Structural Dynamics of Dimethyl Sulfoxide Aqueous Solutions Investigated by Ultrafast Infrared Spectroscopy: Using Thiocyanate Anion as a Local Vibrational Probe

Abstract

The microscopic structure of dimethyl sulfoxide (DMSO) aqueous solutions was investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. The structural dynamics of the binary mixtures were reflected by using thiocyanate anion (SCN–) as a local vibrational probe. FTIR spectra of SCN– anion showed that the hydrogen bond networks of water are affected by the presence of DMSO molecules, and the peak position and bandwidth of SCN– anions are red shifted and narrowed accordingly because of the weak hydration in the binary mixture. The vibrational lifetime of the SCN– anion showed almost linear enhancement with the increase of DMSO, which can be explained by the weak interaction between SCN– and the hydrophobic groups in the DMSO molecule. However, the rotational dynamics of SCN– are slowing down significantly and showed a maximum response at X DMSO (mole fraction) of 0.35, which is mainly caused by the confinement of SCN– anions positioned in the vicinity of the complex structure formed between DMSO and water molecules. The concentration-dependent rotational dynamics of water molecules and SCN– anions are having similar behavior, indicating that the complex structure can be formed between water and DMSO molecules because of the strong interaction. The result also demonstrates that the structural inhomogeneity in aqueous solution can be unraveled by monitoring the vibrational relaxation dynamics of SCN– anion serving as the local vibrational probe.

Liquid–Liquid Phase Separation Produces Fast H-Bond Dynamics in DMSO–Water Mixtures

“…51 Finally, above 40 mol %, the solution again becomes more homogeneous in the DMSO-rich regime, which is consistent with previous measurements. 7,29,44 The decrease in heterogeneity also correlates with our previous study showing that above 35 mol %, DMSO phaseseparates into an "aggregate", which removes DMSO molecules from the water phase, and this produces a more homogeneous environment. 41 Time constants (τ, Figure 4A) represent the frequency fluctuation rates from H-bond dynamics and can be considered a probe of H-bond lifetimes.…”

“…This characteristic bell-shaped curve has been observed in numerous measurements, and its dependence on concentration is correlated with bulk solution properties such as density and viscosity. 7,29,44 Overall, the static inhomogeneity is small for pure water compared to the mixtures. In the 30−50 mol % range, both probes show increased inhomogeneity, as a result of a wider range of environments available between water and DMSO phases within this regime.…”

On the crystal structures and phase transitions of hydrates in the binary dimethyl sulfoxide–water system

“…It is interesting that many physical properties of the liquid have minima or maxima corresponding with the 1:2 composition (e.g. Bordallo et al, 2004;Wei et al, 2018), but that the maxima we observed in T c in the solid corresponds instead with the 1:3 composition. Neutron powder diffraction pattern of -DMSO dihydrate measured on the High Resolution Powder Diffractometer (HRPD) at 175 K, with the background subtracted.…”

“…Many experimental and theoretical investigations have been made into the structure of liquid solutions of DMSO, particularly around the 1:2 composition where the meltingpoint depression is greatest (e.g. Soper & Luzar, 1992;Vishnyakov et al, 2001;Bordallo et al, 2004;McLain et al, 2007;Wallace et al, 2015;Oh et al, 2017;Stachura et al, 2018;Wei et al, 2018;Yang et al, 2020). The prevailing interpretation of DMSO-water liquid data is in terms of 1:3 and 1:2 DMSOwater clusters and possible association into DMSO-(water) n -DMSO chains (Zhang et al, 2009;Idrissi et al, 2015;Verstakov et al, 2020).…”

Vibronic fingerprint of singlet fission in hexacene

“…Our experimental setup for a visible pump and a mid-IR probe measurement is similar to the ultrafast IR spectroscopy measurement for liquid samples, which has been described in previous reports. 52,53 Briefly, a femtosecond amplifier (SPITFIRE ACE-35F1K, Spectra Physics) with an output wavelength of 801 nm and a pulse duration of 40 fs was seeded with the ultrafast short pulse from a Ti-sapphire oscillator (Element 20-1200, Newport Corporation). The femtosecond amplifier pumped an optical parametric amplifier (OPA, TOPAS, Light conversion) to produce a broadband mid-IR pulse with the following specifications: ∼140 fs pulse duration, tunable frequency range from 3 to 15 μm, ∼200 cm −1 bandwidth, and 1 kHz repetition rate.…”