Santanu Bhattacharyya

Detection of Hg2+ and F- ions by using fluorescence switching of quantum dots in an Au-cluster-CdTe QD nanocomposite.

A single probe of an Au nanocluster-CdTe quantum dots nanocomposite has been developed by using tripeptide-capped CdTe quantum dots (QD) and bovine serum albumin (BSA) protein-conjugated Au25 nanocluster (NC) for detection of both Hg(2+) ion and F(-) ion. The formation of Au-NC-CdTe QD nanocomposite has been confirmed by TEM, steady state and time resolved spectroscopy, CD and FTIR studies. A significant signal off (74 % PL quenching at 553 nm) phenomenon of this nanocomposite is observed in presence of 6.56×10(-7)  M Hg(2+) ion, due to salt-induced aggregation. However, a dramatic PL enhancement (128 %) of the Au-NC-CdTe QD nanocomposite is observed in presence of 8.47×10(-7)  M F(-) anion. The calculated limit of detections (LOD) of Hg(2+) ion concentration and F(-) ion concentration are found to be 9 and 117 nM, respectively, which are within the safety range set by the United States Environment Protection Agency. Thus, the simple Au-NC-CdTe QD optical-based sensor is very useful to detect both toxic cations and anions.

Multichromophoric Organic Molecules Encapsulated in Polymer Nanoparticles for Artificial Light Harvesting

We designed a self-assembled multichromophoric organic molecular arrangement inside polymer nanoparticles for light-harvesting antenna materials. The self-assembled molecular arrangement of quaterthiophene molecules was found to be an efficient light-absorbing antenna material, followed by energy transfer to Nile red (NR) dye molecules, which was confined in polymer nanoparticles. The efficiency of the antenna effect was found to be 3.2 and the effective molar extinction coefficient of acceptor dye molecules was found to be enhanced, which indicates an efficient light-harvesting system. Based on this energy-transfer process, tunable photo emission and white light emission has been generated with 14 % quantum yield. Such self-assembled oligothiophene-NR systems encapsulated in polymer nanoparticles may open up new possibilities for fabrication of artificial light harvesting system.

Excited State Features and Dynamics in a Distyrylbenzene-Based Mixed Stack Donor-Acceptor Cocrystal with Luminescent Charge Transfer Characteristics.

Combined structural, photophysical, and quantum-chemical studies at the quantum mechanics/molecular mechanics (QM/MM) level precisely reveal the structure-property relationships in a mixed-stack donor-acceptor cocrystal, which displays vibronically structured fluorescence, strongly red-shifted against the spectra of the parent donor and acceptor, with high quantum yield despite the pronounced CT character of the emitting state. The study elucidates the reasons for this unusual combination, quantifies the ordering and nature of the collective excited singlet and triplet state manifold, and details the deactivation pathways of the initially created Franck-Condon state.

Energy/Hole Transfer Phenomena in Hybrid α-Sexithiophene (α-STH) Nanoparticle–CdTe Quantum-Dot Nanocomposites

Considerable attention has been paid to hybrid organic-inorganic nanocomposites for designing new optical materials. Herein, we demonstrate the energy and hole transfer of hybrid hole-transporting α-sexithiophene (α-STH) nanoparticle-CdTe quantum dot (QD) nanocomposites using steady-state and time-resolved spectroscopy. Absorption and photoluminescence studies confirm the loss of planarity of the α-sexithiophene molecule due to the formation of polymer nanoparticles. Upon photoexcitation at 370 nm, a nonradiative energy transfer (73 %) occurs from the hole-transporting α-STH nanoparticles to the CdTe nanoparticles with a rate of energy transfer of 6.13×10(9) s(-1). However, photoluminescence quenching of the CdTe QDs in the presence of the hole-transporting α-STH nanoparticles is observed at 490 nm excitation, which is due to both static-quenching and hole-transfer-based dynamic-quenching phenomena. The calculated hole-transporting rate is 7.13×10(7) s(-1) in the presence of 42×10(-8)  M α-STH nanoparticles. Our findings suggest that the interest in α-sexithiophene (α-STH) nanoparticle-CdTe QD hybrid nanocomposites might grow in the coming years because of various potential applications, such as solar cells, optoelectronic devices, and so on.

Photoluminescence in Carborane–Stilbene Triads: A Structural, Spectroscopic, and Computational Study

A set of triads in which o- and m-carborane clusters are bonded to two stilbene units through Ccluster -CH2 bonds was synthesized, and their structures were confirmed by X-ray diffraction. A study on the influence of the o- and m- isomers on the absorption and photoluminescence properties of the stilbene units in solution revealed no charge-transfer contributions in the lowest excited state, as confirmed by (TD)DFT calculations. The presence of one or two B-I groups in m-carborane derivatives does not affect the emission properties of the stilbenes in solution, probably due to the rather large distance between the iodo substituents and the fluorophore. Nevertheless, a significant redshift of the photoluminescence (PL) emission maximum in the solid state (thin films and powder samples) compared to solution was observed; this can be traced back to PL sensitization, most probably due to more densely packed stilbene moieties. Remarkably, the PL absolute quantum yields of powder samples are significantly higher than those in solution, and this was attributed to the restricted environment and the aforementioned sensitization. Thus, the bonding of the carborane clusters to two stilbene units preserves their PL behavior in solution, but produces significant changes in the solid state. Furthermore, iodinated species can be considered to be promising precursors for theranostic agents in which both imaging and therapeutic functions could possibly be combined.

Tracking the Source of Carbon Dot Photoluminescence: Aromatic Domains versus Molecular Fluorophores

Carbon dots (CDs) are an intriguing fluorescent material; however, due to a plethora of synthesis techniques and precursor materials, there is still significant debate on their structure and the origin of their optical properties. The two most prevalent mechanisms to explain them are based on polycyclic aromatic hydrocarbon domains and small molecular fluorophores, for instance, citrazinic acid. Yet, how these form and whether they can exist simultaneously is still under study. To address this, we vary the hydrothermal synthesis time of CDs obtained from citric acid and ethylenediamine and show that in the initial phase molecular fluorophores, likely 2-pyridone derivatives, account for the blue luminescence of the dots. However, over time, while the overall size of the CDs does not change, aromatic domains form and grow, resulting in a second, faster decay channel at similar wavelengths and also creating additional lower energetic states. Electrophoresis provides further evidence that the ensemble of CDs consists of several subsets with different internal structure and surface charge. The understanding of the formation mechanism enables a control of the chemical origin of these emitters and the ensuing optical properties of the CDs through synthetic means.

Photophysical properties of ionic liquid-assisted porphyrin nanoaggregate–nickel phthalocyanine conjugates and singlet oxygen generation

In this report, we demonstrate the formation of ionic liquid (IL)-assisted zinc octaethylporphyrin (ZnOEP) nanoaggregates which is confirmed by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) studies. A large red shifted emission of ZnOEP nanoaggregates in comparison to ZnOEP in DCM confirmed the H aggregation which is due to intermolecular porphyrin–porphyrin (such as π–π/hydrophobic) interactions. The steady state and time resolved spectroscopic studies unambiguously confirm the H-aggregation formation of porphyrin molecules during nanoaggregate formation. The significant quenching of the fluorescence spectrum and the shortening of decay time of porphyrin nanoaggregates imply an efficient (89%) energy transfer from porphyrin nanoaggregates to phthalocyanine. Furthermore, the emission band observed at 1270 nm unambiguously confirms the singlet oxygen (1O2) generation from ZnOEP nanoaggregates which opens up further prospects in designing new IL-assisted porphyrin nanoaggregates for their application in photodynamic therapy.

Effect of nitrogen atom positioning on the trade-off between emissive and photocatalytic properties of carbon dots

Carbon dots (CDs) are a versatile nanomaterial with attractive photoluminescent and photocatalytic properties. Here we show that these two functionalities can be easily tuned through a simple synthetic means, using a microwave irradiation, with citric acid and varying concentrations of nitrogen-containing branched polyethyleneimine (BPEI) as precursors. The amount of BPEI determines the degree of nitrogen incorporation and the different inclusion modes within the CDs. At intermediate levels of BPEI, domains grow containing mainly graphitic nitrogen, producing a high photoluminescence yield. For very high (and very low) BPEI content, the nitrogen atoms are located primarily at the edge sites of the aromatic domains. Accordingly, they attract photogenerated electrons, enabling efficient charge separation and enhanced photocatalytic hydrogen generation from water. The ensuing ability to switch between emissive and photocatalytic behavior of CDs is expected to bring substantial improvements on their efficiency for on-demand light emission or energy conversion applications.Carbon dots are an emergent class of nanomaterials that hold promise for innovations in imaging, sensing, and catalytic technologies. Here, Stolarczyk and colleagues control the nitrogen-atom content and location within carbon dots, reporting the resulting impact on emissivity and photocatalytic behaviour.

Photoinduced energy transfer in dye encapsulated polymer nanoparticle–CdTe quantum dot light harvesting assemblies

Here, we have designed organic–inorganic light harvesting assemblies in which highly efficient resonance energy transfer occurs from CdTe quantum dots (donors) to Nile Red dye (acceptor) encapsulated polymer nanoparticles. Our motivation is to develop an assembly where the quantum dots (QDs) will absorb visible light as an antenna material, followed by the funneling of the exciton to an acceptor molecule (the Nile Red dye), which is confined in polymer nanoparticles in order to enhance their energy transfer efficiency. An ionic liquid is used to prepare the positively charged Nile Red (NR) dye encapsulated poly(methyl methacrylate) (PMMA) polymer nanoparticles. Then, negatively charged thioglycolic acid capped CdTe QDs are attached to the surface of the polymer nanoparticles by electrostatic interaction. The drastic quenching of the photoluminescence (60%) and the shortening of the decay time of the CdTe QDs imply an efficient energy transfer (73%) from the CdTe QDs to the NR dye doped PMMA nanoparticles. Time resolved anisotropy decay measurements reveal the rotational motion of the dye molecules inside the PMMA nanoparticles. Interesting findings reveal that the efficient energy transfer in the organic–inorganic assemblies may open up new possibilities for the design of an artificial light harvesting system for future applications.

Photoswitching and Thermoresponsive Properties of Conjugated Multi-chromophore Nanostructured Materials

Conjugated multi-chromophore organic nanostructured materials have recently emerged as a new class of functional materials for developing efficient light-harvesting, photosensitization, photocatalysis, and sensor devices because of their unique photophysical and photochemical properties. Here, we demonstrate the formation of various nanostructures (fibers and flakes) related to the molecular arrangement (H-aggregation) of quaterthiophene (QTH) molecules and their influence on the photophysical properties. XRD studies confirm that the fiber structure consists of >95% crystalline material, whereas the flake structure is almost completely amorphous and the microstrain in flake-shaped QTH is significantly higher than that of QTH in solution. The influence of the aggregation of the QTH molecules on their photoswitching and thermoresponsive photoluminescence properties is revealed. Time-resolved anisotropic studies further unveil the relaxation dynamics and restricted chromophore properties of the self-assembled nano/microstructured morphologies. Further investigations should pave the way for the future development of organic electronics, photovoltaics, and light-harvesting systems based on π-conjugated multi-chromophore organic nanostructured materials.