BITS Faculty Publications
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Item Atomistic modeling of two-dimensional electronic spectra and excited-state dynamics for a light harvesting 2 complex(ACS, 2015-01) Prajapati, Jigneshkumar DahyabhaiThe Light Harvesting 2 (LH2) complex is a vital part of the photosystem of purple bacteria. It is responsible for the absorption of light and transport of the resulting excitations to the reaction center in a highly efficient manner. A general description of the chromophores and the interaction with their local environment is crucial to understand this highly efficient energy transport. Here we include this interaction in an atomistic way using mixed quantum-classical (molecular dynamics) simulations of spectra. In particular, we present the first atomistic simulation of nonlinear optical spectra for LH2 and use it to study the energy transport within the complex. We show that the frequency distributions of the pigments strongly depend on their positions with respect to the protein scaffold and dynamics of their local environment. Furthermore, we show that although the pigments are closely packed the transition frequencies of neighboring pigments are essentially uncorrelated. We present the simulated linear absorption spectra for the LH2 complex and provide a detailed explanation of the states responsible for the observed two-band structure. Finally, we discuss the energy transfer within the complex by analyzing population transfer calculations and 2D spectra for different waiting times. We conclude that the energy transfer from the B800 ring to the B850 ring is mediated by intermediate states that are delocalized over both rings, allowing for a stepwise downhill energy transport.Item 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.Item Photophysical Impact of Diacetylenic Conjugation on Classical Donor–Acceptor Electronic Energy Pair(ACS, 2018-12) Pati, Avik K.Organic fluorophores with extended π-conjugation are important for their widespread applications. The present work provides photophysical insights into a diacetylene bridged classical donor–acceptor electronic energy pair, naphthalene–pyrene, in comparison with its constituents’ molecular structures, naphthyl and pyrenyl acetylenes, as well as parent naphthalene and pyrene chromophores. The diacetylenic dye loses the individual spectral identities of the donor and acceptor fluorophores exhibiting a locally excited (LE) emission (∼411 nm) from the overall molecular entity with high fluorescence quantum yields (0.55–0.84) in nonaqueous media. In contrast to the parent pyrene, the hybrid derivative shows a strongly allowed S0 → S1 transition. In mixed-aqueous media, the dye forms aggregates displaying a new red-shifted absorption (∼425 nm) as well as emission (∼510 nm) band. Unlike the hybrid dye, the naphthyl and pyrenyl acetylenes do not form aggregates. In the aggregate state of the hybrid fluorophore, electronic energy transfer takes place from the naphthyl moiety to pyrenyl ring. The excited-state photophysical properties of the dye are exploited in vapor sensing in the solid state.Item Synthesis of a 3,4-Disubstituted 1,8-Naphthalimide-Based DNA Intercalator for Direct Imaging of Legionella pneumophila(ACS, 2019-03) Sidhu, Jagpreet SinghThe development of organic molecules to target nucleic acid is an active area of research at the interface of chemistry and biochemistry, which involves DNA binding, nuclear imaging, and antitumor studies. These molecules bind with DNA through covalent interactions, electrostatic interactions, or intercalation. However, they are less permeable to membrane, and they have a significant cytotoxicity, which limits their application under in vivo conditions. In the present work, various mono- and disubstituted 1,8-naphthalimides-based derivatives (S-12, S-13, S-15, and S-21) have been synthesized and characterized through various spectroscopic techniques. Among these, 3-amino-4-bromo-1,8-naphthalimide (S-15) was found to have an attractive water solubility and act as a nuclear imaging agent. The spectroscopic absorption and emission data showed that S-15 has a strong affinity for salmon sperm DNA with a binding constant of 6.61 × 104 M–1, and the ratiometric fluorescence intensity (I489/I552) of S-15 has a linear relationship in the 0–50 μM range of DNA concentrations. It intercalates with DNA through the hydrophobic planar naphthalimide core as confirmed through cyclic voltammetry, circular dichroism, 1H NMR titration, and thermal denaturation studies. Positively charged amine groups also participate in H-bonding with the bases and backbone of DNA. The S-15 intercalator showed a large Stokes shift and photostability, which made it attractive for direct imaging of Legionella pneumophila, without the need for a prior membrane permeabilization.Item Long-Range Single-Molecule Förster Resonance Energy Transfer between Alexa Dyes in Zero-Mode Waveguides(ACS, 2020-03) Patra, SatyajitZero-mode waveguide (ZMW) nano-apertures milled in metal films were proposed to improve the Förster resonance energy transfer (FRET) efficiency and enable single-molecule FRET detection beyond the 10 nm barrier, overcoming the restrictions of diffraction-limited detection in a homogeneous medium. However, the earlier ZMW demonstrations were limited to the Atto 550–Atto 647N fluorophore pair, asking the question whether the FRET enhancement observation was an artifact related to this specific set of fluorescent dyes. Here, we use Alexa Fluor 546 and Alexa Fluor 647 to investigate single-molecule FRET at large donor–acceptor separations exceeding 10 nm inside ZMWs. These Alexa fluorescent dyes feature a markedly different chemical structure, surface charge, and hydrophobicity as compared to their Atto counterparts. Our single molecule data on Alexa 546–Alexa 647 demonstrate enhanced FRET efficiencies at large separations exceeding 10 nm, extending the spatial range available for FRET and confirming the earlier conclusions. By showing that the FRET enhancement inside a ZMW does not depend on the set of fluorescent dyes, this report is an important step to establish the relevance of ZMWs to extend the sensitivity and detection range of FRET, while preserving its ability to work on regular fluorescent dye pairs.Item Polar Solvation Dynamics of H2O and D2O at the Surface of Zirconia Nanoparticles(ACS, 1999-08) Pant, Debi D.Time-resolved fluorescence-upconversion spectroscopy has been used to study the polar solvation dynamics of H2O and D2O at the surface of zirconia (ZrO2) nanoparticles. While an isotope effect is observed for the solvation dynamics of bulk D2O, there is no isotope effect on the interfacial solvation dynamics. The interfacial solvation dynamics are the same for H2O and D2O and are faster than the bulk solutions. The bulk isotope effect is due to stronger hydrogen bonding in D2O compared to H2O, slowing the reorientation of the excited-state dipoles in the bulk D2O. The lack of isotope effect for the interfacial dynamics is explained in terms of the solvent interacting with the ZrO2 surface.Item Polar Solvation Dynamics of H2O and D2O at the Surface of Zirconia Nanoparticles(ACS, 1999) Pant, Debi D.Time-resolved fluorescence-upconversion spectroscopy has been used to study the polar solvation dynamics of H2O and D2O at the surface of zirconia (ZrO2) nanoparticles. While an isotope effect is observed for the solvation dynamics of bulk D2O, there is no isotope effect on the interfacial solvation dynamics. The interfacial solvation dynamics are the same for H2O and D2O and are faster than the bulk solutions. The bulk isotope effect is due to stronger hydrogen bonding in D2O compared to H2O, slowing the reorientation of the excited-state dipoles in the bulk D2O. The lack of isotope effect for the interfacial dynamics is explained in terms of the solvent interacting with the ZrO2 surface.Item Polar Solvation Dynamics in Nonionic Reverse Micelles and Model Polymer Solutions(ACS, 2000-12) Pant, Debi D.The effect of confinement on solvation dynamics has been explored in Brij-30/cyclohexane and Triton X-100/cyclohexane nonionic reverse micelles. Inside the reverse micelles, the polar solvation dynamics become slower and show additional slow relaxation modes not observed for bulk water. The solvation dynamics inside the Triton X-100 reverse micelles is slower than the dynamics inside Brij-30 reverse micelles. The results for solvation dynamics in the reverse micelles contrast solvation dynamics in aqueous tri(ethylene glycol) monoethyl ether solutions comparable to the reverse micellar interiors, which show significantly faster response. Measurements in the nonionic reverse micelles are also compared to previous work on ionic reverse micelles. Results reported here show that the interactions of water with the polyoxyethylene ether, as well as the micellar confinement inside these reverse micelles, effectively immobilize the water in the micellar interiors.Item Newly Designed Resorcinolate Binding for Ru(II)– and Re(I)–Polypyridyl Complexes on Oleic Acid Capped TiO2 in Nonaqueous Solvent: Prolonged Charge Separation and Substantial Thermalized 3MLCT Injection(ACS, 2013-01-23) Banerjee, TanmayFemtosecond pump–probe spectroscopic studies on a series of newly synthesized resorcinol-based Ru(II) and Re(I) complexes on oleic acid capped TiO2 nanoparticles have been carried out in chloroform, and the results are compared with those of the catechol analogues. The ruthenium complex shows biexponential injection; the second component arises due to injection from the thermally equilibrated 3MLCT states as a result of a weaker strength of the resorcinolate binding. Also, in comparison with catechol binding, as a result of a greater diffusion of the injected electrons into TiO2, the back electron transfer (BET) is slowed down significantly for the ruthenium complex. These are distinctive observations for any mononuclear ruthenium–polypyridyl–enediol complex reported thus far. However, the rhenium complex educes single exponential ultrafast injection (<120 fs) because of apparent injection in a high density of states and shows the most prominent results with ∼50% slowdown in the charge recombination rate as compared to the analogous catechol bound system. These results exemplify the probable development of highly capable sensitizer dyes with resorcinol as the anchoring group for the development of efficient dye-sensitized solar cells.Item Synthesis, Steady-State, and Femtosecond Transient Absorption Studies of Resorcinol Bound Ruthenium(II)- and Osmium(II)-polypyridyl Complexes on Nano-TiO2 Surface in Water(ACS, 2013-04-05) Banerjee, TanmayThe synthesis of two new ruthenium(II)- and osmium(II)-polypyridyl complexes 3 and 4, respectively, with resorcinol as the enediol anchoring moiety, is described. Steady-state photochemical and electrochemical studies of the two sensitizer dyes confirm strong binding of the dyes to TiO2 in water. Femtosecond transient absorption studies have been carried out on the dye–TiO2 systems in water to reveal <120 fs and 1.5 ps electron injection times along with 30% slower back electron transfer time for the ruthenium complex 3. However, the corresponding osmium complex 4 shows strikingly different behavior for which only a <120 fs ultrafast injection is observed. Most remarkably, the back electron transfer is faster as compared to the corresponding catechol analogue of the dye. The origin and the consequences of such profound effects on the ultrafast interfacial dynamics are discussed. This Article on the electron transfer dynamics of the aforesaid systems reinforces the possibility of resorcinol being explored and developed as an extremely efficient binding moiety for use in dye-sensitized solar cells.