Synthesis and Optical Characterization of Hydrazone-Substituted Push-Pull-Type NLOphores

Two distinct families of NLOphores featuring hydrazone donors were synthesized using click-type [2 + 2] cycloaddition retroelectrocyclizations (CA-RE). Despite the limitations in the substrate scope, it was shown for the first time that hydrazone-activated alkynes could undergo reactions with TCNE/TCNQ. The electrochemical, photophysical, and second-order nonlinear optical (NLO) characteristics of the chromophores were analyzed utilizing experimental and computational approaches. Chromophores 17-21 and 23-27 exhibited two reduction waves, along with one oxidation wave that can be attributed to the hydrazone moiety. All chromophores exhibit charge-transfer bands extending from the visible to the near-infrared region. The λmax of hydrazone-based chromophores falls within the range of 473 to 725 nm. Additionally, all chromophores exhibited positive solvatochromism. Computational studies have been performed to elucidate the origin of the low-energy absorption bands. Parameters such as dipole moment, band gaps, electronegativity, global chemical hardness/softness, average polarizability, and first hyperpolarizability were calculated to obtain information about NLO properties of the target structures. The thermal stabilities of the NLOphores were assessed through TGA. Experimental NLO measurements were conducted using the electric field-induced second harmonic generation (EFISHG) technique. The studied structures demonstrated NLO responses, with μβ values between 520 × 10-48 esu and 5300 × 10-48 esu.

NIR absorbing ferrocenyl perylenediimide-based donor-acceptor chromophores

A set of ferrocenyl-functionalized perylenediimide (PDI) compounds and their 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives 1-5 were designed and synthesized using palladium-catalyzed Sonogashira cross-coupling, followed by a thermally activated [2 + 2] cycloaddition-retroelectrocyclization [CA-RE] reaction with a 1,1,2,2-tetracyanoethylene (TCNE) acceptor in good yields. The TCBD group works as an acceptor, whereas the ferrocenyl group acts as a donor at the central PDI core. The effects of varying the number of ferrocenyl and TCNE groups on the photophysical, thermal, electrochemical, and spectroelectrochemical properties were studied. The di-substituted PDI derivatives 3, 4, and 5 exhibit bathochromic shifts in the absorption spectra compared to 1 and 2, attributed to the extended π-conjugation. The electrochemical analysis of derivatives 2, 4, and 5 shows multiple reduction waves in the low potential region due to the presence of TCBD and perylenediimide acceptor units. Spectroelectrochemical studies were performed, showing that upon applying redox potentials, the absorption spectra shifted from the visible to the near-infrared (NIR) region. Computational calculations indicate that in the HOMO, the electron density is localized on the ferrocene unit, while in the LUMO, it is distributed over the PDI-TCBD unit, indicating a strong D-A interaction.

Recent advances in near-infrared I/II persistent luminescent nanoparticles for biosensing and bioimaging in cancer analysis

Photoluminescent materials (PLNs) are photoluminescent materials that can absorb external excitation light, store it, and slowly release it in the form of light in the dark to achieve long-term luminescence. Developing near-infrared (NIR) PLNs is critical to improving long-afterglow luminescent materials. Because they excite in vitro, NIR-PLNs have the potential to avoid interference from in vivo autofluorescence in biomedical applications. These materials are promising for biosensing and bioimaging applications by exploiting the near-infrared biological window. First, we discuss the biomedical applications of PLNs in the first near-infrared window (NIR-I, 700-900 nm), which have been widely developed and specifically introduce biosensors and imaging reagents. However, the light in this area still suffers from significant light scattering and tissue autofluorescence, which will affect the imaging quality. Over time, fluorescence imaging technology in the second near-infrared window (NIR-II, 1000-1700 nm) has also begun to develop rapidly. NIR-II fluorescence imaging has the advantages of low light scattering loss, high tissue penetration depth, high imaging resolution, and high signal-to-noise ratio, and it shows broad application prospects in biological analysis and medical diagnosis. This critical review collected and sorted articles from the past 5 years and introduced their respective fluorescence imaging technologies and backgrounds based on the definitions of NIR-I and NIR-II. We also analyzed the current advantages and dilemmas that remain to be solved. Herein, we also suggested specific approaches NIR-PLNs can use to improve the quality and be more applicable in cancer research.

Near-IR Capturing N-Methylbenzene Sulfonamide-Phenothiazine Incorporating Strong Electron Acceptor Push-Pull Systems: Photochemical Ultrafast Carrier Dynamics

Unraveling the intriguing aspects of the intramolecular charge transfer (ICT) phenomenon of multi-modular donor-acceptor-based push-pull systems are of paramount importance considering their promising applications, particularly in solar energy harvesting and light-emitting devices. Herein, a series of symmetrical and unsymmetrical donor-acceptor chromophores 1-6, are designed and synthesized by the Corey-Fuchs reaction via Evano's condition followed by [2+2] cycloaddition retroelectrocyclic ring-opening reaction with strong electron acceptors TCNE and TCNQ in good yields (~60-85 %). The photophysical, electrochemical, and computational studies are investigated to explore the effect of incorporation of strong electron acceptors 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and dicyanoquinodimethane (DCNQ) with phenothiazine (PTZ) donor. An additional low-lying broad absorption band extended towards the near-infrared (NIR) region suggests charge polarization after the introduction of the electron acceptors in both symmetrical and asymmetrical systems, leading to such strong ICT bands. The electrochemical properties reveal that reduction potentials of 3 and 6 are lower than those of 2 and 5, suggesting DCNQ imparts more on the electronic properties and hence largely contributes to the stabilization of LUMO energy levels than TCBD, in line with theoretical observations. Relative positions of the frontier orbitals on geometry-optimized structures further support accessing donor-acceptor sites responsible for the ICT transitions. Eventually, ultrafast carrier dynamics of the photoinduced species are investigated by femtosecond transient absorption studies to identify their spectral characteristics and target analysis further provides information about different excited states photophysical events including ICT and their associated time profiles. The key findings obtained here related to excited state dynamical processes of these newly synthesized systems are believed to be significant in advancing their prospect of utilization in solar energy conversion and related photonic applications.

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.

Zinc Tetrapyrrole Coordinated to Imidazole Functionalized Tetracyanobutadiene or Cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene Conjugates: Dark vs. Light-Induced Electron Transfer

Using the popular metal-ligand axial coordination self-assembly approach, donor-acceptor conjugates have been constructed using zinc tetrapyrroles (porphyrin (ZnP), phthalocyanine (ZnPc), and naphthalocyanine (ZnNc)) as electron donors and imidazole functionalized tetracyanobutadiene (Im-TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene (Im-DCNQ) as electron acceptors. The newly formed donor-acceptor conjugates were fully characterized by a suite of physicochemical methods, including absorption and emission, electrochemistry, and computational methods. The measured binding constants for the 1 : 1 complexes were in the order of 104 -105  M-1 in o-dichlorobenzene. Free-energy calculations and the energy level diagrams revealed the high exergonicity for the excited state electron transfer reactions. However, in the case of the ZnNc:Im-DCNQ complex, owing to the facile oxidation of ZnNc and facile reduction of Im-DCNQ, slow electron transfer was witnessed in the dark without the aid of light. Systematic transient pump-probe studies were performed to secure evidence of excited state charge separation and gather their kinetic parameters. The rate of charge separation was as high as 1011  s-1 suggesting efficient processes. These findings show that the present self-assembly approach could be utilized to build donor-acceptor constructs with powerful electron acceptors, TCBD and DCNQ, to witness ground and excited state charge transfer, fundamental events required in energy harvesting, and building optoelectronic devices.

Plasmonic porous micro- and nano-materials based on Au/Ag nanostructures developed for photothermal cancer therapy: challenges in clinicalization

Photothermal therapy (PTT) has developed in recent decades as a relatively safe method for the treatment of cancers. Recently, various species of gold and silver (Au and Ag) nanostructures have been developed and investigated to achieve PTT due to their highly localized surface plasmon resonance (LSPR) effect. Concisely, the collective oscillation of electrons on the surface of Au and Ag nanostructures upon exposure to a specific wavelength (depending on their size and shape) and further plasmonic resonance leads to the heating of the surface of these particles. Hence, porous species can be equipped with tiny plasmonic ingredients that add plasmonic properties to therapeutic cargoes. In this case, a precise review of the recent achievements is very important to figure out to what extent plasmonic photothermal therapy (PPTT) by Au/Ag-based plasmonic porous nanomedicines successfully treated cancers with satisfactory biosafety. Herein, we classify the various species of LSPR-active micro- and nano-materials. Moreover, the routes for the preparation of Ag/Au-plasmonic porous cargoes and related bench assessments are carefully reviewed. Finally, as the main aim of this study, principal requirements for the clinicalization of Ag/Au-plasmonic porous cargoes and their further challenges are discussed, which are critical for specialists in this field.

Superior Photophysical and Photosensitizing Properties of Nanoaggregates of Weakly Emissive Dyes for Use in Bioimaging and Photodynamic Therapy

The development of luminescent dyes based on 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) is an active research area, and a quantum yield (ΦF) of 7.8% has been achieved so far in cyclohexane by appending a fluorophore. Our novel method radically refines weakly emissive 2,3-disubstituted TCBD (phenyl-TCBD 1) (ΦF = 2.3% in CH3CN) into a water-soluble, biocompatible nanoformulation as highly emissive aggregates 1NPs ⊂ PF-127 with ΦF = 7.9% in H2O and without fluorophore conjugation. Characterization of 1NPs ⊂ PF-127 was carried out using various spectroscopic techniques, and its predominant size was found to be 80-100 nm according to transmission electron microscopy and dynamic light scattering techniques. Spectroscopic studies including Fourier transform infrared spectroscopy revealed that aggregated phenyl-TCBD particles were encapsulated in a nonluminescent triblock copolymer (PF-127)-based nanomicelles with the TCBD entrapment efficiency of 77%. With increasing water fraction, the phenyl-TCBD nanoaggregates exhibited a 3-fold higher quantum yield, a greater lifetime, and a red shift (155 nm). This remarkable enhancement in red emissivity enabled them to be used as a bioprobe for bioimaging applications and in photodynamic therapy to selectively target cancer cell lines with singlet oxygen generation capability (ΦΔ = 0.25). According to the MTT assay, compared to the native molecular form (1229 nM), the aggregated 1NPs ⊂ PF-127 (13.51 nM) exhibited dose-dependent cell death when exposed to light with 91-fold increased activity. The histoarchitectures of various vital organs (liver, kidneys, heart, lungs, and spleen) were intact when tested for in vivo biocompatibility. This study has significant implications for developing nonplanar push-pull chromophore-based dyes as biosensors and with potential applications beyond bioimaging.

NIR-Absorbing 1,1,4,4-Tetracyanobuta-1,3-diene- and Dicyanoquinodimethane-Functionalized Donor-Acceptor Phenothiazine Derivatives: Synthesis and Characterization

A series of symmetrical and unsymmetrical donor-acceptor type phenothiazine derivatives 1-18 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions. The incorporation of cyano-based acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the phenothiazine derivatives resulted in systematic variation in the photophysical, thermal, and electrochemical properties. The electronic absorption spectra of the phenothiazine derivatives with strong acceptors 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 show red-shifted absorption as compared to phenothiazine derivatives 1, 4, 7, 10, 13, and 16 in the near-IR region due to a strong intramolecular charge transfer (ICT) transition. The electrochemical analysis of the phenothiazine derivatives 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 reveals two reduction waves at low potential due to the TCBD and DCNQ acceptors. The mono-TCBD-functionalized phenothiazine 2 shows higher thermal stability compared to other phenothiazine derivatives. The computational studies on phenothiazines 1-18 reveal the LUMO is substantially stabilized as acceptor strength increases, which lowers the HOMO-LUMO gap.

Efficient Nitric Oxide Scavenging by Urea-Functionalized Push-Pull Chromophore Modulates NO-Mediated Diseases

The excess nitric oxide (NO) produced in the body in response to bacterial/proinflammatory stimuli is responsible for several pathological conditions. The current approaches that target the production of excess NO, either through the inhibition of nitric oxide synthase enzyme or its downstream mediators have been clinically unsuccessful. With an aim to regulate the excess NO, urea-functionalized push-pull chromophores containing 1,1,4,4-tetracyanobuta-1,3-dienes (TCBD) or expanded TCBD (eTCBD) were developed as NO scavengers. The NMR mechanistic studies revealed that upon NO binding, these molecules are converted to uncommon stable NONOates. The unique emissive property of Urea-eTCBD enables its application in vitro, as a NO-sensor. Furthermore, the cytocompatible Urea-eTCBD, rapidly inactivated the NO released from LPS-activated cells. The therapeutic efficacy of the molecule in modulating NO-mediated pathological condition was confirmed using a carrageenan-induced inflammatory paw model and a corneal injury model. While the results confirm the advantages of scavenging the excess NO to address a multitude of NO-mediated diseases, the promising sensing and bioactivity of Urea-eTCBD can motivate further exploration of such molecules in allied areas of research.

Cycloaddition-Retro-Electrocyclization Click Reaction of Amine End-Capped Oligoynes with Tetracyanoethylene

This study shows the first example of cycloaddition-retro-electrocyclization (CA-RE) click reaction involving nitrogen end-capped push-pull oligoynes. The reported CA-RE reaction with TCNE (tetracyanoethylene) is fully regioselective and leads exclusively to the unprecedented TCBD (tetracyanobuta-1,3-diene-2,3-diyl) end-capped carbon rods. The molecular structure of the products was unambiguously confirmed using X-ray single crystal diffraction and their optical and electronic properties were investigated experimentally and rationalized using DFT (density functional theory) calculations.

Minimalist Design for Solar Energy Conversion: Revamping the π-Grid of an Organic Framework into Open-Shell Superabsorbers

We apply a versatile reaction to a versatile solid: the former involves the electron-deficient alkene tetracyanoethylene (TCNE) as the guest reactant; the latter consists of stacked 2D honeycomb covalent networks based on the electron-rich β-ketoenamine hinges that also activate the conjugated, connecting alkyne units. The TCNE/alkyne reaction is a [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) that forms strong push-pull units directly into the backbone of the framework-i.e., using only the minimalist "bare-bones" scaffold, without the need for additional side groups of alkynes or other functions. The ability of the stacked alkyne units (i.e., as part of the honeycomb mass) to undergo such extensive rearrangement highlights the structural flexibility of these covalent organic framework (COF) hosts. The COF solids remain porous, crystalline, and air-/water-stable after the CA-RE modification, while the resulting push-pull units feature distinct open-shell/free-radical character, are strongly light-absorbing, and shift the absorption ends from 590 nm to around 1900 nm (band gaps from 2.17-2.23 to 0.87-0.95 eV), so as to better capture sunlight (especially the infrared region which takes up 52% of the solar energy). As a result, the modified COF materials achieve the highest photothermal conversion performances, holding promise in thermoelectric power generation and solar steam generation (e.g., with solar-vapor conversion efficiencies >96%).

Accelerated Intramolecular Charge Transfer in Tetracyanobutadiene- and Expanded Tetracyanobutadiene-Incorporated Asymmetric Triphenylamine-Quinoxaline Push-Pull Conjugates

The excited-state properties of an asymmetric triphenylamine-quinoxaline push-pull system wherein triphenylamine and quinoxaline take up the roles of an electron donor and acceptor, respectively, are initially investigated. Further, in order to improve the push-pull effect, powerful electron acceptors, viz., 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded tetracyanobutadiene (also known as expanded-TCBD or exTCBD), have been introduced into the triphenylamine-quinoxaline molecular framework using a catalyst-free [2 + 2] cycloaddition-retroelectrocyclization reaction. The presence of these electron acceptors caused strong ground-state polarization extending the absorption well into the near-IR region accompanied by strong fluorescence quenching due to intramolecular charge transfer (CT). Systematic studies were performed using a suite of spectral, electrochemical, computational, and pump-probe spectroscopic techniques to unravel the intramolecular CT mechanism and to probe the role of TCBD and exTCBD in promoting excited-state CT and separation events. Faster CT in exTCBD-derived compared to that in TCBD-derived push-pull systems has been witnessed in polar benzonitrile.

Charge Resonance and Photoinduced Charge Transfer in Bis(N,N-dimethylaminophenyl-tetracyanobutadiene)-diketopyrrolopyrrole Multimodular System

Intervalence charge transfer (IVCT) or charge resonance is often observed in redox-active systems encompassed of two identical electroactive groups, where one of the groups is either oxidized or reduced and serves as a model system to improve our fundamental understanding of charge transfer. This property has been explored in the present study in a multimodular push-pull system carrying two N,N-dimethylaminophenyl-tetracyanobutadiene (DMA-TCBD) entities covalently linked to the opposite ends of bis(thiophenyl)diketopyrrolopyrrole (TDPP). Electrochemical or chemical reduction of one of the TCBDs promoted electron resonance between them, exhibiting an IVCT absorption peak in the near-infrared area. The comproportionation energy, -ΔG_com, and equilibrium constant, K_com, evaluated from the split reduction peak were, respectively, 1.06 × 10⁴ J/mol and 72.3 M^-1. Excitation of the TDPP entity in the system promoted the thermodynamically feasible sequential charge transfer and separation of charges in benzonitrile, wherein the IVCT peak formed upon charge separation served as a signature peak in characterizing the product. Further, transient data analyzed using Global Target Analysis revealed the charge separation to take place in a ps time scale (k ∼ 10¹⁰ s^-1) as a result of close positioning and strong electronic interaction between the entities. The significance of IVCT in probing excited-state processes is evidenced by the present study.

Ring forming transformations of ynamides via cycloaddition

Ynamides are N-alkyne compounds bearing an electron withdrawing group at the nitrogen atom. They offer unique pathways for the construction of versatile building blocks owing to their exceptional balance between reactivity and stability. Recently several studies have been reported that explore and illustrate the synthetic potential of ynamides and ynamide-derived advanced intermediates in cycloadditions with different reaction partners to yield heterocyclic cycloadducts of synthetic and pharmaceutical value. Cycloaddition reactions of ynamides are the facile and preferable routes for the construction of structural motifs having striking importance in synthetic, medicinal chemistry, and advanced materials. In this systematic review, we highlighted the recently reported novel transformations and synthetic applications that involved the cycloaddition reaction of ynamides. The scope along with the limitations of the transformations are discussed in detail.

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.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor-Acceptor Constructs

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor-acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push-pull systems, PTS2-PTS6 have been newly designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO₂) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2-PTS6, and the subsequent pump-probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push-pull systems.

On the Mechanism of the Formal [2+2] Cycloaddition - Retro-electrocyclization (CA-RE) Reaction

The [2+2] cycloaddition - retro-electrocyclization (CA-RE) reaction is a "click-like" protocol for facile synthesis of donor-acceptor chromophores from an alkyne and tetracyanoethylene. Herein we shed light on the mechanism of this reaction by detailed kinetics studies using ¹ H NMR spectroscopy. By considering several experiments simultaneously, a variety of mechanistic models was evaluated. Surprisingly, a model in which the final 1,1,4,4-tetracyanobuta-1,3-diene product promoted the first step was the only one that described well the experimental data. This autocatalysis model also involved a non-concerted, stepwise formation of the cyclobutene cycloaddition adduct. By proper choice of conditions, we were able to generate the transient cyclobutene in sufficient amount to verify it as an intermediate using ¹³ C NMR spectroscopy. For its final retro-electrocyclization step, simple first-order kinetics was observed and only minor solvent dependence, which indicates a concerted reaction.

Tetracyanobutadiene Bridged Push-Pull Chromophores: Development of New Generation Optoelectronic Materials

This review describes the design strategies used for the synthesis of various tetracyanobutadiene bridged donor-acceptor molecular architectures by a click type [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction sequence. The photophysical and electrochemical properties of the tetracyanobutadiene bridged molecular architectures based on various moieties including diketopyrrolopyrrole, isoindigo, benzothiadiazole, pyrene, pyrazabole, truxene, boron dipyrromethene (BODIPY), phenothiazine, triphenylamine, thiazole and bisthiazole are summarized. Further, we discuss some important applications of the tetracyanobutadiene bridged derivatives in dye sensitized solar cells, bulk heterojunction solar cells and photothermal cancer therapy.

Synthesis of orthogonal push-pull chromophores via click reaction of arylynamines

Herein, we report a catalyst-free 'click' reaction: metal-free [2 + 2] cycloaddition-retro-electrocyclisation (CA-RE) of arylynamines with the sluggish acceptor tetracyanoquinodimethane (TCNQ) to provide orthogonal electron-push-pull light-harvesting small molecules: N-heterocyclic dicyanoquinodimethane-substituted methylene malononitriles. Ynamines are reactive alkynes and tend to induce over-reactions with the CA-RE adducts. The reactivity of arylynamines was balanced properly by ensuring the electron-density of the nitrogen atom was delocalised more over the aromatic rings than the triple bond.

Near-IR Intramolecular Charge Transfer in Strongly Interacting Diphenothiazene-TCBD and Diphenothiazene-DCNQ Push-Pull Triads

Three types of phenothiazines dimers (PTZ-PTZ, 1-3), covalently linked with one or two acetylene linkers, were synthesized by copper-mediated Eglinton and Pd-catalyzed Sonogashira coupling reactions in excellent yields. The dimers 1-3 were further engaged in [2+2] cycloaddition-retroelectrocyclization reactions with strong electron acceptors, tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) to yield tetracyanobutadiene (TCBD, 1 a-3 a), and dicyanoquinodimethane (DCNQ, 1 b-3 b) functionalized donor-acceptor (D-A) conjugates, respectively. The conjugates were examined by a series of spectral, computational, and electrochemical studies. Strong ground state polarization leading to new optical transitions was witnessed in both series of D-A conjugates. In the case of DCNQ derived D-A system 1 b, the optical coverage extended until 1200 nm in benzonitrile, making this a rare class of D-A ICT system. Multiple redox processes were witnessed in these D-A systems, and the frontier orbitals generated on DFT optimized structures further supported the ICT phenomenon. Photochemical studies performed using femtosecond pump-probe studies confirmed solvent polarity dependent excited state charge transfer and separation in these novel multi-modular D-A conjugates. The charge-separated states lasted up to 70 ps in benzonitrile while in toluene slightly prolonged lifetime of up to 100 ps was witnessed. The significance of phenothiazine dimer in wide-band optical capture all the way into the near-IR region and promoting ultrafast photoinduced charge transfer in the D-A-D configured multi-modular systems, and the effect of donor-acceptor distance and the solvent polarity was the direct outcome of the present study.

Synthesis and Photophysical Properties of 1,1,4,4-Tetracyanobutadienes Derived from Ynamides Bearing Fluorophores

1,1,4,4-Tetracyanobutadienes (TCBDs) bearing a large diversity of fluorophores were prepared following a multi-step synthesis. In a crucial last step, all compounds were obtained from the corresponding ynamides, which were particularly suitable for the formation of the TCBDs in the presence of tetracyanoethylene via a [2+2] cycloaddition/retroelectrocyclization step (CA-RE). Several fluorenyl derivatives in addition to phenanthrenyl and terphenyl ones provided ynamide-based TCBDs affording remarkable emission properties covering a large range of wavelengths. Those compounds emit both in solid state and in solution from the visible region to the NIR range, depending on the molecular structures. Quantum yields in cyclohexane reached unforeseen values for such derivatives, up to 7.8 %. A huge sensitivity to the environment of the TCBDs has also been unraveled for most of the compounds since we observed a dramatic fall of the quantum yields when changing the solvent from cyclohexane to toluene, while they are almost non-emissive in dichloromethane.

3-Alkynylindoles as Building Blocks for the Synthesis of Electronically Tunable Indole-Based Push-Pull Chromophores

In this study, two different classes of push–pull chromophores were synthesized in modest to excellent yields by formal [2+2] cycloaddition-retroelectrocyclization (CA-RE) reactions. N-Methyl indole was introduced as a new donor group to activate alkynes in the CA-RE transformations. Depending on the side groups’ size and donor/acceptor characteristics, N-methyl indole-containing compounds exhibited λ_max values ranging between 378 and 658 nm. The optoelectronic properties of the reported D–A-type structures were studied by UV/vis spectroscopy and computational studies. The complete regioselectivity observed in the products was elaborated by one-dimensional (1D) and two-dimensional (2D) NMR studies, and the electron donor strength order of N-alkyl indole and triazene donor groups was also established. The intramolecular charge-transfer characteristics of the target push–pull chromophores were investigated by frontier orbital depictions, electrostatic potential maps, and time-dependent density functional theory calculations. Overall, the computational and experimental results match each other. Integrating a new donor group, N-alkyl indole, into the substrates used in formal [2+2] cycloaddition-retroelectrocyclizations has significant potential to overcome the limited donor-substituted substrate scope problem of CA-RE reactions.

Synthesis of functionalized malononitriles via Fe-catalysed hydrogen atom transfers of alkenes

Described herein is a practical and convenient approach that enabled radical-mediated conjugate addition of unreactive alkenes to electron-deficient alkenes leading to a broad range of substituted malononitriles. These reactions are believed to proceed by Fe-catalysed hydrogen atom transfer (HAT) onto the alkenes affording carbon-centered radical intermediates with Markovnikov selectivity, followed by the capture of electron-deficient alkenes. We explored this synthesis approach under mild conditions with high efficiency and broad substrate scope and the utility is highlighted by the further synthetic transformations of the obtained substituted malononitriles.

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.

Phenothiazine and phenothiazine-5,5-dioxide based push-pull Derivatives: Synthesis, photophysical, electrochemical and computational Studies

A set of phenothiazine (PTZ) and phenothiazine-5,5-dioxide based π-conjugated push–pull chromophores PTZ 1–6 were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition retroelectrocyclic ring opening reaction in...

Polycyclic aromatic hydrocarbon-substituted push-pull chromophores: an investigation of optoelectronic and nonlinear optical properties using experimental and theoretical approaches

A series of new push-pull chromophores were synthesized in moderate to very high yields (65%-97%) by treating TCNE and TCNQ with alkynes substituted by electron-rich diethylaniline and polycyclic aromatic hydrocarbons. Some of the chromophores exhibit strong intramolecular charge-transfer bands in the near-IR region with λ_max values between 695 and 749 nm. With the help of experimental and theoretical analysis, it is concluded that the trend in λ _max values is affected by PAH substituents sterically, not electronically. Steric constraints led to the increased dihedral angles, reducing conjugation efficiencies. The absorption properties of push-pull compounds have been investigated in solvents possessing different polarities. All chromophores exhibited positive solvatochromism. As an additional proof of efficient charge-transfer in push-pull chromophores, quinoid character (dr) values were predicted using calculated bond lengths. Remarkably, substantial dr values (0.045-0.049) were predicted for donor diethylaniline rings in all compounds. The effects of various polycyclic aromatic hydrocarbons on optical and nonlinear optical properties were also studied by computational methods. Several parameters, such as band gaps, Mulliken electronegativity, chemical hardness and softness, dipole moments, average polarizability, first hyperpolarizability, were predicted for chromophores at the B3LYP/6-31++G(d,p) level of theory. The predicted first hyperpolarizability β_(tot) values vary between 198 to 538 × 10^-30 esu for the reported push-pull chromophores in this study. The highest predicted β_(tot) value in this study is 537.842 × 10^-30 esu, 8150 times larger than the predicted β_(tot) value of benchmark NLO material urea, suggests possible utilization of these chromophores in NLO devices. The charge-transfer character of the synthesized structures was further confirmed by HOMO-LUMO depictions and electrostatic potential maps.

Two-photon absorption properties of multipolar triarylamino/tosylamido 1,1,4,4-tetracyanobutadienes

The synthesis and characterization of four new tetracyanobutadiene (TCBD) derivatives (1, 3c and 4b-c) incorporating tosylamido and 4-triphenylamino moieties are reported. Along with those of five closely related or differently branched TCBDs derivatives (2, 3a-b, 4c and 5), their linear and (third-order) nonlinear optical properties were investigated by electronic absorption spectroscopy and Z-scan measurements. Among these compounds, the tri-branched compounds 3c and 5 are the most active two-photon absorbers, with effective cross-sections of 275 and 350 GM at 900 nm, respectively. These properties are briefly discussed with the help of DFT calculations, focussing on structural and electronic factors, and contextualized with results obtained previously for related compounds.

Photoinduced Charge Separation Prompted Intervalence Charge Transfer in a Bis(thienyl)diketopyrrolopyrrole Bridged Donor-TCBD Push-Pull System

Intervalence charge transfer (IVCT), a phenomenon observed in molecular systems comprised of two redox centers differing in oxidation states by one unit, is reported in a novel, newly synthesized, multi-modular donor-acceptor system comprised of central bis(thienyl)diketopyrrolopyrrole (TDPP) hosting two phenothiazine-tetracyanobutadiene (PTZ-TCBD) entities on the opposite sides. One-electron reduction of TCBD promoted electron exchange between the two TCBD resulting in IVCT transition in the near-infrared region. The stabilization energy, -ΔG_com and comproportionation equilibrium constant, K_com calculated from peak potentials of the split reduction waves were found to be 1.06×10⁴  J mol^-1 , and 72.3 M^-1 , respectively. Further, the IVCT transition was also witnessed during the process of thermodynamically feasible electron transfer upon excitation of the TDPP entity in the system, and served as a diagnostic marker to characterize the electron transfer product. Subsequent transient absorption spectral studies and data analysis by Global and Target analyses revealed occurrence of ultrafast charge separation (k_cs ≈10¹⁰  s^-1 ) owing to the close proximity and good communication between the entities of the multi-modular donor-acceptor system. The role of central TDPP in promoting IVCT is borne out from the present investigation.

1,1,4,4-Tetracyanobutadiene-Functionalized Anthracenes: Regioselectivity of Cycloadditions in the Synthesis of Small Near-IR Dyes

Two small 1,1,4,4-tetracyanobutadiene-functionalized chromophores were obtained by careful leverage of the regioselectivity of the cycloaddition reaction of tetracyanoethylene with anthracene-ynamide derivatives, inducing either a [2 + 2] or a [4 + 2] Diels-Alder process. DFT calculations unraveled the mechanism of the [2 + 2] cycloaddition-retroelectrocyclization reaction sequence with ynamides and elucidated the differing mechanisms in the two substrates. The synthesized dyes presented panchromatic absorption extending into the near-IR and far-red/near-IR photoluminescence in the solid state up to 1550 nm.

Design and synthesis of 1,8-naphthalimide functionalized benzothiadiazoles

Push-pull derivatives BTD2–BTD5 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling reaction followed by [2+2] cycloaddition–electrocyclic ring-opening reaction.

Triphenylamine/Tetracyanobutadiene-Based π-Conjugated Push-Pull Molecules End-Capped with Arene Platforms: Synthesis, Photophysics, and Photovoltaic Response

π-Conjugated push-pull molecules based on triphenylamine and 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) have been functionalized with different terminal arene units. In solution, these highly TCBD-twisted systems showed a strong internal charge transfer band in the visible spectrum and no detectable photoluminescence (PL). Photophysical and theoretical investigations revealed very short singlet excited state deactivation time of ≈10 ps resulting from significant conformational changes of the TCBD-arene moiety upon photoexcitation, opening a pathway for non-radiative decay. The PL was recovered in vacuum-processed films or when the molecules were dispersed in a PMMA matrix leading to a significant increase of the excited state deactivation time. As shown by cyclic voltammetry, these molecules can act as electron donors compared to C₆₀ . Hence, vacuum-processed planar heterojunction organic solar cells were fabricated leading to a maximum power conversion efficiency of ca. 1.9 % which decreases with the increase of the arene size.

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.