Biasing the dynamic Δ/Λ equilibrium of a helical and luminescent Tb(iii) complex on chiral amine-functionalized silica surfaces

The pre-existing dynamic Δ/Λ equilibrium of a racemic luminescent terbium (Tb) complex with a tetrakis-ortho-phenylphenoxide-functionalized 1,4,7,10-tetraazacyclododecane (cyclen) ligand (1_Tb) was biased by binding to a newly prepared, chiral (R)- or (S)-1-phenylethylamine (PEA)-functionalized SiO₂ surface ((R)- or (S)-L_NH2/SiO₂). The circular dichroism (CD) and circularly polarized luminescence (CPL) activities of 1_Tb were induced on the SiO₂ surface, indicating that the population ratio of the Δ and Λ helical conformations of 1_Tb was deviated from unity on (R)- or (S)-L_NH2/SiO₂. The induced chiroptical responses were more pronounced at lower surface densities of 1_Tb, suggesting an important role of the local chiral surface environment. Density functional theory (DFT) and time-dependent-DFT calculations suggested that the CD originated from the π-π* transition of the ortho-phenylphenoxide moiety of 1_Tb bound to (R)- or (S)-PEA. Comparison of the simulated and experimental CD spectra suggested preferential population of the Δ form of 1_Tb on (R)-L_NH2/SiO₂ and of the Λ form on (S)-L_NH2/SiO₂.