Chirality driven self-assembly in a fluorescent organogel

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.

Urea vs. carbamate groups: a comparative study in a chiral C2 symmetric organogelator

The effect of the replacement of molecular moieties (carbamates vs. urea) that drive self-assembly for two organogelators with an identical C(2) symmetric molecular structure is described. The main properties of the gels obtained from the urea-based organogelators are also discussed. The proposed organogelators are chiral molecules and are able to express chirality also at the supramolecular level, thus allowing the employment of electronic circular dichroism to gain insight into the molecular-scale structure of fibrillar aggregates. With the same technique, the behavior of enantiomeric mixtures of the urea-based organogelators was investigated, revealing the occurrence of different self-sorting phenomena at the molecular and supramolecular scale. The urea-based organogelators demonstrated to be more efficient gelators with respect to the carbamate-based analogues, showing a high gel-to-sol transition temperature (up to 66 °C) and a very low minimum gelling concentration (0.85 mg mL(-1)). This study is a starting point for a deeper investigation of structure/property relationships and, taking into account the peculiar behavior detected for the enantiomeric mixtures, also of self-sorting and molecular recognition phenomena.

Application of electronic circular dichroism in the study of supramolecular systems

Chiral supramolecular architectures constitute crucial structural and functional elements in living systems and have been long mimicked by chemists to synthesize new artificial systems endowed with desired properties and functions. Among several techniques to study noncovalent chiral assemblies or aggregates, electronic circular dichroism (ECD) plays a key role because many mechanisms responsible for the appearance of ECD bands occur through space, and therefore are intrinsically sensitive to intermolecular interactions, from short to long-range. The aim of this tutorial review is to emphasize the different kinds of information which can be obtained specifically when chiral supramolecular species are characterized by means of ECD spectroscopy. We will survey several typical applications of ECD in the context of supramolecular chemistry, ranging from the simple detection of chiral aggregates or complexes, to the definition of stoichiometric ratios between the partners, the derivation of thermodynamic and kinetic parameters such as binding and rate constants, and ultimately to the refinement of the most plausible structure of the supramolecular species.

Chiral/ring closed vs. achiral/open chain triazine-based organogelators: induction and amplification of supramolecular chirality in organic gels

The purpose of this study is to compare the gelling behavior of two molecules: a chiral compound and its achiral counterpart. The chiral partner is characterized by a rigid, chiral pyrrolidine nucleus, while the achiral one contains a flexible diethanolamine moiety. The chiral compound is an already known good organogelator, but also the achiral compound shows remarkable gelling properties. Very interestingly, a small fraction of the chiral compound induces chirality and strong CD effects in its aggregates with the achiral one. The observed chirality amplification corresponds to a peculiar sergeant-and-soldier effect. Molecular modelling and CD calculations suggested a model for the supramolecular assembly of hetero-aggregates that fits the experimental data.

Strong intermolecular exciton couplings in solid-state circular dichroism of aryl benzyl sulfoxides

A series of 13 enantiopure aryl benzyl sulfoxides () with different substituents on the two aromatic rings has been previously analyzed by means of electronic circular dichroism (CD) spectroscopy. Most of these compounds are crystalline and their X-ray structure is established. For almost one-half of the series, CD spectra measured in the solid state were quite different from those in acetonitrile solution. We demonstrate that the difference is due to strong exciton couplings between molecules packed closely together in the crystal. The computational approach consists of time-dependent density functional theory (TDDFT) calculations run on "dimers" composed of nearest neighbors found in the lattice. Solid-state CD spectra are well reproduced by the average of all possible pairwise terms. The relation between the crystal space group and conformation, and the appearance of solid-state CD spectra, is also discussed.