Bringing to light transient molecular structure and function using advanced vibrational spectroscopy

As chemical synthetic methods have developed so has the complexity and functionality of synthesised compounds. In order to investigate their structure and function, especially when the function results from a perturbation, more sophisticated spectroscopic techniques have to be employed. This thesis is concerned with utilising advanced vibrational spectroscopic techniques to investigate a multitude of complex compounds ranging from molecular rotors and photochromic compounds to chiral structures such as enantioselective supramolecular catalysts. To probe this wide range of molecular systems two methods are employed, time-resolved vibrational spectroscopy and vibrational circular dichroism, which use infrared radiation to probe the vibrational modes of the compounds. Time-resolved vibrational spectroscopy has been used to understand: How a change in the symmetry of a previously successful photochromic compound inhibits its ability to cyclise, the complete life cycle of a prototypical photochromic compound by demystifying the current qualitative description, and finally to probe the dynamics of a sensitive BODIPY-based fluorescent molecular rotor. Vibrational circular dichroism, a technique sensitive to chirality, aided in the discovery of the structure of a supramolecular catalyst that yields the highest enantioselectivity. Also an extension onto the work of amplified vibrational circular dichroism is reported using a very large amplifier that can occupy multiple redox state

Interplay of exciton coupling and large-amplitude motions in the vibrational circular dichroism spectrum of dehydroquinidine

A detailed analysis of the computed structure, energies, vibrational absorption (VA) and circular dichroism (VCD) spectra of 30 low-energy conformers of dehydroquinidine reveals the existence of families of pseudo-conformers, the structures of which differ mostly in the orientation of a single O[BOND]H bond. The pseudo-conformers in a family are separated by very small energy barriers (i.e., 1.0 kcal mol−1 or smaller) and have very different VCD spectra. First, we demonstrate the unreliable character of the Boltzmann factors predicted with DFT. Then, we show that the large differences observed between the VCD spectra of the pseudo-conformers in a family are caused by large-amplitude motions involving the O[BOND]H bond, which trigger the appearance/disappearance of strong VCD exciton-coupling bands in the fingerprint region. This interplay between exciton coupling and large-amplitude-motion phenomena demonstrates that when dealing with flexible molecules with polar bonds, vibrational averaging of VCD spectra should not be neglected. In this regard, the dehydroquinidine molecule considered here is expected to be a typical example and not the exception to the rule

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

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