Dynamics of proton transport through time-resolved vibrational spectroscopy in a protic ionic liquid Sourav Maiti 1,2 , Sunayana Mitra 2 , Clinton A. Johnson 2 , Kai C. Gronborg 2 , Sean Garrett-Roe 2 and Paul M. Donaldson 1 1 Central Laser Facility, RCaH, STFCRutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom, 2 Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States We have investigated the dynamics of excess protons in a protic ionic liquid (Ethylammonium formate, EAF) through time-resolved vibrational spectroscopy. Protic ionic liquids are potential candidates for non-aqueous proton conducting membranes that can operate at temperatures >;120°C desirable for high-temperature fuel cells. Therefore, probing the proton conduction mechanism can provide information about the time and length scale of proton transport crucial for optimizing the proton conduction through the desired ionic liquid. Ultrafast dynamics of non-aqueous proton transport involving ionic liquids have not been explored in detail. Our study develops the application of ultrafast (fs-μs) transient vibrational spectroscopy techniques to give new insights into long-range proton transport. We used the standard photoacid (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS) with EAF to generate protons. Visible excitation of HPTS increases its acidity by six orders of magnitude (pH-jump) making it a source of excess protons to a suitable acceptor, in this case, the formate of EAF. The elementary steps of proton transfer and transport (Figure 1a) can then be probed through mid-infrared transient absorption spectroscopy over an unprecedented time range from femtoseconds to microseconds (Figure 1b) by monitoring the vibrational bands of both the proton donor (HPTS) and acceptor (EAF). Two predominant mechanisms of proton transport are Grotthuss transport involving proton hopping and vehicular transport involving the translation of the proton complex itself. These can be distinguished through the kinetic isotope effect (KIE=k H /k D ) by comparing the rates of the proton (k H ) and deuterium conduction (k D ). In EAF, our results show a KIE~1 for proton transfer and transport suggesting vehicular proton transport where the proton diffuses about 10 solvent shells before regenerating the HPTS. Using these methods we are now able to investigate ionic novel liquids theoretically predicted to have Grotthuss transport.
Figure 1. (a) Schematic of proton transfer and transport steps. ROH is the proton donor (HPTS) and formate (HCOO - ) is the proton acceptor. Upon photoexcitation ROH * donates the proton to the formate.The conjugate base RO *– getsde-excited to RO – through emission.The proton then returns to the RO – from formic acid (HCOOH) to regenerate the ROH, completing the cycle. (b) Transient kinetics of formic acid showing the growth and decay over six orders of magnitude in time. References 1. Maiti, S.; Mitra, S.; Johnson, C. A.; Gronborg, K. C.; Garrett-Roe, S.; Donaldson, P. M. pH Jumps in a Protic Ionic Liquid Proceed by Vehicular Proton Transport. J. Phys. Chem. Lett. 2022, 13 (34), 8104-8110.
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