β-cyclodextrin-anchored C18 brush stationary phases with enhanced shape selectivity for reversed-phase HPLC

Two β-cyclodextrin-anchored reversed-phase stationary phases (β-CD-COOC18 and β-CD-NHCOOC18) were developed using a "rigid anchor-flexible chain" strategy to simultaneously enhance molecular shape selectivity and high-aqueous stability. Octadecyl chains were covalently attached to a rigid β-cyclodextrin scaffold via ester or carbamate linkers and subsequently immobilized onto silica via thiol-ene click chemistry. Tanaka tests revealed that the resulting brush-type architectures possessed markedly improved shape selectivity (α = 2.37 for triphenylene/o-terphenyl) compared to a commercial C18 column (α = 1.41). This exceptional performance is attributed to a synergistic multiple-interaction mechanism, wherein the ordered C18 brush layer forms a flexible "soft cavity" for steric recognition, while the extended CD port (comprising the polar linkers and residual hydroxyls) contributes secondary interactions. Preliminary chiral evaluation using seven diverse probes confirmed that the native CD cavity is chromatographically inaccessible, further supporting this extended-port model. Consequently, the prepared phases enabled improved separation of challenging isomeric systems, including terphenyls, tocopherols, and photo-induced β-carotene geometric isomers. Furthermore, the embedded polar linkers and accessible hydroxyls contributed to outstanding resistance to hydrophobic collapse. In a 1440-min 100% aqueous stop-flow test, both β-CD-C18 phases maintained stable retention and efficiency, in stark contrast to the immediate performance drop of the commercial C18, enabling the effective separation of eleven highly polar nucleosides and bases under extreme aqueous conditions (98-99% H₂O). Overall, utilizing a rigid macrocyclic scaffold to spatially constrain flexible alkyl chains, combined with extended-port functionality, represents an effective approach to integrate specific shape recognition with broad reversed-phase applicability.