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

Abstract

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 (-ΔG r ) and activation barrier (ΔG # ) 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 ~ 4 k B T 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

“…21 The goodness of fit parameter (χ 2 ) for these and other mixtures being nearly unity and the absence of any regular pattern in the residuals strongly suggest the reaction kinetics of P4C in these mixed solvent systems conform to the classical two state reversible reaction mechanism as observed earlier in different reaction environments. [21][22][23][24][25][26][27][28] Interestingly and enhances the amplitude (a 1 ) significantly at this composition of (PrOH + EA) mixture. Moreover, the ratio of the equilibrium constants in the presence and absence of electrolyte determined from the LE decay amplitudes (K time−resolved eq = a 1 /a 2 ) is ∼ 16 which is in close agreement with the value (∼ 15) obtained from the steady state measurements.…”

“…Earlier measurements of reaction time scales of P4C in neat polar solvents 21 indicate that the broad time resolution employed in the present experiments would be able to describe semiquantitatively the effects of mixture composition and electrolyte on reaction time scale in this molecule. As before, [21][22][23][24][25][26][27][28] we have used the twisted intramolecular charge transfer (TICT) model 38,39 to explain the decay kinetics of P4C eventhough alternative models 40,41 are available in the literature. Scheme 1 depicts the reaction in P4C where, in the excited electronic surface (S 1 ), the photo-induced locally excited (LE) state converts to the relatively more polar charge transfer (CT) state with a forward reaction rate constant, k f .…”

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

“…Note that earlier study with these molecules in bulk solvents indicate a reaction activation barrier of ∼3-4 k B T which is not large enough to inhibit breaking down of conventional kinetics observed in the case of high barrier reaction. 48,49 Consequently, it would be interesting if one encounters non-exponential reaction kinetics for these molecules in these reverse micelles, as observed earlier for several bulk polar solvents. 48 Moreover, study with these three TICT molecules is required to establish the generality of the results for a given family of reactant molecules in these complex environments.…”

“…Since solvation time-scale is known to influence reaction rate, 58,59 estimation of the former is crucial for interpreting the experimental results. We shall shortly see that ICT reaction in these molecules also thought to involve twisting [48][49][50][51] and thus expected to couple to the local 'stiffness' of the medium. Fluorescence anisotropy measurements have been carried out to estimate such stiffness or local viscosity experienced by the twisting mode while reacting inside a confined pool.…”

“…We have used the twisted intramolecular charge transfer (TICT) model 50,51,[60][61][62] to explain the decay kinetics of these ICT molecules, although alternative arguments can also be employed. [63][64][65][66] Since the mechanism of TICT reaction is already discussed elsewhere, [48][49][50][51] we describe the mechanism very briefly here with the help of scheme 1. Upon photo-excitation, locally excited (LE) state is produced and converted to the relatively more polar charge transfer (CT) state with a forward reaction rate constant, k f .…”