Reversible Photoisomerization of Norbornadiene-Quadricyclane within a Confined Capsule†.

With the desire to develop a sustainable green method to store and release solar energy via a chemical reaction, we have examined the well-investigated norbornadiene-quadricyclane (NBD-QC) system in water. In this context, we have employed octa acid (OA) as the host that forms a capsule in water. According to 1 H NMR spectra and diffusion constants, OA forms a stable 2:2 complex with both NBD and QC and 1:1:2 mixed complex in the presence of equal amounts of both NBD and QC. The photoconversion of NBD to QC within the OA capsule is clean without side reactions. In this case, OA itself acts as a triplet sensitizer. Recognizing the disadvantage of this supramolecular approach, in the future we plan to look for visible light absorbing sensitizers to perform this conversion. The reverse reaction (QC to NBD) is achieved via electron transfer process with methylene blue as the sensitizer. This reverse reaction is also clean, and no side products were detected. The preliminary results reported here provide "proof of principle" for combining green, sustainable and supramolecular chemistries in the context of solar energy capture and release.

Enzyme-Responsive Theranostic FRET Probe Based on l-Aspartic Amphiphilic Polyester Nanoassemblies for Intracellular Bioimaging in Cancer Cells.

The present study reports one of the first attempts on the design and development of an enzymatic-biodegradable theranostic fluorescence resonance energy transfer (FRET) probe constructed on l-amino acid polymer nanoassemblies and demonstrates the proof-of-concept in live cell bioimaging. l-Aspartic acid was converted into amide or carbamate pendants containing bis-carboxylic acid ester monomers, and they were subjected to melt polymerization along with commercial diols to produce amphiphilic aliphatic polyesters. Nanoparticles of size <200 nm were obtained because of self-assembly of these amphiphilic polyesters in an aqueous medium. These nanoparticles exhibited excellent encapsulation capability for green-fluorescent anti-inflammatory drug curcumin (CUR) and highly luminescent red-fluorophore Nile red (NR) to yield a CUR-NR theranostic FRET probe. Detailed photophysical studies were carried out to demonstrate photoexcitation energy transfer from CUR to NR for the occurrence of the FRET phenomena. The theranostic FRET probe was found to be very stable at extracellular environment and underwent biodegradation at the intracellular regions for delivery of the loaded cargoes. As a result, the theranostic FRET probe functioned as turn-on at the extracellular level and became turn-off at the intracellular level under lysosomal enzyme-responsiveness. The polymer nanoparticle was nontoxic to cells, whereas its CUR encapsulated nanoparticle showed relatively good cytotoxicity in breast cancer cell lines. Live cell confocal microscopy studies using lysotracker staining confirmed the colocalization of CUR as well as NR within the polymer nanoparticles in the lysosomes for enzymatic-biodegradation. Selective photoexcitation experiments in the confocal microscope were carried out to study the FRET probe action in cancer cells. Time-dependent FRET imaging directly supported the occurrence of FRET at the intracellular level and enabled the real-time drug release studies. The present approach opens natural resource-based biodegradable theranostic FRET probes for bioimaging application.

Highly Stable COF-Supported Co/Co(OH)2 Nanoparticles Heterogeneous Catalyst for Reduction of Nitrile/Nitro Compounds under Mild Conditions.

Ordered nanoporosity in covalent organic framework (COF) offers excellent opportunity for property development. Loading nanoparticles (nPs) onto them is one approach to introducing tailor-made properties into a COF. Here, a COF-Co/Co(OH)2 composite containing about 16 wt% of <6 nm sized Co/Co(OH)2 nPs is prepared on a N-rich COF support that catalyzes the release of theoretical equivalence of H2 from readily available, safe, and cheap NaBH4 . Furthermore, the released H2 is utilized for the hydrogenation of nitrile and nitro compounds to amines under ambient conditions in a facile one-pot reaction. The COF "by choice" is built from "methoxy" functionalized dialdehydes which is crucial in enabling the complete retention of the COF structure under the conditions of the catalysis, where the regular Schiff bonds would have hydrolyzed. The N-rich binding pockets in the COF ensure strong nP-COF interactions, which provides stability and enables catalyst recycling. Modeling studies reveal the crucial role played by the COF in exposing the active facets and thereby in controlling the activation of the reducing agent. Additionally, via density functional theory, we provide a rational explanation for how these COFs can stabilize nanoparticles which grow beyond the limiting pore size of the COF and yet result in a truly stable heterogeneous catalyst - a ubiquitous observation. The study underscores the versatility of COF as a heterogeneous support for developing cheap and highly active nonnoble metal catalysts.