Dynamics of Solvent Response in Methanol–Chloroform Binary Solvent Mixture: A Case of Synergistic Solvation

Abstract

Steady-state absorption, emission, and femtosecond transient absorption spectroscopies were used to ascertain the static and dynamic nature of the solvent response of methanol-chloroform binary solvent mixtures of different stoichiometric ratios using 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) as the probe molecule. The appearance of synergistic solvation behavior in the steady-state absorption measurements can be explained in terms of solvent-solvent interactions through an extended hydrogen-bonding network. The disappearance of such synergistic behavior in the excited state of the DCM dye was recently proposed by us to be due to the weak nature of the intermolecular interactions present in binary solvent mixtures (J. Phys. Chem. B 2012, 116, 1345). It was anticipated and subsequently confirmed by the dynamics of the solvent response that the disruption of the weak interactive solvent network is the main reason for the absence of the synergism in the excited state. As expected, we observed the slowest dynamics for the mixture with X(MeOH) = 0.45, with an average solvation time of 12.03 ps, which is much higher than the values for the pure bulk counterparts (⟨τ(s)⟩(Methanol) = 4.32 ps and ⟨τ(s)⟩(Chloroform) = 1.32 ps). The unprecedented slowing of solvation for DCM is probably due to the rigid interactive methanol-chloroform solvent system in the first solvation shell, followed by solvent rearrangements around the solute dipole. Overall interactions present within the methanol-chloroform binary solvent mixture furnish clear evidence of solvent association through weak hydrogen bonding.

Study of Microheterogeneity in Acetonitrile–Water Binary Mixtures by using Polarity‐Resolved Solvation Dynamics

“…Solvation dynamics in a binary solvent has emerged as an important subject in several experimental and theoretical studies over the last few decades . The dynamic features in a binary solvent, in most of the cases, are perturbed by solvent cluster formation, structural fluctuations, conformational changes of the complexes formed by solvent molecules, and solvent‐network formation . Inherent heterogeneity in binary mixtures often produces a slow solvation time, which is absent in the pure solvents .…”

“…Therefore, the solvation dynamics of a mixture of a nonpolar solvent (bad solvator) and a polar solvent (good solvator) must be diffusion assisted; during solvation, the replacement of the nonpolar solvent molecules by polar solvent molecules in the first solvation shell of the excited dipole is a well‐known phenomenon . This slow diffusion may be a major source of the slow kinetics, which is apparent in binary mixtures, but is absent in pure solvents . In this study, we purposely used a binary solvent composed of two highly polar solvents, acetonitrile and water, both of which are known to be good solvators.…”

“…Huppert and co‐workers observed a high Z value (≈1.7) in the hexane–propionitrile binary mixtures, which they explained by considering the diffusion of polar propionitrile molecules into the first solvation shell of the coumarin 153 (C153) dipole . Solvation dynamics in a binary solvent has emerged as an important subject in several experimental and theoretical studies over the last few decades . The dynamic features in a binary solvent, in most of the cases, are perturbed by solvent cluster formation, structural fluctuations, conformational changes of the complexes formed by solvent molecules, and solvent‐network formation .…”

“…In a recent study, we have observed extraordinarily an slow solvation (>5 ns) time for coumarin 343 (C343) in dimethyl sulfoxide (DMSO)–glycerol binary mixtures over the entire concentration range of DMSO . The existence of similar slow solvation components in other binary mixtures has already been reported in experimental and theoretical studies . However, none of these studies rationalize the actual origin of this slow dynamics in a binary mixture.…”

Early Time Solvation Dynamics Probed by Spectrally Resolved Degenerate Pump‐Probe Spectroscopy

“…Thus, using spectrally‐resolved pump‐probe technique, one can also envisage the idea of directly monitoring the time‐dependent gain in stimulated emission spectrum obviating the need of spectral reconstruction and estimation of time‐zero spectrum in non‐polar solvent, which we present in this work. It is worthy to mention here that spectrally‐resolved pump‐probe spectroscopy has been used to study synergistic behaviour in binary solvent mixtures and vibrational wavepacket interferometery measuring solvation in methanol and in proteins; two dimensional electronic spectroscopy has also been used to study solvation . Fluorescence correlation spectroscopy and time‐dependent Stokes shift was also implemented to study solvation dynamics in membranes .…”