A liquid chromatographic charge transfer stationary phase based on the thiol-yne reaction

In the process of developing a pH-stable, highly crosslinked stationary phase using the thiol-yne reaction, a new charge transfer stationary phase was discovered. The first step in the preparation of the crosslinked phase is to attach 1,4-diethynylbenzene (DEB) to thiol functionalized silica particles using the thiol-yne reaction. Upon preparation of that phase, we noticed that the color of the particles was different when the modified particles were wet with aromatic solvents in comparison to wetting with nonaromatic or aqueous solvents. This color change was still apparent upon crosslinking the DEB with 1,6-hexanedithiol to create the crosslinked stationary phase. The chromatographic selectivity for the flat triphenylene over the bulkier o-terphenyl, α_T/OT is an indicator of shape selectivity. The crosslinked phase⬢s α_T/OT is 4.91 ± 0.08, almost twice that of the most shape-selective reversed phase column. The difference of the entropy contributions to retention free energy between the two compounds, ΝTΝS° at 298⬰K, is statistically indistinguishable from zero, (↙0.1⬰±⬰0.9⬰kJ/mol) leading us to believe that the observed shape selectivity is not consistent with the slot model. To test the hypothesis that the DEB-thiol adduct, a 4-ethynyl styryl sulfide (ESS), was responsible for the observed behavior, we prepared a low coverage ESS-containing phase which, unlike higher density, crosslinked, or polymeric phases, should not display shape selectivity based on ⬓slots⬽. With the ESS phase the shape selectivity remained high, with α_T/OT⬰=⬰3.23⬰±⬰0.01. The ESS ligand has electron donating characteristics based on the selectivity for nitrobenzene compared to benzene: 1.83⬰±⬰0.10 on the ESS phase vs 0.64 ± 0.01 on a commercial C18 stationary phase. This shows that the thiol-yne based ESS stationary phase has electron donating charge transfer characteristics.