Chiral diketopyrrolo[3,4-c]pyrrole-based oligothiophenes: Synthesis and characterization of aggregated states in solution and thin films.

In this work, we synthesized a family of three structurally related chiral oligothiophenes containing a 1,4-diketo-3,6-diarylpyrrolo[3,4-c]pyrrole (DPP) unit as the central core; functionalized with the same (S)-3,7-dimethyl-1-octyl chains on the nitrogen atoms of lactam moieties, they only differ in the number of lateral thiophene units. The aggregation modes of these π-conjugated chiral systems were evaluated by means of UV-Vis absorption and ECD spectroscopies in conditions of solution aggregation (CHCl3 /MeOH mixtures) and as thin films, describing in particular the impact of the π-conjugation length on the chiroptical properties. Interestingly, we found that the variable number of thiophene units attached to the DPP core affects not only the propensity to aggregation but also the aggregates' helicity. ECD revealed information about the supramolecular arrangement of these molecules, that one would not obtain by using conventional optical spectroscopy and microscopy techniques. Thin film samples revealed very different aggregation modes with respect to solution aggregates, casting doubts on the common assumption that these latter may serve as simple models of the former ones.

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.

Chiral Oligothiophenes with Remarkable Circularly Polarized Luminescence and Electroluminescence in Thin Films.

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.

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.

Synthesis of 3-Alkylideneisoindolin-1-ones via Sonogashira Cyclocarbonylative Reactions of 2-Ethynylbenzamides.

Cyclocarbonylative Sonogashira reactions of ortho-ethynylbenzamides have been investigated. The process is carried out under CO pressure, in the presence of a very small amount of PdCl2(PPh3)2 (0.4 mol %) as a catalytic precursor and without the need for a Cu salt as the co-catalyst. 2-Ethynylbenzamide reacted successfully with iodoarenes bearing electron-withdrawing and electron-donating groups, giving rise to different classes of compounds depending on the solvent used. On the contrary, N-(4-chlorophenyl)-2-ethynylbenzamide afforded exclusively polyfunctionalized isoindolinones with high stereoselectivity toward (E) isomers.

Stereoselective synthesis of chiral 3-aryl-1-alkynes from bromoallenes and heterocuprates.

The synthesis of chiral 3-aryl-1-alkynes 3 via cross-coupling of 3-alkyl- and 3,3-dialkyl-1-bromo-1,2-dienes 1 and arylbromocuprates (RCuBr)MgBr.LiBr 2 was examined. With phenylcopper reagents and its para-substituted derivatives, as well as with 2-naphthyl cuprates, the reaction gave compounds 3 with high regioselectivity and good yields on the chemically pure product. On the contrary, when ortho-substituted phenyl reagents and 1-naphthyl cuprates were used, the regioselectivity of the process was very dependent upon the steric requirements of the alkyl substituents on the bromoallenic substrate. When the steric bulk was increased, remarkable quantities of isomeric arylallenes 4 were also observed in the reaction mixtures. The high 1,3-anti stereoselectivity of the coupling process allowed us to obtain enantiomerically enriched 3-aryl-1-alkynes from optically active allenic substrates, thus indicating a simple pathway toward the synthesis of quaternary stereogenic centers characterized by an aryl group. A possible cross-coupling mechanism was also suggested to explain the regio- and stereochemical data. For the preparation of omega-functionalized 3-phenyl-1-alkynes, the reaction of 1-bromo-3-phenylpropadiene with Knochel reagents RCu(CN)ZnCl.2LiCl was also studied; this reaction led to the acetylenic compounds in high yields mainly when the R group (also omega-functionalized) on the copper reagent was primary.

Fluoride-promoted rearrangement of organo silicon compounds: a new synthesis of 2-(arylmethyl)aldehydes from 1-alkynes.

A new approach to 2-(arylmethyl)aldehydes 4 based upon a 1,2-anionotropic rearrangement of an aryl group is presented. The synthetic sequence begins with a silylformylation reaction of terminal acetylenes 5 with aryl and heteroaryl silanes 6, followed by treatment of the products (Z)-1 with TBAF. The optimization of the experimental conditions of the fluoride-promoted step is described, together with the synthetic potentialities of the process. A plausible mechanism of the rearrangement reaction is reported that suggests the addition of the fluoride ion to the arylsilicon moiety of beta-silylalkenals (Z)-1 and the consequent migration of the aryl group to the adjacent carbon atom. Both aryl and heteroaryl substituents can rearrange without any loss of configuration. Bromo-functionalized substrates undergo an intramolecular reaction that affords exclusively carbacyclobenzyl aldehydes, further enhancing the high synthetic value of this method.