Chandra Shekhar Purohit

A double-stranded DNA rotaxane

Mechanically interlocked molecules such as rotaxanes and catenanes have potential as components of molecular machinery. Rotaxanes consist of a dumb-bell-shaped molecule encircled by a macrocycle that can move unhindered along the axle, trapped by bulky stoppers. Previously, rotaxanes have been made from a variety of molecules, but not from DNA. Here, we report the design, assembly and characterization of rotaxanes in which both the dumb-bell-shaped molecule and the macrocycle are made of double-stranded DNA, and in which the axle of the dumb-bell is threaded through the macrocycle by base pairing. The assembly involves the formation of pseudorotaxanes, in which the macrocycle and the axle are locked together by hybridization. Ligation of stopper modules to the axle leads to the characteristic dumb-bell topology. When an oligonucleotide is added to release the macrocycle from the axle, the pseudorotaxanes are either converted to mechanically stable rotaxanes, or they disassemble by means of a slippage mechanism to yield a dumb-bell and a free macrocycle. Our DNA rotaxanes allow the fields of mechanically interlocked molecules and DNA nanotechnology to be combined, thus opening new possibilities for research into molecular machines and synthetic biology.

Contrasting crystallographic signatures of Ag(I)- and Cu(II)-N6,N6′- bisadenine complexes: extended vs. foldback geometries

We report crystallographic studies with N6,N6′-crosslinked bisadenine derivative, where complexation with Ag(I) exhibits a pentacoordination mode affording a molecular box, while interaction with Cu(II) results in an intramolecular complex.

Mannosylated self-assembled structures for molecular confinement and gene delivery applications

This paper reports self-assembly of a lysine conjugated with a biantennary mannose to form spherical structures. These supramolecular structures are found to be hollow in nature and they afford effective encapsulation of alkaline phosphatase enzyme, plasmid DNA and a GFP reporter gene, which was transfected in COS-7 cells. Loaded hollow structures also get disrupted upon mild sonication, releasing encapsulated molecules thereby illustrating their potential for confinement and delivery applications.

Bio-inspired self-assembled molecular capsules

Bowl shaped molecules are useful for making molecular capsules with suitable non-covalent bonds. Bowl shaped cyclotriguaiacylene can be suitably modified at its phenolic groups for attaching suitable functionality to construct molecular capsules. In this work, a molecular capsule has been prepared by appending adenine and thymine to cyclotriguaiacylene. These blocks when mixed in a mixture of solvents give a molecular capsule as ascertained by mass and NMR spectroscopy.

Structural systematics of some metal complexes with 4,5-diazafluoren-9-one

AbstractSynthesis and X-ray structural characterization of three complexes of type cis-[M(dafone)2(NCS)2] (M = Co(II), 1; Ni(II), 2; Zn(II), 3; dafone = 4,5-diazafluoren-9-one) and a polymer cis-[Cd(dafone)(NCS)2]n (4) have been reported. Each of the four complexes is crystallized in orthorhombic crystal system. Structural study reveals that each metal(II) centre in the four complexes adopts distorted octahedral geometry with MN6 chromophore in 1-3 and MN4S2 chromophore in 4. The room temperature steady-state fluorescent intensity of dafone in dimethyl formamide at 402 nm is found to be quenched in these reported dafone complexes (1-4). Graphical AbstractSynthesis and X-ray structural characterization of three complexes of type cis-[M(dafone)2(NCS)2] (M = Co(II), 1; Ni(II), 2; Zn(II), 3; dafone = 4,5-diazafluoren-9-one) and a polymer cis-[Cd(dafone)(NCS)2]n (4) have been reported. The fluorescent intensity of dafone in DMF is found to be quenched in these reported dafone complexes (1-4).

A comparative study of optical nonlinearities of trans-A2B-corroles in solution and in aggregated state

A series of novel A3-corrole and trans-A2B-corroles have been synthesized with the aim of developing organic materials with improved nonlinear optical (NLO) properties. All three newly synthesized corroles have been characterized by various spectroscopic techniques including single crystal X-ray structural analysis of the representative one. The crystal structure analysis of 10-(4-hydroxyphenyl)-5,15-bis(2-bromo-5-fluorophenyl) corrole shows several O–H⋯N interactions. The self aggregates of all three corroles were prepared on a silicon wafer as well as on a quartz substrate by using a drop-casting method in a dichloromethane and methanol (1 : 2) solvent mixture. In all three free base corroles, well defined and nicely organized three-dimensional objects with diameter of ca. 320 nm (nanospheres), 450 nm (nanobulbs), and 120 nm (nanodiscs) were obtained. The NLO properties (nonlinear refractive index, n2 and two-photon absorption coefficient, β) of all of the corrole derivatives in toluene solution and in aggregated form were measured by the Z-scan technique. The nonlinear refractive indices, n2 of the free base corroles (in toluene solution) were found out to be −16.8 × 10−18 m2 W−1, −7.8 × 10−18 m2 W−1 and −25.9 × 10−18 m2 W−1 respectively and for the corresponding aggregates (nanoparticles of the free base corroles), it was found out to be −1.1 × 10−15 m2 W−1, −1.9 × 10−15 m2 W−1, and 71.8 × 10−15 m2 W−1 respectively. Similarly, the two-photon absorption coefficient, β of all the synthesized free base corroles (in toluene solution) were found out to be 5.7 × 10−15 m W−1, 1.9 × 10−15 m W−1 and 17.2 × 10−15 m W−1 respectively and for the corresponding aggregates (nanoparticles), the values were 4.0 × 10−13 m W−1, 2.0 × 10−13 m W−1, and 444.0 × 10−13 m W−1 respectively. These NLO properties of the free base corrole derivatives (in solution and in aggregates) have been explored with a specific aim to identify the possibility of their applications in ultrafast switching devices for use in high-speed fiber-optic communications and photonic integrated circuits.

Synthesis, electron transport, and charge storage properties of fullerene–zinc porphyrin hybrid nanodiscs

A donor (D)–acceptor (A) based system has been developed with the help of a newly synthesized Zn–porphyrin complex and C60 fullerene. A series of spectroscopic methods, absorption spectroscopy (UV-Vis-NIR), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements, confirm the existence of a charge transfer based complex between Zn–porphyrin and C60 fullerene. By using this D–A based system, circular disc-like nano objects have been generated. With the help of these nano objects, a diode has been fabricated. Current–voltage (I–V) and the capacitance–voltage (C–V) analyses of the diode were performed. The critical role of the Ag–C60–Zn–porphyrin and C60–Zn–porphyrin–SiOx interfaces are clearly understood from the I–V characteristics of the present device. Existence of a hysteresis loop in the C–V plot indicates that the device is indeed suitable for charge storage applications.

Consecutive introduction of Ag(I) to an anionic homoleptic Co(III) complex: variable Ag(I) coordination mode

Although the pyridine-2,6-diamide ligand is widely reported, coordination chemistry of the analogous pyrazine-2,6-diamide ligand has not been reported to date. To study the complexation behavior, we have synthesized N,N′-dibenzylpyrazine-2,6-dicarboxamide and the heterometallic complexes were in a stepwise manner. After dimerisation with Co(III) complexation, two ring nitrogen atoms are freely available. These two free pyrazine nitrogen atoms were studied for their coordination with Ag(I), resulting in three heterometallic complexes, C40H34AgCoN8O5, C42H38Ag3CoN10O11 and C42H53AgCoN9O13. The mode of coordination could be controlled by varying the metal to ligand ratio and varying the counter anion. Moreover, we found conditions that make the ligand to either coordinate or not coordinate to Ag+. Some of these complexes also display the rare organometallic (C–Ag) bond.

Tetra-kis-(μ-3-meth-oxy-benzoato-κO:O)bis[acetonitrilecopper(II)].

The centrosymmetric binuclear CuII title complex, [Cu2(C8H7O3)4(CH3CN)2], has a paddle-wheel-type structure [Cu—Cu distance = 2.6433 (3) Å]. Each CuII ion is coordinated by four O atoms from two 3-methoxybenzoate ligands and one acetonitrile N atom in a square-pyramidal geometry.

Hydrogen-bonded molecular capsules: probing the role of water molecules in capsule formation in modified cyclotricatechylene

Two new pyridine moiety-appended cavitands, CTC(Py)2(OH)2 and CTC(Py)3, were synthesized and characterized. The solid state structures of both cavitands were studied by single crystal X-ray diffraction. CTC(Py)2(OH)2 resulted in a hydrogen-bonded dimeric molecular capsule, entrapping two molecules of DMSO and intervening water molecules that formed hydrogen bonding to DMSO and CTC(Py)2(OH)2. When crystallized in the absence of water, it forms a 2D polymer by hydrogen bonding of pyridine nitrogen and phenolic hydrogen atoms. CTC(Py)3 forms no such capsule.