Tuning Aqueous Supramolecular Polymerization by an Acid-Responsive Conformational Switch.

Besides their widespread use in coordination chemistry, 2,2'-bipyridines are known for their ability to undergo cis-trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self-assembly has remained unexplored. In this work, the use of 2,2'-bipyridines as acid-responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine-based linear bolaamphiphile, 1, that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2'-bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V-shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine-H+ units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli-responsive supramolecular materials.

Real Time Imaging and Dynamics of Hippocampal Zn2+ under Epileptic Condition Using a Ratiometric Fluorescent Probe.

Zinc, the essential trace element in human body exists either in the bound or free state, for both structural and functional roles. Insights on Zn2+ distribution and its dynamics are essential in view of the fact that Zn2+ dyshomeostasis is a risk factor for epileptic seizures, Alzheimer's disease, depression, etc. Herein, a bipyridine bridged bispyrrole (BP) probe is used for ratiometric imaging and quantification of Zn2+ in hippocampal slices. The green fluorescence emission of BP shifts towards red in the presence of Zn2+. The probe is used to detect and quantify the exogenous and endogenous Zn2+ in glioma cells and hippocampal slices. The dynamics of chelatable zinc ions during epileptic condition is studied in the hippocampal neurons, in vitro wherein the translocation of Zn2+ from presynaptic to postsynaptic neuronal bodies is imaged and ratiometrically quantified. Raman mapping technique is used to confirm the dynamics of Zn2+ under epileptic condition. Finally, the Zn2+ distribution was imaged in vivo in epileptic rats and the total Zn2+ in rat brain was quantified. The results favour the use of BP as an excellent Zn2+ imaging probe in biological system to understand the zinc associated diseases and their management.

Nanosheets of an Organic Molecular Assembly from Aqueous Medium Exhibit High Solid-State Emission and Anisotropic Charge-Carrier Mobility.

A π-conjugated amphiphilic diketopyrrolopyrrole (PDPP-Amphi) forms crystalline 2D supramolecular nanosheets in water when compared to that from methyl cyclohexane. These nanosheets exhibit high fluorescence quantum yield in the solid-state with anisotropic charge-carrier mobility of 0.33 cm2 V-1 s-1 .

A Three-Photon Active Organic Fluorophore for Deep Tissue Ratiometric Imaging of Intracellular Divalent Zinc.

Deep tissue bioimaging with three-photon (3P) excitation using near-infrared (NIR) light in the second IR window (1.0-1.4 µm) could provide high resolution images with an improved signal-to-noise ratio. Herein, we report a photostable and nontoxic 3P excitable donor-π-acceptor system (GMP) having 3P cross-section (σ3 ) of 1.78×10(-80)  cm(6) s(2) photon(-2) and action cross-section (σ3 η3 ) of 2.31×10(-81)  cm(6) s(2) photon(-2) , which provides ratiometric fluorescence response with divalent zinc ions in aqueous conditions. The probe signals the Zn(2+) binding at 530 and 600 nm, respectively, upon 1150 nm excitation with enhanced σ3 of 1.85×10(-80)  cm(6) s(2) photon(-2) and σ3 η3 of 3.33×10(-81)  cm(6) s(2) photon(-2) . The application of this probe is demonstrated for ratiometric 3P imaging of Zn(2+) in vitro using HuH-7 cell lines. Furthermore, the Zn(2+) concentration in rat hippocampal slices was imaged at 1150 nm excitation after incubation with GMP, illustrating its potential as a 3P ratiometric probe for deep tissue Zn(2+) ion imaging.

Heteroaromatic donors in donor-acceptor-donor based fluorophores facilitate zinc ion sensing and cell imaging.

The excited state intra molecular charge transfer (ICT) property of fluorophores has been extensively used for the design of fluorescent chemosensors. Herein, we report the synthesis and properties of three donor­π-acceptor­π-donor (D­π-A­π-D) based molecular probes BP, BT and BA. Two heteroaromatic rings, pyrrole (BP), and thiophene (BT) and a non-heteroaromatic ring N-alkoxy aniline (BA) were selected as donor moieties which were linked to a bipyridine binding site through a vinylic linkage. The heteroaromatic systems BP and BT perform selective and ratiometric emission signalling for zinc ions whereas the non-heteroaromatic probe BA does not. The advantages of the D­π-A­π-D design strategy in the design of ICT based probes for the selective fluorescent ratiometric signalling of zinc ions in biological media is discussed. Further, the use of BP, BT and BA for imaging Zn(2+) ions from MCF-7 cell lines is demonstrated.