A Tunable, Fullerene-Based Molecular Amplifier for Vibrational Circular Dichroism

Absolute configuration and host-guest binding of chiral porphyrin-cages by a combined chiroptical and theoretical approach

Porphyrin cage-compounds are used as biomimetic models and substrate-selective catalysts in supramolecular chemistry. In this work we present the resolution of planar-chiral porphyrin cages and the determination of their absolute configuration by vibrational circular dichroism in combination with density functional theory calculations. The chiral porphyrin-cages form complexes with achiral and chiral viologen-guests and upon binding one of the axial enantiomorphs of the guest is bound selectively, as is indicated by induced-electronic-dichroism-spectra in combination with calculations. This host-guest binding also leads to unusual enhanced vibrational circular dichroism, which is the result of a combination of phenomena, such as rigidification of the host and guest structures, charge transfer, and coupling of specific vibration modes of the host and guest. The results offer insights in how the porphyrin cage-compounds may be used to construct a future molecular Turing machine that can write chiral information onto polymer chains.

Vibrational circular dichroism (VCD) spectroscopy can be useful for determining the absolute configuration of chiral molecules, as long as the signal intensities are high enough. Here, the authors establish the absolute configurations of two large chiral porphyrin cages and, notably, discover that host-guest binding enhances their VCD intensities.

A new horizon for vibrational circular dichroism spectroscopy: a challenge for supramolecular chirality

The development of the solid state and time-step VCD methods opened a new horizon to reveal the mechanism of chirality amplification from microscopic to supramolecular scales.

A Tunable, Fullerene-Based Molecular Amplifier for Vibrational Circular Dichroism

Vibrational circular dichroism (VCD) studies are reported on a chiral compound in which a fullerene C60 moiety is used as an electron acceptor and local VCD amplifier for an alanine-based peptide chain. Four redox states are investigated in this study, of which three are reduced species that possess low-lying electronic states as confirmed by UV/Vis spectroelectrochemistry. VCD measurements in combination with (TD)DFT calculations are used to investigate (i) how the low-lying electronic states of the reduced species modulate the amplification of VCD signals, (ii) how this amplification depends on the distance between oscillator and amplifier, and (iii) how the spatial extent of the amplifier influences amplification. These results pave the way for further development of tailored molecular VCD amplifiers.