methacrylate) matrix. The covalently functionalized polymers contained pendant para-nitroaniline (PNA) moieties. The first, poly(N-(4-nitrophenyl)allylamine), was formed from a poly(allylamine) derivative and is called PPNA. The second was based on poly(hydroxystyrene), with PNA attachment occurring between the phenol group and the PNA hydroxyethyl group; this polymer is named PHS-MENA. The final polymer is a linear epoxy (bisphenol A) with the PNA amino N atoms forming a link in the main chain; it is called Bis A-NA. A sample calculation demonstrated the use of experimental electrochromic spectral data to estimate the electro-optic coefficients.

Corona poling and real‐time second‐harmonic generation study of a novel covalently functionalized amorphous nonlinear optical polymer

Thin films for optical second‐harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p‐nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second‐order nonlinear coefficient, d33=31 pm/V, measured by the Maker‐fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen‐in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a...

Autonomous Nanomotor Based on Copper–Platinum Segmented Nanobattery

We describe an efficient, bubble-free nanoscale motor consisting of a copper-platinum (Cu-Pt) segmented rod that operates as a nanobattery in dilute aqueous Br(2) or I(2) solutions. The motion of the rod is powered by self-electrophoresis caused by redox reactions occurring on the two different metal segments. Asymmetric ratchet-shaped pure copper nanorods were also found to rotate and tumble in aqueous Br(2) solution because of the ion gradient arising from asymmetric dissolution of copper.

One‐Step Catalytic Transformation of Carbohydrates and Cellulosic Biomass to 2,5‐Dimethyltetrahydrofuran for Liquid Fuels

Existing technologies to produce liquid fuels from biomass are typically energy-intensive, multistep processes. Many of these processes use edible biomass as starting material. Carbohydrates, such as mono- and polysaccharides and cellulose, typically constitute 50-80% of plant biomass. Herein, we report that hexose from a wide range of biomass-derived carbohydrates, cellulose, and even raw lignocellulose (e.g., corn stover) can be converted into 2,5-dimethyltetrahydrofuran (DMTHF) in one step, in good yields and under mild conditions in water. Under the same conditions, 2-methyltetrahydrofuran is formed from pentose. The reaction employs a soluble rhodium catalyst, dihydrogen, and HI/HCl+NaI. The catalytic system is robust and can be recycled repeatedly without loss of activity. DMTHF is superior to ethanol and has many of the desirable properties currently found in typical petroleum-derived transportation fuels.

Chemical conversion pathways for carbohydrates

Biomass has emerged as a potential alternative feedstock to dwindling fossil fuel reserves. Starting in the 1990s, extensive research has been directed towards the synthesis of useful platform chemicals from cellulosic biomass. Chemical conversion processes of biomass have evolved as a parallel approach to thermochemical and enzymatic synthetic routes. In this review, we summarize the recent developments in liquid phase chemical conversions of monosaccharides, disaccharides, and polysaccharides. The reaction processes explored are hydrolysis, oxidation, reduction, hydrogenation, hydrogenolysis, esterification, etherification, glycosylation, dehydration, as well as the functionalization of the polysaccharide backbone. Our review follows a “process-driven” approach where the existing carbohydrate conversion pathways are classified according to the types of chemical processes involved.

A Polymerization-Powered Motor†

Research into nanoand micromotors powered by catalytic reactions, or more broadly the study of autonomous motion at the microand nanoscale, has become an area of great current interest. Potential applications include the delivery of materials, self-assembly of superstructures, roving sensors, and other emerging applications. The motors described to date involve the catalytic conversion of small molecules, which typically results in a gradient of charged or neutral species that in turn drives the motor. Polymerization-powered motion has been reported in biological systems, for example, listeria has been observed to move by actin polymerization. However, there have been no reports of motion at the nanoand micrometer scale driven by polymerization. Given the large repertoire of known organometallic polymerization catalysts, the design of polymerization-driven motors would considerably increase the scope of catalytic reactions that could be employed to power autonomous motion. Furthermore, polymerization reactions offer the unique opportunity to both power motion and simultaneously allow the deposition of polymer along the motion track. Herein, we present the first motor to be powered by a polymerization reaction outside biological systems. The motor is powered by ringopening metathesis polymerization (ROMP) of norbornene. These motors show increased diffusion of up to 70 % when placed in solutions of the monomer. Furthermore, the motors were observed to display the phenomenon of chemotaxis when placed in a monomer gradient; an extremely rare example outside biology. Generating motion by polymerization has been previously suggested, although not demonstrated. We chose to employ a form of Grubbs ROMP catalyst for our initial study because of its relatively high stability and high polymerization activity with norbornene (Figure 1). The motors were fashioned by first synthesizing gold–silica Janus particles. This was performed using 0.96 mm silica particles. These particles were deposited as thin films using a published method. Then gold was deposited onto the monolayers creating the asymmetric Janus particles. The particles were then chemically modified with the Grubbs catalyst on the silica side utilizing previously published methods (Supporting Information, Figure S1). XPS confirmed that the catalyst did attach to the motor surface. Catalytic activity was then tested by adding the functionalized particles to norbornene solutions and monitoring monomer consumption by gas chromatography. The turnover frequency (TOF) was found to be proportional to monomer concentration and begins to saturate at 1m norbornene (Supporting Information, Figure S2). SEM images of these particles before and after exposure to a monomer solution shows the formation of polymer at the particle surface (Figure 2). As discussed in the