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

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.

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.

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.

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%).

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.

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...