Department of Chemistry

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End date: 2019Subject: search.filters.subject.Fluorescence

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    Contrasting Solid-State Fluorescence of Diynes with Small and Large Aryl Substituents: Crystal Packing Dependence and Stimuli-Responsive Fluorescence Switching
    (ACS, 2015-09) Pati, Avik K.
    There has been a significant current interest in solid state luminescence of organic molecules and their stimuli responsive fluorescence switching behavior. Although small organic derivatives with olefinic, acetylenic, phenylenevinylenic, phenyleneethynylenic spacers are widely documented as solid state emitters in the literature, the solid state photophysics of organic derivatives with “butadiyne” spacer still remains unexplored. We provide detailed investigation on the solid state fluorescence properties of a series of butadiynyl fluorophores. Replacement of a phenyl ring, which is at periphery of the butadiyne bridge, with a large moiety such as pyrenyl group furnishes contrasting emissions in the solid state. While the butadiyne bridged phenyl derivatives show a blue shift of emission maxima in the solid powder with respect to monomer spectra in solution state, the butadiyne bridged pyrenyl derivatives exhibit a red shift in the solid state. The blue shift of the emission maxima of the butadiyne bridged phenyl derivatives in the solid powder is attributed to allowed excitonic transition in aggregates with nearly parallel transition dipoles. On the other hand, formation of pyrenyl excimer accounts for the red shift of the butadiyne bridged pyrenyl derivatives in the solid powder. In addition to that, the solid state fluorescence of the pyrenyl analogues is reversibly switched between two aggregate forms through external heating and rubbing stimuli.
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    Photophysics of Diphenylbutadiynes in Water, Acetonitrile–Water, and Acetonitrile Solvent Systems: Application to Single Component White Light Emission
    (ACS, 2016-07) Pati, Avik K.
    Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer, and intramolecular charge transfer (ICT) states in acetonitrile–water solvent systems. The LE, excimer, and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (−NMe2)–pull (−CN) substituents and those are closely overlapped for non-push–pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In the case of the push (−NMe2)–pull (−CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push–pull diphenylbutadiyne together cover the visible region (400–700 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push–pull character.
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    White Light Emission in Butadiyne Bridged Pyrene–Phenyl Hybrid Fluorophore: Understanding the Photophysical Importance of Diyne Spacer and Utilizing the Excited-State Photophysics for Vapor Detection
    (ACS, 2016-07) Pati, Avik K.
    Generation of white light emission (WLE) from a single organic fluorophore is challenging because of the need to get fluorescence covering the visible region (400–700 nm) upon excitation of the dye at near-ultraviolet wavelength. Herein, we report WLE from a butadiyne bridged pyrene–phenyl hybrid fluorophore in mixed-aqueous solvents as well as in polymer film matrices. The ability of the butadiynyl dye to emit from multiple excited states such as locally excited (LE; 400–500 nm), aggregate (excimer type; 475–600 nm), and charge transfer (CT; 500–750 nm) states spanning the emission almost throughout the visible range has made the generation of the white light to be possible. In highly polar solvent such as acetonitrile, the butadiynyl dye emits from the LE and CT states, and the WLE is achieved through a control of the dye concentration such that intermolecular CT (exciplex type) contributes along with the intramolecular CT and LE emissions. In mixed-aqueous systems such as water–acetonitrile and water–N,N-dimethylformamide, the CT emission is red-shifted (because of the high dielctric constant of water), and the contribution of the aggregate emission (originated because of the poor solvent water) is important in maintaining the relative distribution of the fluorescence intensities (LE, excimer, and CT) in the entire visible region. The significance of the diyne spacer in achieving the WLE is delineated through a control study with a single acetylenic analogue. The LE, aggregate, and CT emissions are involved in generating bluish-white light in a poly(vinyl alcohol) film matrix of the butadiynyl dye. Blue emission is noted in a poly(methyl methacrylate) (PMMA) film matrix of the dye with a major contribution from the LE and a minor contribution from the aggregate state. Exposure of the PMMA film of the dye to polar aprotic vapors assists in gaining the CT state emission such that the LE, aggregate, CT emissions cover the entire visible region to produce the WLE. This opens a new strategy for selective vapor sensing.
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    A Fluorescence Correlation Spectroscopy, Steady-State, and Time-Resolved Fluorescence Study of the Modulation of Photophysical Properties of Mercaptopropionic Acid Capped CdTe Quantum Dots upon Exposure to Light
    (ACS, 2013-10) Patra, Satyajit
    Light-induced modulation of the fluorescence behavior of mercaptopropionic acid (MPA) capped CdTe quantum dots (QDs) in aqueous solution is studied by a combination of fluorescence correlation spectroscopy (FCS) and steady-state and time-resolved fluorescence techniques. These investigations reveal a dramatic variation in the fluorescence properties of the QDs under exposure to light. In the FCS measurement, a large decrease in amplitude and change in shape of the correlation curves are observed with increasing excitation power. The change in the shape of the correlation curves, particularly at short lag time, e.g., a faster relaxation at high excitation power, is attributed to the increasing contribution of the off state of the QDs. Interestingly, despite this increasing contribution of the off state, which reduces the effective number of emitters in the observation volume and hence should increase the amplitude of the correlation curve, the latter actually decreases at high excitation power. This apparent contradiction is resolved by considering light-induced transformation of the dark QDs to bright QDs due to surface passivation of the QDs with increasing excitation power. Enhancement of the steady-state fluorescence intensity under light irradiation, both in aerated and deaerated environments, supports the mechanism of passivation of the surface trap states by photoadsorption of water molecules. Fluorescence lifetime data is also shown to be consistent with this light-induced surface passivation mechanism.
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    Effect of Capping Agent and Medium on Light-Induced Variation of the Luminescence Properties of CdTe Quantum Dots: A Study Based on Fluorescence Correlation Spectroscopy, Steady State and Time-Resolved Fluorescence Techniques
    (ACS, 2014-07) Patra, Satyajit
    The influence of ligand and solvent on light-induced modulation of the emission behavior of the quantum dots (QDs) has been studied for CdTe QDs capped with hexadecylamine (HDA), mercaptopropionic acid (MPA), and 1-(1-undecanethiol)-3-methyl imidazolium bromide (SMIM) in CHCl3, H2O, and [bmim][PF6] ionic liquid, respectively, using steady state and time-resolved fluorescence and fluorescence correlation spectroscopy techniques. While an aqueous solution of CdTe/MPA QDs exhibits fluorescence enhancement and a small blue shift of the emission peak (λmaxem) in the early stages of the light irradiation, such enhancement could not be observed in the case of CHCl3 solution of CdTe/HDA and [bmim][PF6] solution of CdTe/SMIM. Instead, exposure to light leads to a rapid reduction in luminescence intensity and large blue shift of λmaxem in the case of CHCl3 solution of CdTe/HDA and a very slow decrease of luminescence intensity with negligible shift of λmaxem in the case of an ionic liquid solution of CdTe/SMIM. The time-resolved fluorescence behavior of the QDs is found to be consistent with the steady state results. Fluorescence correlation spectroscopy measurements on the other hand reveal a large decrease of the amplitude of correlation at time zero [G(0)] for the aqueous solution of CdTe/MPA and negligible change in the G(0) value for the ionic liquid solution of CdTe/SMIM with increasing excitation power. The mechanism of these light-induced changes of the luminescence behavior of the QDs is investigated.
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    Diffusion of organic dyes in bovine serum albumin solution studied by fluorescence correlation spectroscopy
    (RSC, 2012-05) Patra, Satyajit
    The understanding of the transport of drugs and naturally occurring molecules in living cells and tissues requires a thorough knowledge of the diffusion behaviour of the molecular systems in these media. In this work, we studied the translational diffusion of three fluorescent molecules, electrically neutral coumarin 102 (C102), cationic rhodamine 6G (R6G) and anionic fluorescein (FL) in phosphate-buffered (pH 7) aqueous solutions of bovine serum albumin (BSA) protein in the absence and presence of common salt and urea using fluorescence correlation spectroscopy (FCS) by monitoring the fluorescence intensity fluctuations in a small confocal observation volume. The diffusion due to both free and BSA-bound molecules is observed in the case of the C102-BSA system. While no exchange between the bound and free states of the molecule is observed in this case, a rapid exchange between the two states is observed in the case of electrically charged hydrophilic dyes R6G and FL. This molecular picture, which is the first of its kind, is a reflection of a weaker binding of R6G and FL compared to C102 with the protein molecule. The binding sites of the probe molecules in BSA were identified based on the urea-induced change of diffusion of the probes in BSA.
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    Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides
    (ACS, 2019-09) Patra, Satyajit
    Single molecule detection provides detailed information about molecular structures and functions but it generally requires the presence of a fluorescent marker which can interfere with the activity of the target molecule or complicate the sample production. Detecting a single protein with its natural UV autofluorescence is an attractive approach to avoid all the issues related to fluorescence labeling. However, the UV autofluorescence signal from a single protein is generally extremely weak. Here, we use aluminum plasmonics to enhance the tryptophan autofluorescence emission of single proteins in the UV range. Zero-mode waveguide nanoapertures enable the observation of the UV fluorescence of single label-free β-galactosidase proteins with increased brightness, microsecond transit times, and operation at micromolar concentrations. We demonstrate quantitative measurements of the local concentration, diffusion coefficient, and hydrodynamic radius of the label-free protein over a broad range of zero-mode waveguide diameters. Although the plasmonic fluorescence enhancement has generated a tremendous interest in the visible and near-infrared parts of the spectrum, this work pushes further the limits of plasmonic-enhanced single molecule detection into the UV range and constitutes a major step forward in our ability to interrogate single proteins in their native state at physiological concentrations.
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    Extending Single-Molecule Förster Resonance Energy Transfer (FRET) Range beyond 10 Nanometers in Zero-Mode Waveguides
    (ACS, 2019-07) Patra, Satyajit
    Single-molecule Förster resonance energy transfer (smFRET) is widely used to monitor conformations and interaction dynamics at the molecular level. However, conventional smFRET measurements are ineffective at donor–acceptor distances exceeding 10 nm, impeding the studies on biomolecules of larger size. Here, we show that zero-mode waveguide (ZMW) apertures can be used to overcome the 10 nm barrier in smFRET. Using an optimized ZMW structure, we demonstrate smFRET between standard commercial fluorophores up to 13.6 nm distance with a significantly improved FRET efficiency. To further break into the classical FRET range limit, ZMWs are combined with molecular constructs featuring multiple acceptor dyes to achieve high FRET efficiencies together with high fluorescence count rates. As we discuss general guidelines for quantitative smFRET measurements inside ZMWs, the technique can be readily applied for monitoring conformations and interactions on large molecular complexes with enhanced brightness.
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    Fluorescence Blinking and Photoactivation of All-Inorganic Perovskite Nanocrystals CsPbBr3 and CsPbBr2I
    (ACS, 2016-01) Patra, Satyajit
    Study of the emission behavior of all-inorganic perovskite nanocrystals CsPbBr3 and CsPbBr2I as a function of the excitation power employing fluorescence correlation spectroscopy and conventional techniques reveals fluorescence blinking in the microsecond time scale and photoinduced emission enhancement. The observation provides insight into the radiative and nonradiative deactivation pathways of these promising substances. Because both blinking and photoactivation processes are intimately linked to the charge separation efficiency and dynamics of the nanocrystals, these key findings are likely to be helpful in realizing the true potential of these substances in photovoltaic and optoelectronic applications.
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    Study on metal nanoparticles synthesis and orientation of gemini surfactant molecules used as stabilizer
    (Elsiever, 2015-05-01) Pande, Surojit; Saha, Subit Kumar; Gangopadhyay, Subhashis
    In the present study, we report the synthesis of gold (Au), silver (Ag), and gold–silver alloy (Au–Ag) nanoparticles (NPs) by seed-mediated method using gemini surfactant, containing diethyl ether spacer group as a stabilizer. As-synthesized NPs are found very much stable and have been characterized using UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and zeta potential techniques. The orientation of gemini surfactant molecules surrounding the metal NPs has been investigated exploiting twisted intramolecular charge transfer (TICT) fluorescence properties of a probe 4-(N,N-dimethylamino) cinnamaldehyde (DMACA). The quenching efficiencies of different NPs have been performed in the fluorescence of DMACA and are found to be different. This effect can be related to the location of DMACA as well as the electro-negativity of the metals as the extent of orientation of the surfactant molecules around NPs controls the location of DMACA in a bilayer. To support the location of DMACA, fluorescence quenching studies with cetylpyridinium chloride (CPC) as an external quencher have also been carried out.