Fluorescence Spectroscopic Studies of (Acetamide + Sodium/Potassium Thiocyanates) Molten Mixtures: Composition and Temperature Dependence

Abstract

Steady state and time-resolved fluorescence spectroscopic techniques have been used to explore the Stokes' shift dynamics and rotational relaxation of a dipolar solute probe in molten mixtures of acetamide (CH(3)CONH(2)) with sodium and potassium thiocyanates (Na /KSCN) at T approximately 318 K and several other higher temperatures. The dipolar solute probe employed for this study is coumarin 153 (C153). Six different fractions (f) of KSCN of the following ternary mixture composition, 0.75 CH(3)CONH(2) + 0.25[(1 - f)NaSCN + fKSCN], have been considered. The estimated experimental dynamic Stokes' shift for these systems ranges between 1800 and 2200 cm(-1) (+/-250 cm(-1)), which is similar to what has been observed with the same solute probe in several imidazolium cation based room temperature ionic liquids (RTIL) and in pure amide solvents. Interestingly, this range of estimated Stokes' shift, even though not corresponding to the megavalue of static dielectric constant reported in the literature for a binary mixture of molten CH(3)CONH(2) and NaSCN, exhibits a nonmonotonic KSCN concentration dependence. The magnitudes of the dynamic Stokes' shift detected in the present experiments are significantly less than the estimated ones, as nearly 40-60% of the total shift is missed due to the limited time resolution employed (full-width at half-maximum of the instrument response function approximately 70 ps). The solvation response function, constructed from the detected shifts in these systems, exhibits triexponential decay with the fastest time constant (tau(1)) in the 10-20 ps range, which might be much shorter if measured with a better time resolution. The second time constant (tau(2)) lies in the 70-100 ps range, and the third one (tau(3)) ranges between 300 and 800 ps. Both these time constants (tau(2) and tau(3)) show alkali metal ion concentration dependence and exhibit viscosity decoupling at higher viscosity in the NaSCN-enriched region. Time dependent rotational anisotropy has been found to be biexponential at all mixture compositions studied. Both the average solvation (<tau(s)>) and rotation (<tau(r)>) times of C153 in these mixtures exhibit fractional power law dependence on medium viscosity (<tau(x)> is proportional to eta(p), x being solvation or rotation). For solvation, p is found to be 0.46, which is very different from that obtained for common polar and nonpolar solvents, and RTILs (p approximately = 1). For rotation, p approximately = 0.65, which is again different from the value (p approximately = 1) obtained for common polar solvents and RTILs but very similar to that (p approximately = 0.6) found for nonpolar solvents. In addition, experimentally measured average rotation times in these mixtures are found to exhibit slip behavior in the low eta/T region, which gradually transforms to subslip as eta/T increases. Calculations using a recently developed semimolecular theory predict a total dynamic Stokes' shift for C153 (dipolar solute) in these molten mixtures near approximately 1600 cm(-1) where the...

Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Choline Chloride + Urea) Melts

“…The initial increase of ) (t Γ followed by decrease to a plateau value with time is a common feature observed for a variety of complex systems that include neat solvents, 60 ionic liquids [88][89][90] and DESs. [14][15][16] Note also that ) (t Γ at ∞ = τ is similar to em Γ obtained from steady state measurements. The overall initial change in ) (t Γ is within ~300-400 cm -1 and amounts to a small percentage (~10%) of the steady state emisison spectral width ( em Γ~ 3000 cm -1 ).…”

“…Similar spectral widths and peak frequencies suggest very similar solute-medium interactions in both the systems under comparison. In this context we would like to mention that (acetamide + electrolyte) DESs [14][15][16][17] were found to be more polar than the present DES as the corresponding spectra resembled those in formamide ( 0 ε ~110). 76 Fig.…”

“…Spectral data for steady state measurements and time resolved studies were collected and analysed by methods described elsewhere. 14 …”

“…8 Extensive inter-species H-bonding and gain in entropy upon melting are the main factors that drive the depression of freezing points, producing liquids that may even sometime show properties reminiscent of supercooled liquids near glass transition. [14][15][16] There exists a wide window of choice for constituents resulting in the formation of neutral and ionic DESs. 5 For example, neutral DESs can be obtained by mixing amides like acetamide and urea, 17 whereas the ionic ones can be obtained by choosing electrolytes as one of the components.…”

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

“…The fast reaction time constant of M6C can also be used to probe the spatial and temporal scales of heterogeneity in molten non-aqueous electrolyte mixtures where the system is expected to behave as super-cooled solutions. 24 Some of the above works are already in progress and we hope to report them soon.…”