Ion-specific structural effects on excited-state proton transfer dynamics in aqueous deep eutectic solvents

Area of Science:

• Photochemistry
• Physical Chemistry
• Supramolecular Chemistry

Background:

• Aqueous deep eutectic solvents (ADES) exhibit unique properties due to water's influence on microstructure.
• Excited-state proton transfer (ESPT) is a crucial photochemical process sensitive to the molecular environment.

Purpose of the Study:

• Investigate the effect of aqueous deep eutectic solvent (ADES) composition on the ESPT reaction of 8-hydroxy-pyrene-1,3,6-trisulfonic acid (HPTS).
• Determine how varying salt cations and anions in ADES influence photochemical reaction rates and proton transfer dynamics.

Main Methods:

• Utilized steady-state and time-resolved fluorescence spectroscopy to study HPTS in 30% v/v acetamide-salt based ADES.
• Employed time-resolved area normalized emission spectra (TRANES) analysis to quantify proton transfer times.

Main Results:

• Water addition to DES significantly facilitates ESPT, which is negligible in neat DES.
• The nature of salt cations and anions in ADES critically impacts ESPT rates, with proton transfer times ranging from 2.7 to 11.8 ns.
• ESPT time increases with decreasing cation size (K+ < Na+ < Li+) and with specific anions (SCN- < NO3- < ClO4-).

Conclusions:

• Cations and anions in ADES specifically interact with water, modulating its proton accepting ability.
• The microstructural organization of ADES, dictated by its ionic components, governs the efficiency of the ESPT reaction.
• This study highlights the tunability of photochemical reactions in ADES by controlling their composition.