Synthesis of Cycloparaphenylenes Bearing Furan-2,5-diyl or 2,2’-Bifuran-5,5’-diyl Units in the Macrocyclic Structures

Grafting Electron-Accepting Fragments on [4]cyclo-2,7-carbazole Scaffold: Tuning the Structural and Electronic Properties of Nanohoops

Since the first applications of nanohoops in organic electronics appear promising, the time has come to go deeper into their rational design in order to reach high-efficiency materials. To do so, systematic studies dealing with the incorporation of electron-rich and/or electron-poor functional units on nanohoops have to be performed. Herein, the synthesis, the electrochemical, photophysical, thermal, and structural properties of two [4]cyclo-2,7-carbazoles, [4]C-Py-Cbz, and [4]C-Pm-Cbz, possessing electron-withdrawing units on their nitrogen atoms (pyridine or pyrimidine) are reported. The synthesis of these nanohoops is first optimized and a high yield above 50% is reached. Through a structure-properties relationship study, it is shown that the substituent has a significant impact on some physicochemical properties (eg HOMO/LUMO levels) while others are kept unchanged (eg fluorescence). Incorporation in electronic devices shows that the most electrically efficient Organic Field-Effect transistors are obtained with [4]C-Py-Cbz although this compound does not present the best-organized semiconductor layer. These experimental data are finally confronted with the electronic couplings between the nanohoops determined at the DFT level and have highlighted the origin in the difference of charge transport properties. [4]C-Py-Cbz has the advantage of a more 2D-like transport character than [4]C-Pm-Cbz, which alleviates the impact of defects and structural organization.

Synthesis and Photophysical Properties of Silole-Fused Cycloparaphenylenes

Herein, we report the introduction of a silole unit into cycloparaphenylenes (CPPs), and two compounds [12]Si3CPP and [16]Si4CPP are obtained by a platinum- and gold-mediated cyclooligomerization strategy. Their optical and electronic properties are studied by UV-vis absorption and fluorescence spectra, which show red shifts and higher photoluminescence quantum yields (PLQYs) compared with the corresponding CPPs.

Transition-Metal-Free Continuous-Flow Synthesis of 2,5-Diaryl Furans: Access to Medicinal Building Blocks and Optoelectronic Materials

The direct transformation of 1,3-dienes into valuable 2,5-diarylfurans using transition-metal-free conditions is presented. By employing a simple oxidation─dehydration sequence on readily accessible 1,3-dienes, important 2,5-diarylfuran building blocks frequently used in medicinal and material chemistry are prepared. The oxidation step is realized using singlet oxygen, and the intermediate endoperoxide is dehydrated under metal-free conditions and at ambient temperature using the Appel reagent. Notably, this sequence can be streamlined into continuous flow, thereby eliminating the isolation of the intermediate, often unstable endoperoxide. This leads to a significant improvement in isolated yields (ca. 27% average increase) of the 2,5-diarylfurans while also increasing safety and reducing waste. Our transition-metal-free synthetic approach to 2,5-diarylfurans delivers several important furan building blocks used commonly in medicinal chemistry and as optoelectronic materials, including short-chain linearly conjugated furan oligomers. Consequently, we also complete a short study of the optical and electrochemical properties of a selection of these novel materials.

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero- or Polycyclic Aromatics

In the last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops. A number of potential applications emerged, including optoelectronic devices, and their use as templates for carbon nanomaterials and in supramolecular chemistry. To tune the structural or optoelectronic properties of carbon nanohoops beyond the size-dependent effect known for [n]CPPs, a variety of aromatic rings other than benzene were introduced. In this Review, we provide an overview of the syntheses, properties, and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or such that contain acceptor, donor, heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.

A BN-Doped Cycloparaphenylene Debuts

The first example of a BN-doped cycloparaphenylene BN-[10]CPP was synthesized and characterized. Its reactivity and photophysical properties were evaluated in direct comparison to its carbonaceous analogues Mes-[10]CPP and [10]CPP. While the photophysical properties of BN-[10]CPP remains similar to its carbonaceous analogues, the electronic structure changes associated with the introduction of a 1,2-azaborine BN heterocycle into a CPP scaffold enables facile and selective late-stage functionalizations that cannot be accomplished with carbonaceous CPPs. Specifically, Ir-catalyzed hydrogenation of BN-[10]CPP selectively reduces the BN heterocyclic ring, which upon hydrolysis produces a rare example of a macrocyclic paraphenylene 6 incorporating the versatile ketone functionality within the macrocyclic ring.

Synthesis, Structure, and Photophysical Properties of m-Phenylene-Embedded Cycloparaphenylene Nanorings

Five m-phenylene-embedded cycloparaphenylenes m₃[9]CPP 1-5 were synthesized by the platinum-mediated cyclooligomerization strategy with high overall yields. The structures of m₃[9]CPP 1-3 were determined by X-ray diffraction analysis. Compared to [9]CPP, m₃[9]CPP 1 caused a significant blueshift in the UV-vis absorption and fluorescence spectra. This result shows that the radial π-conjugation is distorted and partially interrupted. The photophysical properties of m₃[9]CPP 1 were further tuned by the introduction of various substituents for m₃[9]CPP 2-5. Methoxy group substitution at m-phenylene did not change the photophysical properties significantly. Replacement of m-phenylene by tetrafluoro-m-phenylene achieved a significant blueshift. When the carboxyl group was embedded at m-phenylene or the methoxy group was embedded at p-phenylene, significant redshifts were observed with blue color emission. Theoretical calculations revealed that the decrease in the HOMO-LUMO gap in m₃[9]CPP 4 and 5 is favorable for the redshift of the fluorescence spectrum.

Synthesis of Partially Hydrogenated Cycloparaphenylenes with Bent and Fused Structures Bearing Armchair Carbon Nanotube-like Connections

The Diels-Alder reactions between 2 equiv of (E,E)-1,4-bis(4-bromophenyl)-1,3-butadiene and 1,4-benzoquinone led to the formation of a key intermediate with all four 4-bromophenyl substituents cis to one another. The subsequent nickel-mediated homocoupling reactions then produced partially hydrogenated cycloparaphenylenes, including a molecule bearing two units of tetrahydro[6]cycloparaphenylene (4H[6]CPP) fused together through two 1,4-dimethoxybenzene units in an armchair (6,6)carbon nanotube-like connection. Similarly, two 6H[9]CPPs were connected through three 1,4-dimethoxybenzene units in an armchair (9,9)carbon nanotube-like arrangement. A bent 8H[12]CPP and a bent 12H[18]CPP, which were fused intramolecularly with two and three 1,4-dimethoxybenzene units, respectively, to create the bent structures, were likewise synthesized. A molecule containing a bent 8H[12]CPP fused to a 4H[6]CPP was likewise constructed. The structures of these partially hydrogenated CPPs were established by X-ray structure analysis, NMR spectroscopy, and additional independent synthetic pathways.