TY - JOUR

T1 - Stereodiscrimination of guests in chiral organosilica aerogels studied by ESR spectroscopy

AU - Polarz, Sebastian

AU - Krysiak, Yasar

AU - Wessig, Martin

AU - Kuhlmann, Florian

N1 - Publisher Copyright: © 2025 Polarz et al.; licensee Beilstein-Institut. This is an open access article licensed under the terms of the Beilstein-Institut Open Access License Agreement (https://www.beilstein-journals.org/bjnano/terms), which is identical to the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0). The reuse of material under this license requires that the author(s), source and license are credited. Third-party material in this article could be subject to other licenses (typically indicated in the credit line), and in this case, users are required to obtain permission from the license holder to reuse the material.

PY - 2025/11/13

Y1 - 2025/11/13

N2 - Macroporous materials containing surfaces with chiral groups are highly relevant for applications in the chromatographic separation of enantiomers. Despite these materials being highly engineered and commercially available, optimization was often done empirically. A rational design of future and improved solid phases for chiral chromatography requires that one understands how the chemical structure of a surface influences the stereoselectivity of the enantiomers at the surface. Despite the difference in the interaction enthalpies being only in the 1-2 kJ·mol-1 range, an ideal surface would exclusively interact with one enantiomer. However, the question of which selectivity is sufficient or necessary to reach separation is an important point. We have employed the two enantiomers of a chiral, nitroxide-based spin probe as guests in organo-modified macroporous host materials and applied ESR spectroscopy as a tool to investigate their rotational mobility. Using a well-established and commercially available material confirmed the method’s reliability. The data underline how crucial the choice of the right solvent is if one wants to reach sufficient selectivity. Together with a series of custom-made organosilica aerogels, it is shown that adjusting solvent and surface properties so that the two enantiomers (+) and (-) experience a different chemical environment is key. Otherwise, there might be a dynamic equilibrium between surface-adsorbed and mobile spin probes without stereodifferentiation. With this knowledge, it was possible to reach higher selectivity values than for the commercial material. A particularly interesting result was that better performance could be achieved if one attaches bulky, hydrophobic groups directly to the stereocenter. The effect of such neighboring groups on the enantioselectivity highly depends on the distance they have to the stereocenter.

AB - Macroporous materials containing surfaces with chiral groups are highly relevant for applications in the chromatographic separation of enantiomers. Despite these materials being highly engineered and commercially available, optimization was often done empirically. A rational design of future and improved solid phases for chiral chromatography requires that one understands how the chemical structure of a surface influences the stereoselectivity of the enantiomers at the surface. Despite the difference in the interaction enthalpies being only in the 1-2 kJ·mol-1 range, an ideal surface would exclusively interact with one enantiomer. However, the question of which selectivity is sufficient or necessary to reach separation is an important point. We have employed the two enantiomers of a chiral, nitroxide-based spin probe as guests in organo-modified macroporous host materials and applied ESR spectroscopy as a tool to investigate their rotational mobility. Using a well-established and commercially available material confirmed the method’s reliability. The data underline how crucial the choice of the right solvent is if one wants to reach sufficient selectivity. Together with a series of custom-made organosilica aerogels, it is shown that adjusting solvent and surface properties so that the two enantiomers (+) and (-) experience a different chemical environment is key. Otherwise, there might be a dynamic equilibrium between surface-adsorbed and mobile spin probes without stereodifferentiation. With this knowledge, it was possible to reach higher selectivity values than for the commercial material. A particularly interesting result was that better performance could be achieved if one attaches bulky, hydrophobic groups directly to the stereocenter. The effect of such neighboring groups on the enantioselectivity highly depends on the distance they have to the stereocenter.

KW - chirality

KW - confinement chemistry

KW - ESR spectroscopy

KW - organic inorganic hybrids

KW - porous materials

UR - http://www.scopus.com/inward/record.url?scp=105021855429&partnerID=8YFLogxK

U2 - 10.3762/bjnano.16.140

DO - 10.3762/bjnano.16.140

M3 - Article

AN - SCOPUS:105021855429

VL - 16

SP - 2034

EP - 2054

JO - Beilstein Journal of Nanotechnology

JF - Beilstein Journal of Nanotechnology

SN - 2190-4286

ER -