Sandipan Dawn

University of South Carolina, Columbia, South Carolina, United States

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Publications (4)28.7 Total impact

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    ABSTRACT: Confined environments can be used to alter the selectivity of a reaction by influencing the organization of the reactants, altering the mobility of trapped molecules, facilitating one reaction pathway or selectively stabilizing the products. This manuscript utilizes a series of potentially photoreactive guests to interrogate the utility of the one-dimensional nanochannels of a porous host to absorb and to facilitate the reaction of encapsulated guests. The host is a columnar self-assembled phenylethynylene bis-urea macrocycle, which absorbs guests including coumarin, 6-methyl coumarin, 7-methyl coumarin, 7-methoxy coumarin, acenaphthylene, cis-stilbene, trans-stilbene and trans-β-methyl styrene to afford crystalline inclusion complexes. We examine the structure of the host:guest complexes using powder X-ray diffraction, which suggests that they are well-ordered highly crystalline materials. Investigations using solid state cross-polarized magic angle spinning 13C{1H}CP-MAS NMR spectroscopy indicate that the guests are mobile relative to the host. Upon UV-irradiation, we observed selective photodimerization reactions for coumarin, 6-methyl coumarin, 7-methyl coumarin, and acenaphthylene, while the other substrates were unreactive even under prolonged UV-irradiation. Grand Canonical Monte Carlo simulations suggest that the reactive guests were close paired and preorganized in con-figurations that facilitate the photodimerization with high selectivity while the unreactive guests did not exhibit similar close pair-ing. A greater understanding of the factors that control diffusion and reaction in confinement could lead to the development of better catalysts.
    Full-text · Article · Jul 2014 · The Journal of Physical Chemistry A
  • Sandipan Dawn · Linda S. Shimizu · Harry Bermudez

    No preview · Conference Paper · Sep 2013
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    ABSTRACT: We report herein the construction of a metal organic polymeric framework from a bipyridine macrocyclic ligand and silver triflate. The ligand consists of two triazinanone protected urea groups and two bipyridine units that coordinate with the Ag atoms in crystallographically different manners. In a unit cell of the crystal there are three independent Ag atoms and two ligands that form a basket shape enclosing two of the silver atoms. Two silver atoms are 3.13 Å apart, suggesting an Ag–Ag bond interaction. We probe the characteristics of the complex by solid-state spectroscopic techniques including UV–vis, Fluorescence, IR, NMR and Raman spectroscopy. The Raman spectra suggest that a bonding interaction indeed exists between two Ag atoms and that this proximity is not merely a packing artifact. A third silver atom helps organize these boxes into an elongated polymeric chain, and ultimately these chains stack into layers with the adjacent chains to give an intriguing supramolecular structure.
    Full-text · Article · Jan 2012 · Inorganic Chemistry Communications
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    ABSTRACT: There is much interest in designing molecular sized containers that influence and facilitate chemical reactions within their nanocavities. On top of the advantages of improved yield and selectivity, the studies of reactions in confinement also give important clues that extend our basic understanding of chemical processes. We report here, the synthesis and self-assembly of an expanded bis-urea macrocycle to give crystals with columnar channels. Constructed from two C-shaped phenylethynylene units and two urea groups, the macrocycle affords a large pore with a diameter of ∼9 Å. Despite its increased size, the macrocycles assemble into columns with high fidelity to afford porous crystals. The porosity and accessibility of these channels have been demonstrated by gas adsorption studies and by the uptake of coumarin to afford solid inclusion complexes. Upon UV-irradiation, these inclusion complexes facilitate the conversion of coumarin to its anti-head-to-head (HH) photodimer with high selectivity. This is contrary to what is observed upon the solid-state irradiation of coumarin, which affords photodimers with low selectivity and conversion.
    No preview · Article · May 2011 · Journal of the American Chemical Society
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    ABSTRACT: We report herein the synthesis and metal complexation properties of two macrocyclic hosts that contain two 2,2'-bipyridines and two urea groups. These hosts take advantage of the conformationally mobile 5,5'-positions of the bipyridines to give metal binding sites that are dynamic. By simple bond rotation, these hosts can exchange an interior (endo) situated metal binding site for an exterior (exo) binding site. We examine the solid-state structures of the two free hosts and two coordination complexes ([Cd(host 1)(H(2)O)(NO(3))(2)] and [Ag(2)(host 2)](SO(3)CF(3))(2)) using X-ray crystallography. Analysis of these crystal structures suggests that the bipyridine groups within the hosts are able to rotate to access multiple conformations including the desired exo and endo conformations. We also investigate the binding affinity of these new ligands in solution by UV-vis titrations with a series of metal nitrate salts (Ag, Cd, Zn, Ni, Mn, Fe, Co, Cr, and Cu) to afford discrete metal complexes. Some complexes showed a slow subsequent assembly to yield coordination polymers. Thus, these systems may afford unique insights into the process of metal organic framework formation.
    No preview · Article · Nov 2009 · Journal of the American Chemical Society