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.
A new chiral benzo[1,2-b:4,5-b']dithiophene-based oligothiophene has been synthesized and spectroscopically characterized in solution and in the solid state. The electronic circular dichroism (ECD) spectra of the thin films revealed the occurrence of an uncommon phenomenon, consisting of the inversion of the circular dichroism sign by sample flipping. To the best of our knowledge, this is unprecedented in samples where no preferential orientation of the chromophores has been induced, and must be due to spontaneous local anisotropies. It reveals peculiar features upon the usual solvent annealing procedure, but finally results in stable and very strong signals, which are perfectly reproduced in sign, and relative and even absolute magnitude. Although the recorded signal cannot be considered as a true ECD spectrum, it corresponds to a very large discrimination of light circular polarization, which is well explained by means of the Mueller matrix analysis and does not depend on instrumental defects. It is a genuine property of this material, which correlates with polarized optical microscopy (POM) images and only depends on the self-assembling properties of this molecule. This phenomenon may have relevant implications, on account of its strength and of the interest of oligothiophenes in current optoelectronic devices
A chiral benzo[1, 2‐b:4,5‐b′]dithiophene‐based oligothiophene with the very uncommon chiroptical property of signal inversion on sample flipping was recently described. This property is due to the interference between linear dichroism and linear birefringence, called LDLB effect, which is theoretically well understood, but to date very rarely reported in the literature. Samples with very large LDLB effect lead to the unique possibility of discriminating the direction of sample illumination (i. e., from the front or back). This article presents a set of analogue compounds, in which small structural changes are introduced to uncover which factors determine the very large LDLB effect or true circular dichroism connected to supramolecular chirality. It also reveals the impact of the deposition technique, in which spin coating may exert a primary role, and results in films with outstanding chiroptical features. These features are made even more relevant due to the semiconducting properties of oligothiophenes, which are interesting for optical sensing applications.
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.
A detailed investigation of the circular dichroism imaging (CDi) technique on thin films of a chiral 1,4-dialkoxyphenylene-based oligothiophene with outstanding chiroptical features revealed the primary role of local supramolecular structures.
Silk fibroin supported Pd (Pd/SF) has been prepared and used as catalyst in Suzuki–Miyaura cross‐coupling reactions in water/ethanol (4:1 v/v) mixture. The reactions proceed rapidly with aryl iodides and boronic acids with different electronic properties using low metal loading (0.38 mol‐%). Pd/SF exhibits better recyclability compared to other biopolymer‐supported Pd catalysts, up to nineteen cycles without loss of activity.
This work reports the first observation of circularly polarized electroluminescence (CPEL) in thin films of self‐organized oligothiophenes. Four new 1,4‐phenylene and 9H‐carbazole‐based oligothiophenes were ad hoc designed to ensure efficient spontaneous formation of chiral supramolecular order. They were easily synthesized and their chiroptical properties in thin films were measured. Circularly polarized luminescence (CPL) spectra revealed glum in the order of 10−2 on a wide wavelengths range, originating from their self‐organized chiral supramolecular organization. These molecules have reasonable properties as organic semiconductors and for this reason they can constitute the active layer of circularly‐polarized organic light‐emitting diodes (CP‐OLEDs). Thus, we could investigate directly their electroluminescence (EL) and CPEL, without resorting to blends, but rather in a simple multilayer device with basic architecture. This is the first example of a CP‐OLED with active layer made only of a small organic compound.
New luminophores having different aryl nuclei and propynones moieties have been obtained via Sonogashira reactions. Their optical properties were evaluated and indicated that carbonyl groups are responsible for significant bathochromic effects and high Stokes shifts. The insertion of-OMe groups on the central benzene unit gives to the fluorophore high optical efficiency (7.7%) when homogeneously dispersed in a poly(cyclohexyl methacrylate) (PCMA) film and connected to a PV cell.
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