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Research
Functional Materials
We use synthetic chemistry to prepare structurally and functionally encoded molecular building blocks, molecular recognition and self-assembly to assemble these building blocks into functional materials, and various spectroscopic, structural, and analytical techniques to explore their properties and functions. Our interdisciplinary research program seeks to unveil fundamental knowledge in materials and supramolecular chemistry and apply this knowledge to solve current problems in the following areas.

Stimuli-Responsive Metal–Organic Frameworks Based on Functionally Tunable Ligands
We are developing stimuli-responsive metal–organic frameworks (MOFs) using chemically and optically tunable ligands that can (1) control network interpenetration, (2) bind analytes selectively, and (3) display discrete guest-induced optical signals. The introduction of optically tunable, multifunctional ligands into MOFs will help expand their utility for a range of new applications, including sensing, imaging, light-harvesting, and light-emitting that would entail optically tunable materials. 
Furthermore, we have demonstrated that (ChemComm 2013, 49, 6629) the anion binding property of π-acidic naphthalenediimide (NDI) ligands can be exploited to develop non-interpenetrated square-grid MOFs. The anion exchange behavior of NDI-based cationic MOFs and their ability to sequester charge-diffused anions, such as perchlorate (an explosive used in rocket fuel), pertechnetate (a radioactive anion present in nuclear wastes), and perrhenate (used in nuclear medicine) via selective crystallization are currently under investigation.

Anion Recognition and Sensing by π-Acidic Naphthalenediimide Receptors
We have discovered that (JACS 2010, 2011, 2012; CrystEngComm 2012; OBC 2013) depending on the electron-donating ability of anions (i.e., Lewis basicity) and the electron-accepting ability (i.e., π-acidity) of π-electron deficient receptors, such as naphthalenediimides (NDIs) and perylene diimides (PDIs), the nature of their interactions in aprotic solvents varies from anion–π interactions to charge-transfer (CT) and formal electron transfer (thermal and photoinduced ET) events. While weak anion–π interactions involving non-Lewis basic anions do not perturb the electronic and optical properties of π-acidic receptors, formal ET from strongly Lewis basic anions, such as hydroxide, fluoride, and cyanide reduce colorless NDIs to orange colored NDI•– radical anion and pink colored NDI2– dianion in stepwise fashion. Preorganization of two NDI units into overlapping positions in tweezers-shaped receptors improves their fluoride sensitivity to nM level. These supramolecular/electronic interactions of Lewis basic anions can be exploited for visually detecting some of these toxic anions. We are currently investing the possibility of cooperative binding and stimuli-driven release of salts (ion-pairs) using NDI-based macrocyclic and foldameric receptors.

Multichromophoric Supramolecular Solar Cells and Photocatalytic Oxidation of Water
In order to enhance the efficiency of light to electricity throughout the visible region, we have constructed dye-sensitized solar cells (DSSCs) by assembling multiple chromophores and electron donors and acceptors, such as Zn-porphyrin, Zn-phthalocyanine, and perylene dyes on titania-coated ITO electrodes (Chem Comm 2012, 48, 8775). These dyad-based DSSCs show much higher energy conversion efficiencies than those comprised of  individual dyes. We are trying to extend this work for photocatalytic water oxidation for hydrogen fuel production. 

Stimuli-Responsive Vesicles and Nanotubes Based on Amphiphilic Macrocycles
We have constructed amphiphilic hexaamide macrocycles (ChemComm 2013, 49, 4601) via template directed synthesis and self-assembled them into open and closed vesicles by controlling their functionalities and solvent medium. Protonation these vesicles transforms them into nanotube and fibers, while nonpolar solvents convert them into flakes and foils. The stimuli-responsive morphological changes of these nanostructures can be exploited for controlled loading and delivery of drugs and catalysts. 


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