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Nanographenes are a popular area of research due to their promising properties for electronics. Over the last twenty years there has been a significant increase in interest in the development of contorted nanographenes. While many top-down techniques are employed in the synthesis of these planar nanographenes, the use of alkynes in bottom-up syntheses allows for easy functionalization and the development of contorted nanographenes. The syntheses of contorted nanographenes with a focus on utilizing alkynes is reviewed here.

The synthesis of pyreno[a]pyrene-based helicene hybrids was achieved in good yield via a four-fold alkyne benzannulation reaction that was promoted by Bronsted acid. The molecules are configuration...

The properties of nanographenes can be tuned by changing their shapes, therefore the development of new methods suitable for the synthesis of various nanographenes is highly desirable. Described herein is an intramolecular InCl3 /AgNTf2 -catalyzed regioselective domino benzannulation reaction of buta-1,3-diynes to build irregular nanographenes. Different nanographene compounds were easily obtained in moderate to high yields through careful design of the precursor compounds. This new domino reaction was successfully applied to a fourfold alkyne benzannulation of dimethoxy-1,1'-binaphthalene derivatives to arrive at novel chiral butterfly ligand precursors. The regioselectivity of the benzannulation reaction was unambiguously confirmed by X-ray crystallography. Moreover, this new method enables us to synthesize different nanographene isomers and study their optical properties as a function of shape.

A highly regioselective synthesis of 2,3-disubstituted chromen-4-one derivatives is accomplished from readily available internal alkynes and 2-methoxybenzoyl chlorides. The reaction proceeds via a domino intermolecular Friedel-Crafts acylation/intramolecular vinyl carbocation trapping (or oxa-Michael addition)/demethylation reaction sequence. This Lewis acid promoted method features relatively mild reaction conditions to synthesize a variety of 2,3-disubstituted chromen-4-one derivatives in one pot with up to 93% yield. The chromen-2-one (coumarin) product was obtained when 2,6-dimethoxybenzoyl chloride was used as a starting material via an electrophilic aromatic substitution/rearrangement process.

A highly concise one-pot method towards biologically pertinent compounds having a [6-5-6] tricyclic framework has been established. The multicomponent reaction utilizes a tandem, double Diels-Alder cycloaddition followed by a Nazarov reaction to furnish the [6-5-6] backbone. This method produces three fused rings evolving from the construction of five new carbon-carbon bonds, a quaternary carbon, and stereogenic centers in a one-pot reaction. The products are produced with excellent regio- and diastereocontrol.

We synthesized thiophene-functionalized pyrene, peropyrene, and teropyrene through a two- or four-fold alkyne annulation reaction promoted by triflic acid. All of the target compounds were fully characterized spectroscopically, and the structure of the peropyrene analogue was unambiguously confirmed by X-ray crystallography. A significant red shift in the absorption and emission properties as a function of extended conjugation was observed by UV–vis and fluorescence spectroscopy. This alkyne annulation strategy is a useful method for the conversion of smaller polycyclic aromatics to larger ones, such as nanographenes.

The synthesis of pyreno[

The rational synthesis of atomically precise polycyclic aromatic hydrocarbons (PAHs) and graphene nanoribbons (GNRs) has been achieved by using ‘bottom-up’ strategies. The creation of new methods to make functional PAHs and GNRs under mild conditions is of significant value. Recently, we have described the synthesis of pyrene, peropyrene, and teropyrene derivatives through the double or quadruple cyclization reaction of alkynes promoted by Bronsted acids. The successful synthesis of GNRs further exemplified the application of this method to the rapid construction of large PAHs.

The design of a relatively simple and efficient method to extend the π-conjugation of readily available aromatics in one-dimension is of significant value. In this paper, pyrenes, peropyrenes, and teropyrenes were synthesized through a double or quadruple benzannulation reaction of alkynes promoted by Brønsted acid. This novel method does not involve cyclodehydrogenation (oxidative aryl-aryl coupling) to arrive at the newly incorporated large arene moieties. All of the target compounds were synthesized in moderate to good yields and were fully characterized with the structures unambiguously confirmed by X-ray crystallography. As expected, photophysical characterization clearly shows increasing red-shifts as a function of extended conjugation within the fused ring systems.

Since the discovery of graphene, there is an increasing amount of research devoted to graphene materials, namely, graphene nanoribbons (GNRs). The "top-down" production of narrow (10 nm wide), unoxidized, and easily processable GNRs with atomically precise edges is challenging, and therefore, new methods need to be developed. We have designed a "bottom-up" approach for the synthesis of very narrow (ca. 0.5 nm) and soluble GNRs using a nonoxidative alkyne benzannulation strategy promoted by Brønsted acid. Suzuki polymerization was used to produce the GNR precursor, a poly(2,6-dialkynyl-p-phenylene) (PDAPP), with a weight-average molecular weight of 37.6 kg mol(-1). Cyclization of the ethynylaryl side chains on PDAPP was efficiently achieved using Brønsted acids to ultimately produce the GNRs. Infrared and Raman spectroscopic characterization of the GNRs matches very well with calculated results. The formation of the GNRs was also supported by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM).

The development and utilization of highly functionalized and reactive dienophiles in the Diels–Alder cyclization reaction is of value in producing diversely functionalized, and therefore useful, cyclic products. We have developed a Diels–Alder reaction of conjugated 2,4-diynones, promoted by Lewis acids, to produce substituted 2-alkynyl-1,4-cyclohexadiene (‘skipped’ cyclohexadiene) products in good to excellent yields. The reaction was successful with a variety of cyclic and acyclic dienes as well as with a diversity of 2,4-diynones.

Twisted system: A photochemical cyclodehydrochlorination (CDHC) reaction is a relatively mild and efficient way to access highly contorted, helically coiled graphene nanoribbons. The material is characterized by a number of techniques and boasts a relatively large optical band gap of 2.15 eV.

The stereoselective anti addition of diatomic halogens to alkynes has been well studied. A method is reported that utilizes the β-silyl effect to override this typically observed anti selectivity and provides halogenation products that result from syn addition. The reaction involves the addition of iodine monochloride to trialkylsilyl-substituted alkynes to produce tetrasubstituted (Z)-dihaloalkenes in good to excellent yields and with excellent regio- and diastereoselectivity.

Symmetrically and unsymmetrically endcapped polyynes (triisopropylsilyl – TIPS – and super trityl endcapping groups) have been studied with density functional theory calculations and Raman and infrared spectroscopy. This spectroscopic investigation provides general guidelines for studying the vibrational and structural features of sp-carbon-based materials, which nowadays are at the center of growing interest for the potential applications of nanostructured carbon materials in molecular electronics. On the basis of the description of CC stretching modes of polyynes in terms of longitudinal optical phonons of the parent infinite sp-carbon chain, we have carried out the complete assignment of the observed infrared and Raman features in the 2300–1900 cm−1 range, for chain lengths in the range 4–20 triple bonds. This allows demonstration of polarization effects induced by an unsymmetric structure and deviations from chain linearity in long polyynes. Copyright © 2013 John Wiley & Sons, Ltd.

Linear sp-carbon polyynes are ideal molecules for the direct investigation of the physical properties of one-dimensional conjugated systems. Traditionally, the main obstacle to the study of these systems has been their instability under normal laboratory conditions. Thanks to improved chemical syntheses, a broadened range of polyynes is now available for study, including the hydrocarbon series endcapped with adamantyl groups, Ad[n]. In this work, the quantitative Raman spectroscopic analysis is reported for each member of this homologous series. The Raman scattering intensities corresponding to the major features of the polyyne segment (i.e., the effective conjugation coordinate or Я lines) are then used to evaluate the nonlinear optical (NLO) behavior via determination of the vibrational contribution to the second hyperpolarizability (γvib). This study shows that γvib values as a function of length are of the same order of magnitude as those arising from the electronic contribution to the molecular second hyperpolarizability, γele, as reported for triisopropylsilyl polyynes, TIPS[n], using the differential optical Kerr effect technique. The nonlinear response for Ad[n] polyynes, supported by the analogous response for TIPS[n] polyynes, confirms that linear sp-carbon molecules have large second hyperpolarizabilities that show a power-law increase in γ-values versus length that is larger than other known π-conjugated systems, such as polyenes. Furthermore, the NLO response of polyynes versus length approaches the theoretical limit described by Kuzyk more closely than other conjugated organic oligomers. Copyright © 2012 John Wiley & Sons, Ltd.

Molecules composed of sp-hybridized carbon chains (polyynes) are the simplest of the known conjugated organic oligomers. In comparison to their counterparts such as polyacetylene and polydiacetylene, however, the formation of polyynes has traditionally posed a difficult synthetic challenge. In particular, there is no reliable method to form end-capped polyethynylene, and monodisperse polyynes have therefore been assembled. As a result, structure–property relationships for shorter polyynes have been relatively well established in recent years, while extension of these trends toward longer polyynes has remained a difficult task. Using the Fritsch–Buttenberg–Wiechell (FBW) rearrangement, the formation of diynes through decaynes has become possible and has provided a unique chance to explore the physical characteristics of conjugated polyyne chains. This paper highlights recent advances in the synthesis of extended polyynes, as well as interesting aspects of their NMR, Raman, and UV/vis spectroscopic analyses. These synthetic achievements offer the opportunity to predict some of the properties of the carbon allotrope carbyne. In particular, a set of X-ray crystallographic analyses of t-Bu end-capped polyynes (tBu[n]) shows a definitive experimental trend in reduced bond-length alternation (BLA).

The mechanism of the Fritsch-Buttenberg-Wiechell rearrangement of (13)C labeled precursors has been examined to determine the propensity of the alkynyl (R-CC-) group to migrate in an alkylidene carbenoid species. Reaction of dibromoolefins with n-BuLi and ketones with Me(3)SiC(Li)N(2) both demonstrate that the alkynyl moiety readily undergoes 1,2-migration from carbenoid intermediates.

Rearrangement of a carbene/carbenoid intermediate to form an acetylene moiety, known as the Fritsch–Buttenberg–Wiechell (FBW) rearrangement, was developed for the formation of polyynes and polyyne frameworks within highly conjugated organic materials. Necessary precursors can be prepared through formation of an alkynyl ketone, followed by dibromoolefination under Corey–Fuchs conditions. The carbenoid rearrangement is brought about by treatment of the dibromoolefin with BuLi under mild conditions. The success of these FBW reactions is quite solvent-dependent, and nonpolar hydrocarbon solvents (e.g., hexanes, toluene, benzene) work quite well, while use of ethereal solvents such as diethyl ether and tetrahydrofuran (THF) does not provide the desired polyyne product. This protocol was successfully applied to the formation of silyl, alkyl, alkenyl, and aryl polyynes, including di-, tri-, and tetrayne products, as well as the construction of two-dimensional carbon-rich molecules. A one-pot variant of this procedure is being developed and is particularly applicable toward the synthesis of polyyne natural products. Formation of a series of triisopropylsilyl end-capped polyynes, from the triyne to decayne, was achieved. Third-order nonlinear optical properties of these polyynes were evaluated. This study shows that the molecular second hyperpolarizabilities for the polyynes as a function of length increase at a rate that is higher than all other nonaromatic organic oligomers. © 2006 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 6: 169–182; 2006: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20081

Carbyne is an allotrope of carbon composed of sp-hybridized carbon atoms. Although its formation in the laboratory is suggested, no well-defined sample is described. Interest in carbyne and its potential properties remains intense because of, at least in part, technological breakthroughs offered by other carbon allotropes, such as fullerenes, carbon nanotubes and graphene. Here, we describe the synthesis of a series of conjugated polyynes as models for carbyne. The longest of the series consists of 44 contiguous acetylenic carbons, and it maintains a framework clearly composed of alternating single and triple bonds. Spectroscopic analyses for these polyynes reveal a distinct trend towards a finite gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for carbyne, which is estimated to be ∼485 nm (∼2.56 eV). Even the longest members of this series of polyynes are not particularly sensitive to light, moisture or oxygen, and they can be handled and characterized under normal laboratory conditions.