Expanding the chemical structure space of opto-electronic molecular materials: unprecedented push-pull chromophores by reaction of a donor-substituted tetracyanofulvene with electron-rich alkynes

Perspectives on push-pull chromophores derived from click-type [2 + 2] cycloaddition-retroelectrocyclization reactions of electron-rich alkynes and electron-deficient alkenes

Various push-pull chromophores can be synthesized in a single and atom-economical step through [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reactions involving diverse electron-rich alkynes and electron-deficient alkenes. In this review, a comprehensive investigation of the recent and noteworthy advancements in the research on push-pull chromophores prepared via the [2 + 2] CA-RE reaction is conducted. In particular, an overview of the physicochemical properties of the family of these compounds that have been investigated is provided to clarify their potential for future applications.

A study of [2 + 2] cycloaddition-retroelectrocyclization in water: observation of substrate-driven transient-nanoreactor-induced new reactivity

Organic solvents limit [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) in biological fields. We examined the formation of 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) through CA-RE reactions and their unusual reactivity to produce N-heterocyclic compounds when the nature of the surfactant and the concentrations were varied in the aqueous phase. An environment in which transient self-assemblies (vesicles) were induced by the substrate and surfactant molecules initiated new reactivity through H₂O addition on the TCBD, generating the enol form of the intermediate, which results in the formation of the 6,6-dicyano-heteropentafulvene (amidofulvene) compound, while lamellar sheets at higher concentrations favored TCBD generation. Interestingly, the amidofulvene underwent a clean transformation to 6-membered heterocycles that resemble cardiotonic drugs (milrinone, amrinone) via keto-enol tautomerism mediated by a polar aprotic solvent, opening up a new avenue for drug discovery. Unlike organic-solvent-mediated CA-RE reactions, the present nanoreactor-mediated approach enabled the selective production of TCBDs as well as new heterocycles using H₂O as a green solvent. In addition to the widely explored organic electronics/materials, we believe that this study will help to overcome the long-standing limitation of CA-RE reaction applicability in biological fields.

Nanotechnology-Assisted, Single-Chromophore-Based White-Light-Emitting Organic Materials with Bioimaging Properties

White-light-emitting (WLE) organic materials, especially small molecules comprising a single chromophoric unit, have received much attention due to their tremendous use in modern-day electronic devices and biomaterials. They can increase the efficiency and lifetime of devices compared to the currently used combination approach. Herein, we explored a small symmetric push-pull organic molecule Hexyl-TCBD with a single 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) chromophoric unit containing urea as a key functional group on an acceptor-donor∼donor-acceptor (A-D∼D-A) backbone for its ability to show white-light emission in solution as well as in the solid state. The luminescence was absent in the solid state due to the H-bonding- and π-stacking-driven quenching processes, while emission behavior in solution was tunable with variable CIE chromaticity index values via hydrogen (H)-bonding-governed disaggregation phenomena. Translation of WLE from the Hexyl-TCBD solution to a solid state was demonstrated by utilizing nonemissive polystyrene (80 wt % with respect to the chromophore) as the matrix to obtain WLE nanofibers (made by the electrospun technique) via segregating the molecules. The optical microscopy study validated the WLE nanofibers. The presence of multicolor photoluminescence, including white light, could be fine-tuned through various excitation wavelengths, solvent polarities, and polystyrene matrices. Furthermore, the detailed photophysical studies, including lifetime measurements, indicated that the inherent intramolecular charge transfer (ICT) bands of Hexyl-TCBD exhibit better ICT state stabilization by space charge distribution through the modulation of H-bonding between urea groups. Finally, a cytotoxicity study was performed for Hexyl-TCBD on normal and cancer cell lines using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay to explore bioimaging applications in biosystems. MTT results revealed significant toxicity toward cancer cells, whereas normal cells exhibited good biocompatibility.

Chemical transformations of push-pull fluorenones: push-pull dibenzodicyanofulvenes as well as fluorenone- and dibenzodicyanofulvene-tetracyanobutadiene conjugates

Push-pull fluorenones (FOs) were synthesized by treating a benzopentalenequinone (BPO) derivative with alkynes that bear an electron-rich aniline moiety via a regioselective [4 + 2] cycloaddition (CA) followed by a [4 + 1] retrocycloaddition (RCA). The resulting FOs were readily converted into dibenzodicyanofulvenes (DBDCFs) by treatment with malononitrile in the presence of TiCl4 and pyridine. The FOs and DBDCFs exhibit intramolecular charge-transfer (ICT) that manifests in absorptions at 350-650 nm and amphoteric electrochemical behavior. Furthermore, FOs and DBDCFs that contain a C[triple bond, length as m-dash]C bond react with tetracyanoethylene in a formal [2 + 2] CA followed by a retro-electrocyclization to afford sterically congested tetracyanobutadiene (TCBD) conjugates. The substituent (H or Me) on the aromatic ring adjacent to the butadiene moiety thereby determines whether the butadiene adopts an s-cis or s-trans conformation, and thus controls the physicochemical properties of the resulting TCBDs. The TCBD conjugates exhibit ICT absorptions (≤800 nm) together with up to four reversible reduction steps.

An overview of the cycloaddition chemistry of fulvenes and emerging applications

The unusual electronic properties and unique reactivity of fulvenes have interested researchers for over a century. The propensity to form dipolar structures at relatively low temperatures and to participate as various components in cycloaddition reactions, often highly selectively, makes them ideal for the synthesis of complex polycyclic carbon scaffolds. As a result, fulvene cycloaddition chemistry has been employed extensively for the synthesis of natural products. More recently, fulvene cycloaddition chemistry has also found application to other areas including materials chemistry and dynamic combinatorial chemistry. This highlight article discusses the unusual properties of fulvenes and their varied cycloaddition chemistry, focussing on applications in organic and natural synthesis, dynamic combinatorial chemistry and materials chemistry, including dynamers, hydrogels and charge transfer complexes. Tables providing comprehensive directories of fulvene cycloaddition chemistry are provided, including fulvene intramolecular and intermolecular cycloadditions complete with reactant partners and their resulting cyclic adducts, which provide a useful reference source for synthetic chemists working with fulvenes and complex polycyclic scaffolds.

Crystal structures of a series of 6-aryl-1,3-diphenyl-fulvenes

The synthesis and crystal structures of a series of 6-aryl-fuvlenes (fulvene is 5-methyl-idene-cyclo-penta-1,3-diene) with varying methyl-ation patterns on the 6-phenyl substituent are reported, namely 6-(3-methyl-phen-yl)-1,3-di-phenyl-fulvene (C25H20), 6-(4-methyl-phen-yl)-1,3-di-phenyl-fulvene (C25H20), 6-mesityl-3-di-phenyl-fulvene (C27H24) and 6-(2,3,4,5,6-penta-methyl-phen-yl)-1,3-di-phenyl-fulvene (C29H28). The bond lengths are typical of those observed in related fulvenes. A network of C-H⋯π ring inter-actions consolidates the packing in each structure.

Disilanylene-bridged BODIPY-based D–σ–A architectures: a novel promising series of NLO chromophores

A novel series of disilanylene-bridged BODIPY-based chromophores have been reported.

Design, synthesis and electronic properties of push–pull–push type dye

Alkyne conjugated push–pull–push type dye where alkyne π-spacer and push units monitor the electronic properties of the dye.

6,6-Dicyanopentafulvenes: teaching an old dog new tricks

6,6-Dicyanopentafulvene (DCF) is a fascinating molecular entity that consists of a cyclopentadiene ring conjugated to an exocyclic double bond bearing two cyano groups on its periphery. Herein, we give a brief history of the chemistry of DCFs prior to our arrival to the field in 2011, followed by a summary of our work. We show how substitution on the ring and the exocyclic bond affects the HOMO and LUMO energies of pentafulvenes and how the design of DCFs was exploited computationally for the first time. Shortly after the report of the first rational synthesis of DCFs, we discovered that DCFs had a vast and astonishing array of reactivities to form new molecular entities. Simple, catalyst-free reactions between DCF acceptors and electron-rich donors led to the formation of scaffolds of exceptional complexity. Furthermore, our discovery that DCFs are capable of undergoing mild pentafulvene-to-benzene rearrangements challenges previous conventions of fulvene chemistry.