BITS Faculty Publications

Permanent URI for this communityhttp://localhost:4000/handle/123456789/1867

Browse

Has files: NoSubject: search.filters.subject.Absorption

Search Results

Now showing 1 - 10 of 21
  • No Thumbnail Available
    Item
    Meta Effect of Absorption Energy in Donor–Acceptor Substituted Benzenoids: A Computational Study of Its Dependence on Acceptor Strength, Solvent Polarity, and Conjugation Length
    (ACS, 2014-08) Pati, Avik K.
    The present work focuses on theoretical understanding of electronic absorption energies of N,N-dimethylaniline with different ortho-, meta-, and para-substituted acceptor groups. The meta isomers exhibit the lowest absorption energy compared to the ortho and para derivatives. This unusual behavior of absorption energies of the meta isomers is related to the “meta effect” well-known in organic photochemical reactions. The meta effect of absorption energy of the derivatives is found to depend on the strength of acceptors, solvent polarity, and conjugation length. The meta derivatives with strong acceptor groups generally exhibit the lowest absorption energy over the other isomers irrespective of solvent polarity. However, the meta isomers with weak acceptor groups exhibit the meta effect only in highly polar solvents. The trend of the lowest absorption energies of the meta isomers is observed to change if the acceptor group is bridged through π conjugation unit (n) with the core moiety. The normal pattern of absorption energy that is the para isomer is of lowest energy is observed to occur for the derivatives where the repeated conjugation units (n) are between 2 and 4. The normal pattern of absorption energy is continued to observe from n > 4 for all the derivatives.
  • No Thumbnail Available
    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.
  • No Thumbnail Available
    Item
    Experimental and Theoretical Investigation into the Polaron Structure of K-Doped Polyfluorene Films
    (ACS, 2020-12) Ghosh, Sarbani
    The evolution of the electronic structure and optical transition upon n-doping of poly(9,9-dioctylfluorene) (PFO) films is elucidated with photoelectron spectroscopy, optical absorption, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations. Optical absorption measurements extending into near infrared show two low-energy absorption features at low doping ratios and an additional peak at a higher energy of ∼2.2 eV that disappears with increasing doping ratios. A gap state (i.e., polaronic state) close to the Fermi level and a significantly destabilized highest valence band appear in the experimentally measured ultraviolet photoelectron spectra. These experimental results are interpreted by the TD-DFT calculations, which show that the lower energy peaks originate from the excitation from polaronic states to the conduction band, while the higher energy peak mainly originates from the destabilized valence band to conduction band transitions and only appears at low doping ratios (cred ≤ 50%, 0.5 potassium atom per fluorene monomer). The DFT calculations further indicate that polaron pairs rather than bipolarons are preferentially formed at high doping ratios. Comparing the results of doped glassy and β-phase films, we find that the ordered segments in the β-phase film disappear due to the dopant (potassium) insertion, resulting in a similar polaronic structure.
  • No Thumbnail Available
    Item
    Synthesis of a 3,4-Disubstituted 1,8-Naphthalimide-Based DNA Intercalator for Direct Imaging of Legionella pneumophila
    (ACS, 2019-03) Sidhu, Jagpreet Singh
    The 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.
  • No Thumbnail Available
    Item
    Solvent effect on absorption and fluorescence spectra of cinchonine and cinchonidine dications: Estimation of ground and excited state dipole moments by experimental and numerical studies
    (Elsevier, 2015-06) Pant, Debi D.
    Absorption and fluorescence spectra of dications of cinchonine (C2 +) and cinchonidine (Cd2 +) have been measured at room temperature in solvents of different polarities. Ground and excited state electric dipole moments are determined experimentally using solvatochromic shift method based on bulk solvent properties. Theoretical calculations are done using B3LYP/6-31G(D) and CIS/6-31G(D) levels of theory for ground and excited states, respectively. Both experimental and theoretical studies reveal that excited state dipole moment (μe) values are higher than corresponding ground state dipole moment (μg) value of C2 + and Cd2 +, which is attributed to the higher polarity of excited states compared to the ground state polarity of these molecules.
  • No Thumbnail Available
    Item
    Solvatochromatic shift of absorption and fluorescence spectra of 6-methoxyquinoline: Estimation of ground and excited state dipole moments
    (Elsevier, 2013-03) Pant, Debi D.
    Electronic absorption and fluorescence spectra of protonated 6-methoxyquinoline (6MQ+) and neutral 6-methoxyquinoline (6MQ) have been recorded at room temperature in wide range of solvents of different polarities. The absorption maximum remains almost unchanged with the increase in solvent polarity, whereas a red shift in fluorescence emission maximum was observed. The ground-state dipole moment and the first excited singlet state dipole moment of 6MQ+ and 6MQ were obtained from Bakhshiev's and Bilot–Kawski's equations by means of solvatochromic shift method. High values of dipole moment are observed for excited state as compared to the corresponding ground state dipole moment values of 6MQ+ and 6MQ and this is attributed to the more polar excited states of 6MQ+ and 6MQ.
  • No Thumbnail Available
    Item
    Estimation of ground and excited state dipole moments of quinidine and quinidine dication: Experimental and numerical methods
    (Elsevier, 2013-03) Pant, Debi D.
    Absorption and fluorescence spectra of quinidine (QD) and quinidine dication (QD2 +) have been measured at room temperature in solvents of different polarities. Ground and excited state electric dipole moments are determined experimentally using solvatochromic shift method based on bulk solvent properties. Numerical calculations are also performed using B3LYP/6-31G(D) level of theory for ground state and CIS/6-31G(D) level of theory for first excited singlet state. From both experimental and numerical studies it has been observed that dipole moment values of excited states (μe) are higher than corresponding ground state value (μg), of QD and QD2 +, which is attributed to the higher polarity of excited states of QD and QD2 + molecules.
  • No Thumbnail Available
    Item
    Steady state and time-resolved fluorescence spectroscopy of quinine sulfate dication bound to sodium dodecylsulfate micelles: Fluorescent complex formation
    (Elsevier, 2014-01) Pant, Debi D.
    Interaction of quinine sulfate dication (QSD) with anionic, sodium dodecylsulphate (SDS) surfactant has been studied at different premicellar, micellar and postmicellar concentrations in aqueous phase using steady state, time-resolved fluorescence and fluorescence anisotropy techniques. At premicellar concentrations of SDS, the decrease in absorbance, appearance of an extra fluorescence band at lower wavelengths and tri-exponential decay behavior of fluorescence, are attributed to complex formation between QSD molecules and surfactant monomers. At postmicellar concentrations the red shift in fluorescence spectrum, increase in quantum yield and increase in fluorescence lifetimes are attributed to incorporation of solute molecules to micelles. At lower concentrations of SDS, a large shift in fluorescence is observed on excitation at the red edge of absorption spectrum and this is explained in terms of distribution of ion pairs of different energies in the ground state and the observed fluorescence lifetime behavior corroborates with this model. The temporal fluorescence anisotropy decay of QSD in SDS micelles allowed determination of restriction on the motion of the fluorophore. All the different techniques used in this study reveal that the photophysics of QSD is very sensitive to the microenvironments of SDS micelles and QSD molecules reside at the water-micelle interface.
  • No Thumbnail Available
    Item
    Estimation of ground and excited state dipole moments of synthesized coumarin derivative [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane
    (Elsevier, 2014-12) Pant, Debi D.
    Electronic absorption and fluorescence spectra of coumarin derivative [N-(2-oxo-2H-chromen-4-yl)imino] triphenyl phosphorane have been recorded at room temperature in wide range of solvents of different polarities. The absorption maximum remains almost unchanged with the increase in solvent polarity, whereas, a red shift in fluorescence emission maximum was observed. Synthesized compound [N-(2-oxo-2H-chromen-4-yl)imino] triphenyl phosphorane was characterized by 1H and 13C NMR, and FT-IR spectral studies. The ground and excited state dipole moments of molecule were obtained from Bakhshiev's and Bilot–Kawski's equations by means of the solvatochromic shift method. Very high value of dipole moment is observed for excited state as compared to ground state value and this is attributed to more polar excited state of the molecule. Numerical calculations are performed using B3LYP/6-31G+(d) and B3LYP/6-31G(d) level of theory for ground state in Gaussian 03. Studies in different solvents are also done using the CPCM method and UA0 radii with the same level of theory. A critical analysis between the values of dipole moment in gas phase and various solvents is also carried out for ground state.
  • No Thumbnail Available
    Item
    Solvatochromic shift and estimation of dipole moment of quinine sulfate
    (Elsevier, 2012-02) Pant, Debi D.
    The absorption and fluorescence spectra of quinine sulfate (QS) have been recorded at room temperature in wide range of solvents of different polarities. The ground-state dipole moment of QS was obtained from quantum mechanical calculations and the excited state dipole moment of QS was estimated from Bakhshiev's and Bilot–Kawski's equations by means of solvatochromic shift method. Higher value of dipole moment has been observed for excited state as compared to the corresponding ground state value and this is attributed to the more polar excited state of QS.