π‐Electron‐System‐Layered Polymer: Through‐Space Conjugation and Properties as a Single Molecular Wire

[2.2]Paracyclophane‐based through‐space conjugated oligomers and polymers were prepared, in which poly(p‐arylene–ethynylene) (PAE) units were partially π‐stacked and layered, and their properties in the ground state and excited state were investigated in detail. Electronic interactions among PAE units were effective through at least ten units in the ground state. Photoexcited energy transfer occurred from the stacked PAE units to the end‐capping PAE moieties. The electrical conductivity of the polymers was estimated using the flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC) method and investigated together with time‐dependent density functional theory (TD‐DFT) calculations, showing that intramolecular charge carrier mobility through the stacked PAE units was a few tens of percentage larger than through the twisted PAE units.     

Control of Circularly Polarized Luminescence by Orientation of Stacked π‐Electron Systems

Planar chiral building blocks based on 4,7,12,15‐tetrasubstituted [2.2]paracyclophanes were obtained via a synthetic route involving an optical resolution step. Planar chiral enantiomers, comprising two fluorophores that were stacked to form a V‐shaped higher‐ordered structure, were synthesized from these building blocks. The V‐shaped molecules emitted intense circularly polarized luminescence (CPL). Their chiroptical properties were compared with those of X‐shaped molecules bearing the same two fluorophores stacked together. The CPL sign of the X‐shaped molecule was opposite to that of the V‐shaped molecule, which is supported by the theoretical results, indicating that the CPL sign can be controlled by the orientation of the stacked fluorophores.     

Synthesis of through‐space conjugated polymers containing [2.2]paracyclophane and thieno[3,4‐b]pyrazine in the main chain

We synthesized through‐space conjugated polymers with [2.2]paracyclophane and thieno[3,4‐b]pyrazine units in the main chain by the Sonogashira–Hagihara coupling reaction. The obtained polymers were soluble in common organic solvents, and homogeneous thin films were readily obtained from the polymer solutions by spin‐coating techniques. The polymers exhibited the extension of the conjugation length via the through‐space interaction. The polymers showed orangish‐red emission with peak maxima of around 610 nm in diluted solutions and their thin films, which were derived from the thieno[3,4‐b]pyrazine moieties. The optical and electrochemical behaviors of the polymers containing pseudo‐para‐ and pseudo‐ortho‐linked [2.2]paracyclophane were identical. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

New Access to the Chemistry of Piperonylidinyl‐4‐[2.2]‐Paracyclophanylamine and Some Electron π‐Deficients

Piperonylidinyl‐4‐[2.2]paracyclophanylamine 1 reacted with some electron π‐deficients via charge‐transfer complexation and afforded amino derivatives of [2.2[paracyclophane 2–9 . Transannular electronic interaction existing in a cyclophane molecule plays an essential role for product formation.     

Conformationally Complex π‐Conjugated Molecular and Polymeric Materials: New Challenges for Organic Synthesis

No protection? A unique, albeit fairly specialized, example of what promises to be a useful retrosynthetic consideration for those designing complex π‐molecular systems, namely, using conformation as a protecting group to shield a given reactive site in lieu of protecting‐group manipulation, is described (see scheme).

     

[2.2]paracyclophane‐based through‐space conjugated polymers with fluorescence quenchers

New [2.2]paracyclophane‐based through‐space conjugated polymers containing fluorescence quenchers such as anthraquinone and ferrocene units at the polymer termini were designed and synthesized. Their optical properties were investigated in detail. Fluorescence emission from the stacked π‐electron systems was effectively quenched by the stacked π‐electron systems at the polymer termini due to the energy and electron transfer through a single polymer chain; thus, the polymers acted as the molecular wire. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Through‐Bond/Through‐Space Anion Relay Chemistry Exploiting Vinylepoxides as Bifunctional Linchpins

The development of new bifunctional linchpins that permit the union of diverse building blocks is essential for the synthetic utility of anion relay chemistry (ARC). The design, synthesis, and validation of three vinylepoxide linchpins for through‐bond/through‐space ARC are now reported. For negative charge migration, this class of bifunctional linchpins employs initial through‐bond ARC by an S_N2′ reaction, followed by through‐space ARC exploiting a 1,4‐Brook rearrangement. The trans‐disubstituted vinylepoxide linchpin yields a mixture of E/Z isomers, whereas the cis‐disubstituted and the trans‐trisubstituted vinylepoxide linchpins proceed to deliver three‐component adducts with excellent E selectivity.     

Electron Transport through Single π‐Conjugated Molecules Bridging between Metal Electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron‐transport properties of single π‐conjugated molecule junctions, which include benzene, fullerene, and π‐stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single‐molecule junctions. The effect of the metal–molecule contact on the conductance of the single π‐conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π‐stacked system using π‐stacked aromatic molecules enclosed within self‐assembled coordination cages is reviewed. The electron transport in the π‐stacked systems is found to be efficient at the single‐molecule level, thus providing insight into the design of conductive materials.     

Synthesis and properties of through‐space conjugated polymers based on cyano‐substituted poly(p‐arylenevinylene)s

New through‐space cyano‐substituted poly(p‐arylenevinylene)s containing a [2.2]paracyclophane unit were synthesized by the Knoevenagel reaction. Polymers 5 and 7 have cyano groups at α‐positions and β‐positions from the dialkoxyphenylene unit, respectively. Their optical and electrochemical behaviors were investigated in detail in comparison with their model compounds. Polymers 5 and 7 exhibited through‐space conjugation via the cyclophane units. Polymer 5 showed greenish blue emission (λ_max = 477 nm) in diluted solution with fluorescence quantum efficiency (?_F) of only 0.007, whereas polymer 7 emitted in the bluish green region (λ_max = 510 nm) with ?_F of 0.32. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5979–5988, 2009     

Achieving Efficient Multichannel Conductance in Through‐Space Conjugated Single‐Molecule Parallel Circuits

Constructing single‐molecule parallel circuits with multiple conduction channels is an effective strategy to improve the conductance of a single molecular junction, but rarely reported. We present a novel through‐space conjugated single‐molecule parallel circuit (f‐4Ph‐4SMe) comprised of a pair of closely parallelly aligned p‐quaterphenyl chains tethered by a vinyl bridge and end‐capped with four SMe anchoring groups. Scanning‐tunneling‐microscopy‐based break junction (STM‐BJ) and transmission calculations demonstrate that f‐4Ph‐4SMe holds multiple conductance states owing to different contact configurations. When four SMe groups are in contact with two electrodes at the same time, the through‐bond and through‐space conduction channels work synergistically, resulting in a conductance much larger than those of analogous molecules with two SMe groups or the sum of two p‐quaterphenyl chains. The system is an ideal model for understanding electron transport through parallel π‐stacked molecular systems and may serve as a key component for integrated molecular circuits with controllable conductance.     

Energy Transfer from Locally Excited π* to Charge Transfer Ground States in a Silylene–π Hetero‐Junction Polymer

Summary: The silylene–π conjugating polymer, poly(di‐n‐hexylsilylenephenylene‐ethynylenephenylene) ( 1 ) adopted a fairly flexible coil‐like conformation due to the bent structure of silylene moiety and showed a unique photoexcited energy transfer behavior. The UV‐vis absorption and steady‐state/time‐resolved photoluminescence studies revealed the occurrence of an intramolecular photoexcited energy transfer (IET) between locally excited π* to charge transfer ground states as well as an intramolecular charge transfer (ICT).

The silylene–π conjugating polymer, poly(di‐n‐hexylsilylenephenylene‐ethynylenephenylene) showing a unique photoexcited energy transfer behavior.     

Metal‐Catalyzed Annulation Reactions for π‐Conjugated Polycycles

The progress of the metal‐catalyzed annulation reactions toward construction of various π‐conjugated polycyclic cores with high conjugation extension is described. This article gives a brief overview of various annulation reactions promoted by metal catalysts including C?H bond functionalization, [2+2+2] cycloaddition, cascade processes, ring closing metathesis, electrophilic aromatization, and various cross‐coupling reactions. A variety of conjugated polycycles with planar, bowl‐shaped, and helical structures have been constructed in high efficiency and selectivity.     

Preparation and Acid‐Responsive Photophysical Properties of T‐Shaped π‐Conjugated Molecules Containing a Benzimidazole Junction

T‐shaped π‐conjugated molecules with an N‐methyl‐benzimidazole junction have been synthesized and their acid‐responsive photophysical properties owing to the change in the π‐conjugation system are discussed. T‐shaped π‐conjugated molecules consist of two orthogonal π‐conjugated systems including a phenyl thiophene extended from the 2‐position and alkyl phenylenes connected through various π‐spacers from the 4,7‐positions of the N‐methyl‐benzimidazole junction. The π‐spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited‐state energy and charge‐transfer (CT) characteristics. In particular, the π‐conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature‐dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations.     

Dissymmetrical U‐Shaped π‐Stacked Supramolecular Assemblies by Using a Dinuclear CuI Clip with Organophosphorus Ligands and Monotopic Fully π‐Conjugated Ligands

Reactions between the U‐shaped binuclear Cu^I complex A that bears short metal–metal distances and the cyano‐capped monotopic π‐conjugated ligands 1 – 5 that carry gradually bulkier polyaromatic terminal fragments lead to the formation of π‐stacked supramolecular assemblies 6 – 10 , respectively, in yields of 50–80 %. These derivatives have been characterized by multinuclear NMR spectroscopic analysis and X‐ray diffraction studies. Their solid‐state structures show the selective formation of U‐shaped supramolecular assemblies in which two monotopic π‐conjugated systems present large ( 6 , 7 , and 9 ) or medium ( 8 and 10 ) intramolecular π overlap, thus revealing π–π interactions. These assemblies self‐organize into head‐to‐tail π‐stacked dimers that in turn self‐assemble to afford infinite columnar π stacks. The nature, extent, and complexity of the intermolecular contacts within the head‐to‐tail π‐stacked dimer depend on the nature of the terminal polyaromatic fragment carried by the cyano‐capped monotopic ligand, but it does not alter the result of the self‐assembling process. These results demonstrate that the dinuclear molecular clip A that bears short metal–metal distances allows selective supramolecular assembly processes driven by the formation of intra‐ and intermolecular short π–π interactions in the resulting self‐assembled structures; thus, demonstrating that their shape is not only dictated by the symmetry of the building blocks. This approach opens perspectives toward the formation of extended π‐stacked columns based on dissymmetrical and functional π‐conjugated systems.