• Subhadip Ghosh

      Articles written in Journal of Chemical Sciences

    • Excited-state proton transfer from pyranine to acetate in methanol

      Sudip Kumar Mondal Subhadip Ghosh Kalyanasis Sahu Pratik Sen Kankan Bhattacharyya

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      Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulphonate, HPTS) to acetate in methanol has been studied by steady-state and time-resolved fluorescence spectroscopy. The rate constant of direct proton transfer from pyranine to acetate ($k_1$) is calculated to be $\sim 1 \times 10^9$ M-1 s-1. This is slower by about two orders of magnitude than that in bulk water ($8 \times 10^{10}$ M-1 s-1) at 4 M acetate.

    • Ultrafast fluorescence resonance energy transfer in a bile salt aggregate: Excitation wavelength dependence

      Ujjwal Mandal Subhadip Ghosh Dibyendu Kumar Das Aniruddha Adhikari Shantanu Dey Kankan Bhattacharyya

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      Fluorescence resonance energy transfer (FRET) from Coumarin 153 (C153) to Rhodamine 6G (R6G) in a secondary aggregate of a bile salt (sodium deoxycholate, NaDC) is studied by femtosecond up-conversion. The emission spectrum of C153 in NaDC is analysed in terms of two spectra-one with emission maximum at 480 nm which corresponds to a non-polar and hydrophobic site and another with maximum at ∼ 530 nm which arises from a polar hydrophilic site. The time constants of FRET were obtained from the rise time of the emission of the acceptor (R6G). In the NaDC aggregate, FRET occurs in multiple time scales -4 ps and 3700 ps. The 4 ps component is assigned to FRET from a donor (D) to an acceptor (A) held at a close distance ($R_{\text{DA}} \sim 17$ Å) inside the bile salt aggregate. The 3700 ps component corresponds to a donor-acceptor distance ∼ 48 Å. The long (3700 ps) component may involve diffusion of the donor. With increase in the excitation wavelength ($\lambda_{\text{ex}}$) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (∼ 4 ps) increases from 3 to 40% with a concomitant decrease in the contribution of the ultraslow component (∼3700 ps) from 97 to 60%. The $\lambda_{ex}$ dependence is attributed to the presence of donors at different locations. At a long $\lambda_{\text{ex}}$ (435 nm) donors in the highly polar peripheral region are excited. A short $\lambda_{\text{ex}}$ (375 nm) `selects’ donor at a hydrophobic location.

    • Evaluation of intramolecular charge transfer state of 4-𝑁, 𝑁-dimethylamino cinnamaldehyde using time-dependent density functional theory

      Surajit Ghosh K V S Girish Subhadip Ghosh

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      Intramolecular charge transfer of 4-𝑁,𝑁-dimethylamino cinnamaldehyde (DMACA) in vacuum and in five different aprotic solvents has been studied by using time-dependent density functional theory (TDDFT). Polarizable continuum model (PCM) was employed to consider solvent-solute interactions. The potential energy curves were constructed at different torsional angle of 𝑁,𝑁-dimethylamino moiety with respect to the adjacent phenyl ring. A large bathochromic shift in our calculated emission and absorption energies for polar solvents is a clear reminiscent of charge transfer nature of the excited state. Finally, the reported results are in agreement with experimental findings.

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