• Harun Al Rasid Gazi

      Articles written in Journal of Chemical Sciences

    • Excited state intramolecular charge transfer reaction of 4-(morpholenyl) benzonitrile in solution: Effects of hetero atom in the donor moiety

      Tuhin Pradhan Harun Al Rasid Gazi Ranjit Biswas

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      An intramolecular charge transfer (ICT) molecule with an extra hetero atom in its donor moiety has been synthesized in order to investigate how ICT reaction is affected by hetero atom replacement. Photo-physical and photo-dynamical properties of this molecule, 4-(morpholenyl)benzonitrile (M6C), have been studied in 20 different solvents. The correlation between the reaction driving force ($-\Delta G_r$) and activation barrier ($\Delta G$#) has been explored in order to understand the solvent effects (static and dynamic) on the photo-excited ICT reaction in this molecule. A Kramer’s model analysis of the experimentally observed reaction rate constants indicates a solvent-averaged activation barrier of $\sim 4 k_BT$ in the absence of solvent dynamical control. The reaction in M6C is therefore not a barrier-less reaction but close to the limit where conventional kinetics might break down.

    • Excited state charge transfer reaction in (mixed solvent + electrolyte) systems: Role of reactant-solvent and reactant-ion interactions

      Harun Al Rasid Gazi Ranjit Biswas

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      Fluorescence spectroscopic techniques have been used to study the excited state intramolecular charge transfer reaction of 4-(1-azetidinyl)benzonitrile (P4C) in two sets of mixed solvents, (1-propanol + ethyl acetate) and (propylene carbonate + acetonitrile), in the absence and presence of a strong electrolyte, lithium perchlorate. These two sets of mixed solvent systems represent binary solvent mixtures of low and high polarities, respectively. Density, sound velocity and viscosity measurements indicate that these two mixed solvent systems are structurally different. Stronger ion-reactant interaction is evidenced in the mole fraction independence of emission frequencies in electrolyte solutions of low polar binary solvent mixtures. For both these mixtures, the reaction driving force ($-\Delta G_r$) decreases with increase in mole fraction of the relatively less polar solvent component of the mixture. Interestingly, $-\Delta G_r$ increases significantly on addition of electrolyte in low polar mixtures and exhibits mixture composition dependence but, in contrast, $-\Delta G_r$ in high polar mixtures does not sense variation in mixture composition in presence of electrolyte. This insensitivity to mixture composition for high polar mixtures is also observed for the measured reaction time constant. In addition, the reaction time constant does not sense the presence of electrolyte in the high polar solvent mixtures. The reaction time constant in low polar mixtures, which becomes faster on addition of electrolyte, lengthens on increasing the mole fraction of the relatively less polar solvent component of the mixture. These observations have been qualitatively explained in terms of the measured solvent reorganization energy and reaction driving force by using expressions from the classical theory of electron transfer reaction.

    • Excited state intramolecular charge transfer reaction in non-aqueous reverse micelles: Effects of solvent confinement and electrolyte concentration

      Tuhin Pradhan Harun Al Rasid Gazi Biswajit Guchhait Ranjit Biswas

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      Steady state and time resolved fluorescence emission spectroscopy have been employed to investigate the effects of solvent confinement and electrolyte concentration on excited state intramolecular charge transfer (ICT) reaction in 4-(1-pyrrolidinyl) benzonitrile (P5C), 4-(1-piperidinyl) benzonitrile (P6C), and 4-(1-morpholenyl) benzonitrile (M6C) in AOT/n-heptane/acetonitrile and AOT/n-heptane/methanol reverse micelles. Dramatic confinement effects have been revealed via a huge reduction (factor ranging between 100 and 20) over bulk values of both equilibrium and reaction rate constants. A strong dependence on the size of the confinement ($W_s$) of these quantities has also been observed. $W_s$ dependent average static dielectric constant, viscosity and solvation time-scale have been determined. Estimated dielectric constants for confined methanol and acetonitrile show a decrease from the respective bulk values by a factor of 3-5 and viscosities increased by a factor of 2 at the highest $W_s$ considered. Addition of electrolyte at $W_s = 5$ for acetonitrile is found to produce a linear increase of confined solvent viscosity but leads to a non-monotonic electrolyte concentration dependence of average solvation time. Reaction rate constant is found to decrease linearly with electrolyte concentration for P5C and P6C but non-monotonically for M6C, the highest decrease for all the molecules being ∼ 20% over the value in the absence of added electrolyte in the solvent pool. The observed huge reduction in reaction rate constant is attributed to the effects of decreased solution polarity, enhanced viscosity and slowed-down solvent reorganization of the solvent under confinement in these non-aqueous reverse micelles.

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