• N Periasamy

      Articles written in Journal of Chemical Sciences

    • Singlet molecular oxygen quantum yield measurements of some porphyrins and metalloporphyrins

      R Venkatesan N Periasamy T S Srivastava

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      Hematoporphyrin IX dimethyl ester (HPDME), tetraphenylporphyrin (TPP), tetra(4-methoxyphenyl)porphyrin (TMPP), tetra(3,4-dimethoxyphenyl)porphyrin (TDMPP), tetra(3,4,5-trimethoxyphenyl)porphyrin (TTMPP), tetraanthrylporphyrin (TAP) and tetraacridylporphyrin (TACP), and their Zn2+, SnX22+, Pd2+ and Pt2+ complexes have been prepared and characterized. The singlet molecular oxygen quantum yield (ϕΔ) values of the above porphyrins and their metal derivatives in N,N-dimethyl formamide (DMF) have been measured in the presence of 1,3-diphenylisobenzofuran (DPBF) as1O2 acceptor using steady state technique after correcting for the intensity of light absorbed by the photosensitizers. The ϕΔ values for the free base porphyrins are usually around 0.60. Hematoporphyrin IX dimethyl ester and its metal derivatives follow the order: HPDME (ϕΔ = 0.60) > Zn HPDME (0.40) > PdHPDME (0.34) > Sn(OH)2 HPDME (0.28) > PtHPDME (0.24). Zinc(II) complexes of tetraarylporphyrins show about 65% efficiency in ϕΔ values are compared to the ϕΔ values of their corresponding free base porphyrins. These results can be explained on the basis of catalyzed intersystem crossing to the ground state. The ϕλ values of the above free base porphyrins and their metal complexes in DMF in presence of DPBF using single-pulsed laser excitation technique follow similar trends.

    • The deexcitation of the S1 state of aminoanthraquinones: A steadystate and timeresolved study

      V J P Srivatsavoy B Venkataraman N Periasamy

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      The non-radiative processes of deactivation from the lowest singlet excited state of aminoanthraquinones have been studied using steady-state and time-resolved methods. The fluorescence decay rate constant, kf correlates well with the solvent polarity parameter, ET(30), in nonhydrogen bonding solvents. Large deuterium isotope effects in fluorescence lifetimes (τf) and quantum yields (ϕf) are observed in the case of 1-amino (AAQ) and 1-methylaminoanthraquinones (MAQ), where the S1 state is mainly deactivated through internal conversion to the ground state. The temperature-dependence of the fluorescence quantum yields of various aminoanthraquinones was also investigated. The ϕf and τf exhibited strong temperature-dependence in the case of 1-acetylaminoanthraquinone (ACAQ). In the case of ACAQ, the intersystem crossing to the triplet state is a major deactivation channel from the S1 and in this derivative a close-lying T2 state seems to be responsible for the high kisc rate.

      The fluorescence properties of 1,5-diaminoanthraquinone (DAQ) are affected by intermolecular hydrogen bonding with alcohols. Increasingn-alkyl chain length in the case of l-(n-alkyl)aminoanthraquinones from methyl to butyl does not produce any change in the fluorescence properties, whereas a hydroxypropyl substitution results in a small decrease of ϕf and τf in these compounds, indicating an interaction of the hydroxyl group with the carbonyl group of the aminoanthraquinones.

    • Foreword

      N Periasamy

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    • Identification of intermediate species in protein-folding by quantitative analysis of amplitudes in time-domain fluorescence spectroscopy

      Anoop M Saxena G Krishnamoorthy Jayant B Udgaonkar N Periasamy

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      In protein-folding studies it is often required to differentiate a system with only two-states, namely the native (N) and unfolded (U) forms of the protein present at any condition of the solvent, from a situation wherein intermediate state(s) could also be present. This differentiation of a two-state from a multi-state structural transition is non-trivial when studied by the several steady-state spectroscopic methods that are popular in protein-folding studies. In contrast to the steady-state methods, time-resolved fluorescence has the capability to reveal the presence of heterogeneity of structural forms due to the `fingerprint’ nature of fluorescence lifetimes of various forms. In this work, we establish this method by quantitative analysis of amplitudes associated with fluorescence lifetimes in multiexponential decays. First, we show that we can estimate, accurately, the relative population of species from two-component mixtures of non-interacting molecules such as fluorescent dyes, peptides and proteins. Subsequently, we demonstrate, by analysing the amplitudes of fluorescence lifetimes which are controlled by fluorescence resonance energy transfer (FRET), that the equilibrium folding-unfolding transition of the small singledomain protein barstar is not a two-step process.

    • Synthesis, photoluminescence and electrochemical properties of 2,7-diarylfluorene derivatives

      Neeraj Agarwal Pabitra K Nayak N Periasamy

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      A new class of highly fluorescent 9,9'-bis-(alkyl)-2,7-diarylfluorene having electron withdrawing or electron donating groups on the 𝑝-phenyl positions were synthesized and characterized. The highly luminescent fluorene derivatives, 1-6 showed blue emission (376-416 nm), narrow FWHM (∼ 50 nm), high quantum yield ($\phi_{\text{F}} = 0.12-0.87$) and short fluorescence lifetimes, $\tau_{\text{F}} = 0.23-1.14$ ns. The HOMO levels of 9,9'-bis-(alkyl)-fluorene were tuned by the p-substituents at 2,7-phenyl group. Hammett correlation with $E_{\text{HOMO}}$ of these new molecules provides an effective tool to predict the HOMO level of similar molecules prior to the synthesis. These data indicate that they are useful as emitting materials for organic light emitting devices, OLEDs.

    • Synthesis, photophysical and electrochemical properties of 2,8-diaryl-dibenzothiophene derivatives for organic electronics

      Pabitra K Nayak Neeraj Agarwal N Periasamy

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      A series of 2,8-𝑝-diaryldibenzothiophene derivatives were synthesized and characterized. These molecules have electron withdrawing or electron donating groups at the para phenyl position, which alters the electronic properties of these derivatives. The quantum yield, fluorescence lifetime, singlet, triplet and EHOMO energy levels of these compounds were determined by fluorescence, phosphorescence and cyclic voltammetry. A plot of Hammett constants of the para substituents vs EHOMO revealed a linear relationship. The usefulness of these molecules in organic light emitting diodes, OLEDs is discussed vis-à-vis the energy levels and properties.

    • Blue and white light electroluminescence in a multilayer OLED using a new aluminium complex

      Pabitra K Nayak Neeraj Agarwal Farman Ali Meghan P Patankar K L Narasimhan N Periasamy

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      Synthesis, structure, optical absorption, emission and electroluminescence properties of a new blue emitting Al complex, namely, bis-(2-amino-8-hydroxyquinolinato), acetylacetonato Al(III) are reported. Multilayer OLED using the Al complex showed blue emission at 465 nm, maximum brightness of ∼ 425 cd/m2 and maximum current efficiency of 0.16 cd/A. Another multilayer OLED using the Al complex doped with phosphorescent Ir complex showed `white’ light emission, CIE coordinate (0.41, 0.35), maximum brightness of ∼ 970 cd/m2 and maximum current efficiency of 0.53 cd/A.

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