Manas Kumar Bera

Charge-Transfer-Induced Fluorescence Quenching of Anthracene Derivatives and Selective Detection of Picric Acid.

2,6-Divinylpyridine-appended anthracene derivatives flanked by two alkyl chains at the 9,10-position of the core have been designed, synthesized, and characterized by NMR, MALDI-TOF, FTIR, and single-crystal XRD. These anthracene derivatives are able to recognize picric acid (2,4,6-trinitrophenol, PA) selectively down to parts per billion (ppb) level in aqueous as well as nonaqueous medium. Fluorescence emission of these derivatives in solution is significantly quenched by adding trace amounts of PA, even in the presence of other competing analogues, such as 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (NP), nitrobenzene (NB), benzoic acid (BA), and phenol (PH). The high sensitivity of these derivatives toward PA is considered as a combined effect of the proton-induced intramolecular charge transfer (ICT) as well as electron transfer from the electron-rich anthracene to the electron-deficient PA. Moreover, visual detection of PA has been successfully demonstrated in the solid state by using different substrates.

Fluorene-based chemodosimeter for “turn-on” sensing of cyanide by hampering ESIPT and live cell imaging

A new salicylaldehyde appended fluorene-based chemodosimeter (FSal) has been designed by taking consideration of the special nucleophilicity of cyanide ion. FSal shows selective affinity towards CN- over other anions (namely F-, Br-, NO3 -, ClO4 -, N3 -, H2PO4 -, AcO-, I-, Cl-, and NO2 -) through turn-on fluorescence with a minimum detection limit of 0.06 ppm. The turn-on fluorescence of the FSal-CN complex resulting from hampering ESIPT is also supported by DFT and TDDFT calculations. Biological compatibility and live cell imaging of this unique probe have also been explored.

Amphiphilic and Thermoresponsive Conjugated Block Copolymer with Its Solvent Dependent Optical and Photoluminescence Properties: Toward Sensing Applications.

Herein, we present a new class of amphiphilic, thermoresponsive rod-coil conjugated block copolymer having regioregular poly(3-hexyl thiophene) and poly(N-isopropylacrylamide). Optical and luminescence properties of theses polymers significantly depend on the self-assembled nanostructures formed in different solvent and are easily tailored by chnaging the solvent composition or external stimuli like heat. Unique optical and electronic properties of this block copolymer are believed to make it promising for applications like sensor, fluorescence thermometer, optoelectronic, and bioelectronics devices.

Selective detection of cyanide by a polyfluorene-based organoboron fluorescent chemodosimeter

Organoboron-based polymeric fluorescent probes have been designed and synthesized for the sensitive and selective detection of toxic cyanide ions by a fluorescence “turn off” mechanism. Bis-(bromohexyl)-polyfluorenes have been synthesized by Pd catalyzed Suzuki cross-coupling polymerization. Its treatment with (Mes)2BF achieved organoboron-appended polyfluorene (PFBP). PFBP that is soluble in common organic solvents shows bright blue luminescence under UV-irradiation. The substantial quenching of the luminescence of a PFBP solution in the presence of toxic cyanide ions (CN−) has been observed. This is due to the formation of strong covalent bonds between the cyanide ions and trivalent boron centres of the side chains of the PFBP, which is further supported by NMR studies. The polyfluorene backbone of the PFBP acts as the fluorophore, and the boron centre as the cyanide acceptor. This novel type of polymeric material will find a potential application in sensor technology, as the detection limit is very low at ∼0.5 μM.

How the stereochemistry decides the selectivity: an approach towards metal ion detection

The stereochemistry of the coordination sites of a ligand plays a specific role in its binding with metals in a specific geometry. Herein, we designed and successfully prepared three different fluorene-based (A–B)n-type salen polymers (achiral FSP1, FSP2, and chiral FSP3), wherein the A-part is fluorophore and the B-part is the receptor. In the receptor, the coordination sites have four atoms (ONNO) that can bind any metal ion, but the orientation of ONNO differs in the three polymers. This orientation of the coordination site (i.e., the stereochemistry) into the receptor part of the polymer makes them more selective for a particular metal ion. In this study, it is shown that the orientation of the coordination sites of the receptor in the main chain polymer significantly determines the selective detection behavior for metal ions. Among the three polymers, FSP1 and FSP2 are sensitive towards different metal ions but are not selective towards any particular metal ion. However, in contrast, FSP3 is highly sensitive and selective to Zn2+ ions over other metal ions with a turn-on visible bright blue fluorescent color. This turn-on detection of the polymer is possibly due to the suppression of photo-induced electron transfer (PET) upon binding with Zn2+ ions. Theoretical calculations were also performed to show the orientation of the coordination sites. In FSP3, the coordination sites orient in a distorted tetrahedral fashion, which is very much prone to bind Zn2+ in a nearly tetrahedral geometry and that makes it more selective for Zn2+ ions only. The coordination geometry was also supported by 2D NMR studies. This report provides a template for the suitable design of a Zn2+ sensor, depending on the nature of the receptor incorporated into the main chain polymer.

A polyfluorene based zwitterionic fluorescent probe for response towards biological species in aqueous media

Water soluble zwitterionic fluorescent conjugated boronic acid-bearing polyfluorene (PFBA) has been prepared from poly(9,9′-(6′′-bromohexyl)fluorene-co-alt-1,4-phenylene) (polymer 3) through a post-polymerized quaternization with 3-pyridineboronic acid. Titration of diol containing monosaccharides (D-glucose and D-fructose), L-ascorbic acid and L-DOPA with PFBA polymer in 0.1 M phosphate buffer (pH 7.4) solution results in significant concentration dependent quenching of the blue fluorescence of the polymer due to static quenching as the bio-analytes form a ground state boronate ester complex with PFBA. Upon addition of D-glucose, the emission colour of PFBA changes to greenish-yellow owing to the effective induced aggregated polymer structure. PFBA also exhibits maximum response to the biological analytes at pH 7.4 which provides a scope for PFBA to be used in biological systems.

Solid state emissive organic fluorophores with remarkable broad color tunability based on aryl-substituted buta-1,3-diene as the central core

Multicolor emissive organic solid fluorophores are scarcely observed because of molecular aggregation in the condensed phase. However, rational molecular design is crucial to develop solid state emissive fluorophores from a simple core skeleton. Herein, the development of a series of small molecular derivatives and a conjugated copolymer having a (1Z,3Z)-1,4-diphenyl-1,3-butadiene unit as the central core has been reported. Notably, all fluorophores are highly emissive in the solid state and the emission colors cover the whole visible region from blue to red. Colour tuning has been simply achieved by varying the substituents attached with the aryl group of the central core. In addition, a partially twisted structure of the central core helps to reduce the intramolecular interaction and improve the photophysical properties of the fluorophores. Furthermore, electrochemical measurements indicate that polymer based fluorophores are potential candidates for organic photovoltaic (OPV) applications.

Salen-based enantiomeric polymers for enantioselective recognition

This paper describes how the spatial arrangement of the building blocks in a polymer chain influences the recognition properties of polymers. Here, we have designed and synthesized a series of fluorene based (A–B)n type salen polymers where the A-part is a fluorophore and the B-part is a recognition site using the condensation reaction of a fluorene based di(salicylaldehyde) compound (FSal) with diamines. Among the polymers, two are chiral (P1 and P2) which are enantiomeric (1R,2R vs. 1S,2S) to each other and three are achiral polymers (P3, P4 and P5). Circular dichroism studies of two chiral polymers reveal more or less equal and opposite CD intensities, indicating the induction of handedness in the polymer backbone. All polymers (P1–P5) contain the same recognition site but the spatial arrangements of the recognition sites are different from one another. We have studied the recognition properties of the polymers towards phenylglycinol. Only P1 and P2 exhibit good enantioselective recognition towards phenylglycinol through “turn-on” fluorescence enhancement because the microenvironment of the recognition sites is well organized to accommodate guest molecules. Three achiral polymers P3, P4 and P5 have no capacity to capture the guest molecules due to the lack of a suitable microenvironment at the recognition sites. This observation supports the phenomenon that the spatial arrangement of the building blocks in a main chain polymer determines its recognition properties. Moreover, P1 and P2 recognize the guest molecules through “turn-on” fluorescence (bright blue) in solution which gives a good platform for real application purposes.