Tetraethynylethene Molecular Scaffolding

Unlocking Diverse π-Bond Enrichment Frameworks by the Synthesis and Conversion of Boronated Phenyldiethynylethylenes

The π-bond enrichment frameworks not only serve as a crucial building block in organic synthesis but also assume a pivotal role in the fields of materials science, biomedicine, photochemistry, and other related disciplines owing to their distinctive structural characteristics. The incorporation of various substituents into the C═C double bonds of tetrasubstituted alkenes is currently a highly significant research area. However, the synthesis of tetrasubstituted alkenes with diverse substituents on double bonds poses a significant challenge in achieving stereoselectivity. Here, we reported an efficient and convergent route of Cu-catalyzed borylalkynylation of both symmetrical and unsymmetrical 1,3-diynes, B2pin2, and acetylene bromide to the construction of boronated phenyldiethynylethylene (BPDEE) derivatives with excellent chemo-, stereo-, and regioselectivities. BPDEE derivatives could transform into novel tetrasubstituted organic π-conjugated gem-diphenyldiethynylethylene (DPDEE), vinylphenyldiethynylethylene (VPDEE), and phenyltriethynylethylene (PTEE) derivatives by a stepwise process, which provides a flexible platform for the synthesis of complex π-bond enrichment frameworks that were difficult to synthesize by previous methods. The initial optical characterization revealed that the synthesized molecules exhibited aggregation-induced emission (AIE) properties, which further establishes the groundwork for future applications and enriches and advances the field of functional π-conjugated frameworks research.

Substituted Tetraethynylethylene-Tetravinylethylene Hybrids

A general synthetic approach to molecular structures that are hybrids of tetraethynylethylene (TEE) and tetravinylethylene (TVE) is reported. The synthesis permits the controlled preparation of many previously inaccessible structures, including examples with different substituents on each of the four branching arms. Most substituted TVE-TEE hybrids are found to be significantly more robust compounds than their unsubstituted counterparts, enhancing the prospects of their deployment in conducting materials and devices. Their participation in pericyclic reaction cascades, leading to sp³-rich polycycles, is demonstrated. The utilization of TEE-TVE hybrids as building blocks for larger acyclic, through/cross-conjugated hydrocarbon frameworks is also established. Aryl-substituted TEEs, TVEs, and their hybrids are fluorescent, with some exhibiting aggregation-induced emission enhancement. The structural requirements are defined and explained, setting the scene for applications as fluorescent probes and organic light-emitting diodes.

Novel synthetic strategy towards subphthalocyanine-functionalized acetylenic scaffolds via various dibromo-enynes

Boron subphthalocyanine (SubPc) is a strong chromophore with interesting applications in the field of functional materials and can be synthetically modified in both the peripheral and axial positions, allowing tuning of optical and redox properties. Herein we present novel acetylenic scaffolds where SubPc units are bridged via acetylenic moieties at the boron atoms. Specifically, we show that dibromo-functionalized enyne and enediyne units (vinylic dibromides) can be attached to one or two SubPc boron atoms using an AlCl3-mediated alkynylation protocol of trimethylsilyl-protected alkynes, and such compounds can conveniently be employed for further Sonogashira or Glaser-Hay coupling reactions. Thereby, new tetraethynylethene (TEE) - SubPc scaffolds were obtained. The degree of communication between two SubPc units incorporated in dimeric scaffolds was investigated by cyclic voltammetry. When bridged by one TEE unit, the oxidations of the SubPc units occurred sequentially, while the two SubPc units behaved as independent redox centers when separated by two TEE units.

Unlocking Acyclic π-Bond Rich Structure Space with Tetraethynylethylene-Tetravinylethylene Hybrids

Literature reports describe tetraethynylethylene (TEE) as unstable but tetravinylethylene (TVE) as stable. The stabilities of these two known compounds are reinvestigated, along with those of five unprecedented TEE-TVE hybrid compounds. The five new C₁₀ hydrocarbons possess a core, tetrasubstituted C═C bond carrying all possible combinations of vinyl and ethynyl groups. A unified strategy is described for their synthesis, whereupon cross-conjugated ketones are dibromo-olefinated then cross-coupled. Due to an incorrect but nonetheless widely held belief that acyclic π-bond rich hydrocarbons are inherently unstable, a standardized set of robustness tests is introduced. Whereas only TVE survives storage in neat form, all seven hydrocarbons are remarkably robust in dilute solution, generally surviving exposure to moderate heat, light, air, and acid. The first X-ray crystal structure of TEE is reported. Subgroups of hybrids based upon conformational preferences are identified through electronic absorption spectra and associated computational studies. These new acyclic π-bond rich systems have extensive, untapped potential for the production of stable, conjugated carbon-rich materials.

All-carbon scaffolds by rational design

The search for new molecular and regular polymeric allotropes of carbon has greatly stimulated the preparation and investigation of pi-conjugated acetylenic macrocycles, which often represent substructures of proposed 2D carbon networks. Perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes with large, multi-nanometer-sized all-carbon cores are potent electron acceptors, and their optoelectronic as well as stability and solubility properties are greatly enhanced by peripheral donor substitution. Acetylenic scaffolding into three dimensions has generated an expanded cubane with a C(56) core, the first representative of a new class of "platonic" objects. Exceptional chiroptical properties displayed by enantiomerically pure alleno-acetylenic, shape-persistent macrocycles promise fascinating perspectives for the development of molecular and supramolecular chiroptical materials.

Controlling the conformational changes in donor-acceptor [4]-dendralenes through intramolecular charge-transfer processes

The synthesis of two [4]-dendralene compounds incorporating thiophene-(p-nitrophenyl) donor-acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron-donating components of the dendralenes are represented by bromothienyl (in 13) and ethylenedioxythiophene(EDOT)-thienyl (in 15) end-groups. The most facile transformation involves the isomerisation of donor-acceptor conjugated systems (a conformers) into structures in which only the thiophenes are conjugated (b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single-crystal X-ray diffraction studies and the structural changes in both compounds have been monitored by 1H NMR spectroscopy and absorption studies. The transformations were found to be first-order processes with rate constants of k=0.0027 s(-1) and k=0.00022 s(-1) for 13 and 15, respectively. Density functional theory calculations at the B3LYP/6-31G* level give credence to the proposed mechanism for the a-->b conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative (15) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.

Acetylene-derived strong organic acceptors for planar and nonplanar push-pull chromophores

Though investigated for decades, interest in push-pull chromophores (D-pi-A), strong electron donors (D) connected by pi-conjugating spacers to strong electron acceptors (A), continues to grow. Such chromophores are of substantial interest for optoelectronic devices such as waveguides. Also, strong donors and acceptors form bimolecular charge-transfer (CT) complexes and salts, some of which exhibit electrical conductivity and magnetic behavior. Furthermore, strong organic acceptors are increasingly explored as dopants in the fabrication of organic light-emitting diodes (OLEDs) and solar cells. This Account describes systematic efforts pursued over the past decade in our laboratory to generate new families of organic electron acceptors (A) and conjugate them via pi-spacers to electron donors (D) under formation of push-pull systems with intense intramolecular CT interactions and high third-order optical nonlinearities. First, we describe donor-acceptor-substituted tetraethynylethenes (TEEs). In these chromophores, the peripherally attached p-nitrophenyl acceptors and N,N-dimethylanilino donors behave as nearly independent redox centers. Acetylenic scaffolding using TEE building blocks produces large all-carbon sheets, such as perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes with potent electron-acceptor properties. Arylated TEEs act as molecular switches allowing two-way photochemical interconversion that is not perturbed by thermal isomerization pathways. Upon sequential substitution of the acetylene moieties in TEEs, we formed another family of potent acceptors, the cyanoethynylethenes (CEEs). Donor-substituted CEEs are planar CT chromophores with very high third-order optical nonlinearities. Their high environmental stability allows for the formation of thin films by vapor-phase deposition. Through careful analysis of the physicochemical properties of CEEs, we established useful guidelines for evaluating and tuning the optical gap in strong push-pull chromophores: increasing the length of the pi-spacer in D-pi-A systems reduces ground-state D-A conjugation and lowers the HOMO-LUMO gap. By taking advantage of "click-chemistry"-type [2 + 2] cycloadditions of tetracyanoethene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) with appropriately activated alkynes, followed by retro-electrocyclization, the formation of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs), 1,1,2,4,4-pentacyanobuta-1,3-dienes (PCBDs), and novel TCNQ adducts is possible. Some of these stable, nonplanar CT chromophores form high optical quality amorphous thin films by vapor-phase deposition. Despite donor substitution, the new acceptors (TCBDs, PCBDs, and the TCNQ adducts) rival TCNE and TCNQ in their ease for reversible electron uptake. High-yielding cycloaddition/retro-electrocyclization cascades provide access to multivalent, dendritic chromophores acting as "molecular batteries" with a remarkable capacity for multiple electron uptake in a narrow potential range. Finally, we used a one-pot protocol for electronically controlled consecutive TCNE and tetrathiafulvalene (TTF) additions to end-capped polyynes to form [AB]-type oligomers with a dendralene-type backbone.

Conjugation and optoelectronic properties of acetylenic scaffolds and charge-transfer chromophores

Our group started a research program in acetylene chemistry in 1987; since then, an intense research effort led to a fascinating journey into acetylenic scaffolding, aimed at exploring conjugative and optoelectronic properties of acetylenic chromophores. This journey included the generation of a unique molecular construction kit for acetylenic scaffolding, consisting of (E)-1,2-diethynylethenes [DEEs, (E)-hex-3-ene-1,5-diynes], tetraethynylethenes (TEEs, 3,4-diethynylhex-3-ene-1,5-diynes), chiral 1,3-diethynylallenes (DEAs, hepta-3,4-diene-1,6-diynes), 1,4-di and 1,1,4,4-tetraethynylbutatrienes, chiral trialkynylmethanes, and 1,1,2,2-tetraethynylethanes. These building modules were subsequently applied to the synthesis of carbon-rich architectures extending into one, two, and three dimensions. They include multinanometer-long monodisperse oligomers as models for infinite acetylenic polymers, molecular switches, perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes, and an octamethoxy-substituted expanded cubane with a central C56 core. Donor-substituted cyanoethynylethenes (CEEs) and 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) were introduced as new push-pull chromophores featuring intense intramolecular charge-transfer (CT) interactions. Dendritic multivalent CT chromophores were constructed using atom-economic, "click"-like reactions, and these systems were shown to behave as "molecular batteries", featuring exceptional electron uptake and storage capacity. The research finally led to the development of an unprecedented cascade reaction for the preparation of dendritic and oligomeric donor-acceptor (D-A) molecules. New [AB]-type oligomers become accessible in domino reactions involving repetitive sequences of [2+2] cycloadditions of tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF) to polyynes, followed by retro-electrocyclizations.

Redox-active polyiron complexes with tetra(ethynylphenyl)ethene and [2,2]paracyclophane spacers containing ethynylphenyl units: extension to higher dimensional molecular wire

Redox active polyiron complexes with higher dimensional spacers, the tetrairon complex with the two-dimensional tetra(ethynylphenyl)ethene spacer, (C=C)[p-C(6)H(4)-C triple bond C-FeCp*(dppe)](4), and the diiron complex with the three-dimensional spacer (pseudo-m-[2,2]paracyclophane)[C triple bond C-FeCp*(dppe)](2), are prepared, and their performance as molecular wires is evaluated on the basis of their comproportionation constants for the mixed valence state.

Palladium-Catalyzed Dimerization of Conjugated Diynes: Synthesis of (E)-1,2-Divinyldiethynylethenes Having Donor and Acceptor Chromophores at the Terminus of Alkyne

(E)-1,2-Divinyldiethynylethenes (DVDEEs) having donor and acceptor chromophores at the terminals of alkyne unit 7' were synthesized by the palladium/HOAc-catalyzed dimerization of the conjugated diynes 8 bearing an Si group at the terminus of alkyne (R(1) position), followed by desilylation and subsequent Sonogashira reaction of the resulting terminal alkyne 10 with aryl iodides.

Advanced opto-electronics materials by fullerene and acetylene scaffolding

Functional π-systems with unusual opto-electronic properties are intensively investigated from both fundamental research and technological application viewpoints. This article reports on novel π-conjugated systems obtained by acetylenic and fullerene scaffolding. Linearly conjugated acetylenic nanorods, consisting of monodisperse poly(triacetylene) (PTA) oligomers and extending up to 18 nm length, were prepared to investigate the limits of effective conjugation and to explore at which length a monodisperse oligomer reaches the properties of an infinite polydisperse polymer. With the cyanoethynylethenes (CEEs), a powerful new class of electron acceptors is introduced that undergo intense intramolecular charge-transfer (CT) interactions with appended donors. Macrocyclic scaffolds with unusual opto-electronic properties are perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes bearing peripheral dialkylanilino donor groups. Extended porphyrin-fullerene conjugates are investigated for their novel photophysical and efficient multicharge storage properties. Self-assembly of fullerenes and porphyrins, governed by weak interactions between the two components, leads to unprecedented nanopatterned surfaces that are investigated by scanning tunneling microscopy (STM).

Nanopatterning of Alkynes on Hydrogen-Terminated Silicon Surfaces by Scanning Probe-Induced Cathodic Electrografting

The electrochemical cathodic electrografting reaction, previously demonstrated on bulk silicon surfaces, can be patterned on the nanoscale utilizing conducting probe atomic force microscopy (CP-AFM). Alkyne electrografting is a particularly useful chemical technique since it leads to direct covalent attachment of conjugated alkynes to silicon. In addition, application of a forward bias during the reaction renders the surface less sensitive to oxidation and the resulting monolayers are very stable in air and basic aqueous solution. Alkyne monolayer lines can be drawn down to 40 nm resolution using a Pt-coated AFM tip, and the heights of the monolayers scale with the molecular length of the alkyne. The tip is biased (+) and the surface is biased (-) to drive the cathodic electrografting reaction under ambient conditions. The resistance of the monolayers to fluoride, as well as friction force microscopy, indicate that the alkynes are covalently bonded to the surface, not oxide-based, and hydrophobic. The reaction does not work with alkenes, and therefore hydrosilylation is not the primary mode of reaction. Wider lines (300 nm) can be produced using broadened Pt-coated AFM tips. This reaction could be important for the interfacing of conjugated molecules directly to silicon in a spatially controlled fashion.

Synthesis of (E)-1,2-divinyl-1,2-diethynylethene (DVDEE) via the palladium-catalyzed reaction of conjugated diynes. A new building block for molecular scaffolding

Conjugated diynes undergo dimerization in the presence of acetic acid and catalytic amounts of Pd(PPh3)4. The unprecedented reaction gives a novel chromophore containing the (E)-1,2-divinyl-1.2-diethynylethene (DVDEE) module, which is a potentially viable building block for acetylenic scaffolding.

Highly functionalized dimeric tetraethynylethenes and expanded radialenes: strong evidence for macrocyclic cross-conjugation

A selection of dimeric tetraethynylethenes (TEEs) and perethynylated expanded radialenes, containing different donor/acceptor substitution patterns, have been prepared and fully characterized. The first X-ray crystal structure of an expanded [6]radialene, with twelve peripheral 3,5-di(tert-butyl)phenyl substituents, is presented. This macrocycle, the all-carbon core of which is isomeric with fullerene C60, adopts a non-planar, "chair-like" conformation. Also a TEE dimer, carrying N,N-dimethylaniline donor substituents, has been subjected to an X-ray crystallographic analysis. The electronic properties were studied by UV/Vis spectroscopy and electrochemistry, providing fundamental insight into mechanisms of pi-electron delocalization in the acyclic and macrocyclic chromophores. Donor or donor-acceptor-substituted dimeric TEE derivatives show very strong absorptions extending over the entire UV/Vis region; their longest wavelength absorption bands have high charge-transfer character. Macrocyclic cross-conjugation in the expanded radialenes becomes increasingly efficient with increasing donor-acceptor polarization. A dual, strongly solvent-polarity-dependent fluorescence was observed for a tetrakis(N,N-dimethylaniline)-substituted dimeric TEE; this interesting emission behavior is explained by the twisted intramolecular charge-transfer (TICT) state model. Donor-substituted expanded radialenes display huge resonance-enhanced third-order nonlinear optical coefficients.

Pt-tetraethynylethene molecular scaffolding: synthesis and characterization of a novel class of organometallic molecular rods

The series of monodisperse Pt-bridged TEE oligomers 3a-f was prepared by oxidative Glaser-Hay oligomerization of monomer 7 under end-capping conditions. These novel molecular rods extend in length from 3.3 nm (monomeric 3a) to 12.1 nm (hexameric 3 f). Their isolation was achieved by high performance gel permeation chromatography (GPC), and their purification was best monitored by analytical GPC in combination with matrix-assisted laser-desorption-ionization mass spectrometry (MALDI-TOF MS). The mass spectra of each oligomer revealed the molecular ion or its sodium complex as parent ion together with a clean, highly characteristic fragmentation pattern. Delayed addition of the end-capping reagent PhC(triple bond)CH to the oligomerization mixture afforded polymer 10 with an average of approximately 32 repeat units and a remarkably narrow molecular weight distribution (Mw/Mn=1.06), which is indicative of a living polymerization process. UV/Vis spectral data as well as measurements of the second hyperpolarizability gamma by third harmonic generation (THG) revealed a nearly complete lack of pi-electron delocalization along the oligomeric backbone. The Pt atoms act as true insulating centers, and the Pt-C(sp) bonds hardly possess any pi character. The synthesis of the molecular rods 3a-f provides another demonstration of the power of oxidative acetylenic homocouplings for the preparation of unusual nanoarchitecture.