Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Assignment of Vibrational Circular Dichroism Cross-Referenced Electronic Circular Dichroism Spectra of Flexible Foldamer Building Blocks: Towards Assigning Pure Chiroptical Properties of Foldamers

Assignment of the most established electronic circular dichroism (ECD) spectra of polypeptides and foldamers is either “evidence based” or relies on the 3D structures of longer oligomers of limited internal dynamics, which are derived from NMR spectroscopy (or X‐ray) data. Critics warn that the use of NMR spectroscopy and ECD side by side has severe limitations for flexible molecules because explicit knowledge of conformational ensembles is a challenge. Herein, an old–new method of comparing ab initio computed and measured vibrational circular dichroism (VCD) data is presented to validate both the structures (conf(i)) and their relative weights (c(i)) that make up the conformational ensemble. Based on the array of {conf(i), c(i)}, the pure ECD spectra, g(i)^conf(i), can be ab initio calculated. The reconstructed spectrum Σc(i)g(i)^conf(i) can thus help to assign any experimental ECD counterparts. Herein, such a protocol is successfully applied to flexible foldamer building blocks of sugar β‐amino acid diamides. The epimeric pair of the model system was selected because these molecules were conformationally tunable by simple chemical modification, and thus, the robustness of the current approach could be probed. The initial hydrogen bond (NH⋅⋅⋅O) eliminated by N‐methylation reorients the amide plain, which influences the chiroptical properties of the foldamer building block; this structural change is successfully monitored by changes to the VCD and ECD transitions, which are now assigned to pure conformers. The current method seems to be general and effective without requiring extensive CPU and spectroscopic resources.

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.

Chirality Induction from a Chiral Guest to the Hydrogen-Bonding Network of Its Hexameric Resorcinarene Host Capsule

The hexameric capsule of resorcin[4]arene 1 is capable of encapsulating tertiary amines, which has recently been used in the application of [(1)₆ (H₂ O)₈ ] as (co-)catalyst in various asymmetric reactions. However, not much is known about the highly asymmetric but conformationally very dynamic structure of the capsule after uptake of chiral molecules. Therefore, in this contribution, we utilize electronic circular dichroism and vibrational circular dichroism spectroscopy to investigate how several chiral guest molecules affect the structural preferences of the capsule [(1)₆ (H₂ O)₈ ]. In particular, we show that one small chiral amine encapsulated in [(1)₆ (H₂ O)₈ ] is sufficient to control and dictate the stereochemical preferences of the entire capsule. Furthermore, neither strong π-π interactions nor a significant steric bulk are required for this induction. The observation of such a chiral imprint of the guest's stereochemistry onto its host molecule is expected to have implications also for other supramolecular capsules.

Exploring the role of molecular chirality in the photo-responsiveness of dipeptide-based gels

Chiral effect: upon UV light irradiation, the l-gel has a markedly faster gel–sol transition than the d-gel.

Vibrational optical activity as probe for intermolecular interactions

A detailed VCD spectroscopic analysis of well-selected chiral model systems can give valuable and unprecedented insights into intermolecular interactions such as solvation or reactant–substrate binding in catalysis.