Magnetic Circular Dichroism Elucidates Molecular Interactions in Aggregated Chiral Organic Materials.

Chiral materials formed by aggregated organic compounds play a fundamental role in chiral optoelectronics, photonics and spintronics. Nonetheless, a precise understanding of the molecular interactions involved remains an open problem. Here we introduce magnetic circular dichroism (MCD) as a new tool to elucidate molecular interactions and structural parameters of a supramolecular system. A detailed analysis of MCD together with electronic circular dichroism spectra combined to ab initio calculations unveils essential information on the geometry and energy levels of a self-assembled thin film made of a carbazole di-bithiophene chiral molecule. This approach can be extended to a generality of chiral organic materials and can help rationalizing the fundamental interactions leading to supramolecular order. This in turn could enable a better understanding of structure-property relationships, resulting in a more efficient material design.

Spatially Resolved Chiroptical Spectroscopies Emphasizing Recent Applications to Thin Films of Chiral Organic Dyes.

Instrumental techniques able to identify and structurally characterize the aggregation states in thin films of chiral organic π-conjugated materials, from the first-order supramolecular arrangement up to the microscopic and mesoscopic scale, are very helpful for clarifying structure-property relationships. Chiroptical imaging is currently gaining a central role, for its ability of mapping local supramolecular structures in thin films. The present review gives an overview of electronic circular dichroism imaging (ECDi), circularly polarized luminescence imaging (CPLi), and vibrational circular dichroism imaging (VCDi), with a focus on their applications on thin films of chiral organic dyes as case studies.

Chiral Diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole Dyes with Highly Tuneable Properties in Solution and Thin Films.

We have studied the impact of achiral substituents on the chiral supramolecular architectures of diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole (DPP) dyes. We decorated the same chiral DPP motif with substituent groups on the nitrogen atoms of the lactam moiety: the hydrophobic n-octyl alkyl chain, the hydrophilic tri(ethylene glycol) (TEG) chain and the thermo-cleavable tert-butoxycarbonyl (t-Boc) carbamate group. In spite of having identical conjugated chromophore and chiral appendages, in aggregated form the three dyes displayed profoundly different optical, chiroptical, electrochemical and thermal features. ECD measurements revealed differences in the aggregation modes, which would be inaccessible by most other techniques. We found strong chiroptical features, which would have major implications in the context of chiral organic opto-electronics and in the development of other highly innovative technological applications.

Chiral Diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole Dyes with Highly Tuneable Properties in Solution and Thin Films.

Invited for the cover of this issue are the research groups of Lorenzo Di Bari at the University of Pisa and Gianluca Maria Farinola at the University of Bari Aldo Moro. The image depicts three diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole dyes with the same chiral appendage R* but different achiral substituent groups Y showing profoundly different features in their aggregated form. Read the full text of the article at 10.1002/chem.202300291.

Rational Design of New Conjugated Polymers with Main Chain Chirality for Efficient Optoelectronic Devices: Carbo[6]Helicene and Indacenodithiophene Copolymers as Model Compounds.

The unique properties of conjugated polymers (CPs) in various optoelectronic applications are mainly attributed to their different self-assembly processes and superstructures. Various methods are utilized to tune and control CP structure and properties with less attention paid to the use of chirality. CPs with main chain chirality are rare and their microscopic and macroscopic properties are still unknown. In this work, the first experimental results are provided along these lines by synthesizing a series of racemic and enantiopure CPs containing statistical and alternating carbo[6]helicene and indacenodithiophene moieties and evaluating their microscopic (optical, energy levels) and macroscopic properties (hole mobilities, photovoltaic performance). It is demonstrated that a small statistical insertion of either the racemic or enantiopure helicene into the polymer backbone finely tunes the microscopic and macroscopic properties as a function of the statistical content. The microscopic properties of the enantiopure versus the racemic polymers with the same helicene loading remain similar. On the contrary, the macroscopic properties, and more interestingly those between the two enantiomeric forms, are altered as a function of the statistical content. Once incorporated into a solar cell device, these chiral CPs display better performance in their enantiopure versus racemic forms.

Circularly Polarized Microscopy of Thin Films of Chiral Organic Dyes.

We introduce an optical microscopy technique, circularly polarized microscopy or CPM, able to afford spatially resolved electronic circular dichroism (ECD) of thin films of chiral organic semiconductors through a commercial microscope equipped with a camera and inexpensive optics. Provided the dichroic ratio is sufficiently large, the spatial resolution is on the order of the µm and is only limited by the magnification optics integrated in the microscope. We apply CPM to thin films of small chiral π-conjugated molecules, which gave rise to ordered aggregates in the thin layer. Primarily, conventional ECD can reveal and characterize chiral supramolecular structures and possible interferences between anisotropic properties of solid samples; however, it cannot generally account for the spatial distribution of such properties. CPM offers a characterization of supramolecular chirality and of commingling polarization anisotropies of the material, describing their local distribution. To validate CPM, we demonstrated that it can be adopted to quantify the local ECD of samples characterized by intense signals, virtually on any standard optical microscope.

Emergent Nonreciprocal Circularly Polarized Emission from an Organic Thin Film.

Controlling circularly polarized (CP) emission is key for both fundamental understanding and applications in the field of chiral photonics and electronics. Here, a completely new way to achieve this goal is presented. A luminescent thin film, made from a chiral conjugated phenylene bis-thiophenylpropynone able to self-assemble into ordered structures, emits highly circularly polarized light with opposite handedness from its two opposite faces. Such emergent nonreciprocal behavior in CP emission, so far unprecedented, represents a fundamental advance, opening new opportunities in design, preparation, and applications of CP emitting materials.