Enhanced Stability and Solubility of Photoacids in Binary Solvent Mixtures for Light-Driven Reversible CO₂ Capture-Release

Rapid rise of atmospheric CO₂ levels urgently requires more affordable carbon capture technologies with innovations beyond present-day energy intensive regeneration processes such as temperature or pressure swing [1]. Recently, pH-swing was proposed as a promising low energy alternative approach for CO₂ release. A pH-swing - i.e. cycling between CO₂ absorption at high pH, forming HCO₃^-, and CO₂ desorption at low pH - omits the need to increase temperature or decrease pressure during desorption for sorbent regeneration [2]. Controlled and reversible acidification of a solution can be achieved by the excitation of photoacids. Metastable photoacids are organic molecules that can reversibly lower pH of a solution under UV-visible light. Merocyanine-type photoacids can decrease the pH by up to 3.5 units in less than 30 seconds (pH-jump), and then reverse to the original pH in darkness within 15 minutes through thermal relaxation [3, 4]. The non-invasive nature of these light switchable proton emitters renders highly relevant applications in functionalized surfaces, such as membranes [5] and sensors [6], or pH-driven reactions such as hydrogel formation [7] or CO₂ absorption/release [8]. Merocyanine-type photoacids are generally limited by their low chemical stability due to hydrolysis (half-life is 16 hours) and low solubility in water (0.19 mM) [5]. In aprotic organic solvents such as dimethyl sulfoxide (DMSO), photoacids display higher chemical stability and solubility, however pH-jump in such solvents is not reversible [3].<br/>Herein we propose to dissolve photoacids in water-DMSO solvent mixtures to achieve the ‘best of both worlds’: high reversibility of pH-jump as well as high chemical stability and solubility. By combining pH-jump, solubility and stability studies, we demonstrate that solubility can be enhanced by 2 orders of magnitude, and half-life increased by 1 order of magnitude in DMSO-water mixtures. UV-Vis absorption, Nuclear Magnetic Resonance spectroscopy, Small-Angle X-ray Scattering and Polarisable Molecular Dynamics simulations were used to reveal the role of H-bonding networks and modified solvent structure to enable these enhancements. Besides optimizing for specific properties, binary solvent mixtures provide an additional degree of freedom to tune photoacid properties such as pK_a and relaxation kinetics that can also be applied to other types of photoacids. Finally, the photoacid in an optimal solvent mixture of 15% DMSO in water enables a highly reversible, stable and controlled CO₂ capture-release system. With higher stability and solubility, photoinduced pH-swing has the potential to drastically reduce energy demand, material degradation and infrastructure of mature carbon capture technologies by ambient CO₂ desorption that is controlled by visible light.<br/>[1] S. E. Renfrew, D. E. Starr, P. Strasser, <i>ACS Catal.</i> 10, 21, 13058–13074 (2020) https://doi.org/10.1021/acscatal.0c03639<br/>[2] S. Jin, M. Wu, Y. Jing, R. G. Gordon, M. J. Aziz, <i>Nat Commun</i> 13, 2140 (2022) https://doi.org/10.1038/s41467-022-29791-7<br/>[3] C. Berton, D. Maria Busiello, S. Zamuner, E. Solari, R. Scopelliti, F. Fadaei-Tirani, K. S. C. Pezzato, <i>Chem. Sci..,</i> 11, 8457-8468 (2020) https://doi.org/10.1039/D0SC03152F<br/>[3] L. Wimberger, J. Andréasson, J. Beves, <i>Chem. Commun.</i> 58, 5610-5613 (2022) https://doi.org/10.26434/chemrxiv-2022-wnts7<br/>[4] W. White, C. D. Sanborn, D. M. Fabian, S. Ardo, <i>Joule,</i> 2(1), 94-109 (2018) https://doi.org/10.1016/j.joule.2017.10.015<br/>[5] T. Khalil, A. Alharbi, C. Baum, Y. Liao, <i>Macromolecular Rapid Commun </i>39(15), 1800319 (2018) https://doi.org/10.1002/marc.201800319<br/>[6] A. Meeks, M. M. Lerch, T. B. H. Schroeder, A. Shastri, J. Aizenberg, <i>J. Am. </i><i>Chem. Soc. 144</i>, 219-227 (2022) https://doi.org/10.1021/jacs.1c08778<br/>[7] R. Bennett, S. Clifford , K. Anderson , G. Puxty, <i>Energy Procedia</i> 114, 1-6 (2017) https://doi.org/10.1016/j.egypro.2017.03.1139<br/>[8] N. Abeyrathna, Y. Liao, <i>J. Phys. Org. </i><i>Chem.</i> 30.8, e3664 (2017) https://doi.org/10.1002/poc.3664