The Importance of Hydrogen Bonded Networks in the Dynamic Heterogeneity of Deep Eutectic Solvents.

We have examined the composition-dependent rotational diffusion dynamics of the chromophores perylene (neutral), oxazine 725 (cationic) and disodium fluorescein (anionic) in a series of choline chloride/1,3-propanediol (ChCl/PD) deep eutectic solvents (DESs). We find that the rotational diffusion dynamics of all chromophores do not correlate with the bulk viscosities of any of these ChCl/PD DES compositions, consistent with dynamic heterogeneity in all cases. Comparison with other DES systems─choline chloride/ethylene glycol (ChCl/EG) and choline chloride/glycerol (ChCl/Gly)─enables evaluation of the influence of hydrogen bond donor (HBD) structure and hydroxyl group density. This comparison highlights the critical role of hydrogen-bonded networks in these DESs and underscores the pivotal function of ChCl in disrupting these networks at ca. 15 mol % ChCl across all three DES systems. Taken collectively, these data point to the constituent structural factors that mediate the properties of DESs on local and bulk scales.