Donor−Acceptor-Substituted Phenylethenyl Bithiophenes: Highly Efficient and Stable Nonlinear Optical 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.

Synthesis and Characterization of Tetraphenylethene AIEgen-Based Push-Pull Chromophores for Photothermal Applications: Could the Cycloaddition-Retroelectrocyclization Click Reaction Make Any Molecule Photothermally Active?

Three new tetraphenylethene (TPE) push-pull chromophores exhibiting strong intramolecular charge transfer (ICT) are described. They were obtained via [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) click reactions on an electron-rich alkyne-tetrafunctionalized TPE (TPE-alkyne) using both 1,1,2,2-tetracyanoethene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) as electron-deficient alkenes. Only the starting TPE-alkyne displayed significant AIE behavior, whereas for TPE-TCNE, a faint effect was observed, and for TPE-TCNQ and TPE-F₄-TCNQ, no fluorescence was observed in any conditions. The main ICT bands that dominate the UV-Visible absorption spectra underwent a pronounced red-shift beyond the near-infrared (NIR) region for TPE-F₄-TCNQ. Based on TD-DFT calculations, it was shown that the ICT character shown by the compounds exclusively originated from the clicked moieties independently of the nature of the central molecular platform. Photothermal (PT) studies conducted on both TPE-TCNQ and TPE-F4-TCNQ in the solid state revealed excellent properties, especially for TPE-F4-TCNQ. These results indicated that CA-RE reaction of TCNQ or F₄-TCNQ with donor-substituted are promising candidates for PT applications.

Scaffold Hopping Strategy to Identify Prostanoid EP4 Receptor Antagonists for Cancer Immunotherapy

Cancer cells can effectively suppress the natural immune response in humans, and prostaglandin E₂ (PGE₂) is a key mediator in the development of tumor cell resistance to immunotherapy. As a major contributor to PGE₂-elicited immunosuppressive activity, the EP4 receptor promotes tumor development and progression in the tumor microenvironment, and the development of selective and potent EP4 receptor antagonists should have promising potential for tumor immunotherapy. Aiming at improving the drug-like properties, a series of 4,7-dihydro-5H-thieno[2,3-c]pyran derivatives were designed and synthesized through a scaffold hopping strategy. The most promising compound 47 exhibited good EP4 antagonistic activity and excellent subtype selectivity, as well as favorable drug-like properties. It effectively suppressed the expression of multiple immunosuppression-related genes in macrophages. Meanwhile, oral administration of compound 47, alone or in combination with anti-PD-1 antibody, significantly enhanced the antitumor immune response and inhibited tumor growth in the mouse CT26 colon carcinoma model.

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.

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.

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.

Characterization of Conjugation Effects in the Series of Quinoxaline-2-ones by Means of Vibrational Raman Spectroscopy

A broad series of quinoxalinone-based π-conjugated donor-acceptor fluoro- and NLO-phores is characterized by means of Raman spectroscopy and single-crystal X-ray analysis supported by quantum chemical computations. Intense Raman spectroscopic markers that allow the differentiation of even closely related structures are identified. The intensities of these bands are shown to be related to the conjugation of the different molecular moieties, and they can provide an estimation of its extent. The intensity redistribution between these markers serves as a source of auxiliary structural information capable of pointing to a distortion of the conjugation or to the influence of aggregation effects in the condensed state. A simple relation between the intensity of the marker and the position and oscillator strength of the lowest-energy electronic absorption band of quinoxalinones allows a linking of the Raman effect with the optical properties of these compounds, which can be used for the rational design of novel species with improved optical characteristics.

Building Conjugated Donor-Acceptor Cross-Links into Metal-Organic Frameworks for Photo- and Electroactivity

We convert a coordination network into a covalent solid, while maintaining the crystallinity and greatly enhancing the framework rigidity and redox-active and photochemical properties. Specifically, intensely light-absorbing push-pull functions are postsynthetically installed by reacting the electrophilic TCNE (tetracyanoethylene) guests and the electron-rich alkyne side arms on a microporous Zr-organic framework, generating black microporous crystallites with a band gap smaller than 1.0 eV. The reaction proceeds in the known [2 + 2] cycloaddition-retroelectrocyclization mechanism and extensively establishes conjugated (polyene) bridges across the linker molecules. The donor (4-methoxyphenyl) and acceptor (dicyanovinyl) couples of the polyene bridges also act as an efficient fluorescent quencher and can be selectively installed in a thin outer layer of the host crystallite to form a core-shell assembly for turn-on fluorescent sensing of small amine molecules in water solutions.

Synthesis, characterization and unusual near-infrared luminescence of 1,1,4,4-tetracyanobutadiene derivatives

Two 1,1,4,4-tetracyanobutadiene (TCBD) derivatives were prepared by reaction of tetracyanoethylene with ynamides bearing either a pyrene or a perylene unit. They display luminescence that could be detected up to 1350 nm in the solid state.

Raposo MM, Martínez-Máñez R, Sancenón F. N,N-Diphenylanilino-heterocyclic aldehyde-based chemosensors for UV-vis/NIR and fluorescence Cu(ii) detection

Cu(ii) coordination with aldehyde-containing probes induced the appearance of NIR bands coupled with remarkable colour changes.

New D–π-A push–pull chromophores as low band gap molecular semiconductors for organic small molecule solar cell applications

Low band gap molecular semiconductors based on push–pull systems with appropriate HOMO–LUMO energy levels for organic photovoltaic applications were accomplished.

Novel quinoxalinone-based push–pull chromophores with highly sensitive emission and absorption properties towards small structural modifications

The photophysical properties of a series of novel push–pull quinoxalinone-based chromophores that strongly absorb and emit light in the broad visible spectrum were comprehensively studied both experimentally and through quantum chemical methods.

Synthesis and characterisation of push–pull flavin dyes with efficient second harmonic generation (SHG) properties

We report push–pull flavins bearing different donor groups that exhibit high hyperpolarisabilities (β = 355–9550 × 10⁻³⁰ esu) and decomposition temperatures thereby indicating their potential application as non-linear optical device materials.

Computational study of the electronic structures, UV-Vis spectra and static second-order nonlinear optical susceptibilities of macrocyclic thiophene derivatives

Using thiophene (which has a moderate resonance energy) as a spacer rather than benzene permits better π-electron delocalization and leads to a large nonlinear optical response. Thus, the nonlinear optical coefficients of a series of macrocyclic thiophene derivatives (C[3T_DA](n) with C(n) symmetry) were studied, and their electronic structures, UV-Vis spectra and static second-order nonlinear optical susceptibilities (β(0)) were computed. The calculated results showed that ΔE(H-L) increased and the UV-Vis spectrum redshifted as the number of C[3T_DA] units increased (one C[3T_DA] unit consists of trithiophene and diacetylene). The value of β(0) calculated by either the ZINDO-SOS or the FF method showed the same trend: the absolute value of β(0) increased as the number of units increased. The value of β(0) predicted by ZINDO-SOS was an order of magnitude larger than that predicted by the FF method. However, the results suggest that macrocyclic thiophene compounds potentially exhibit large static second-order nonlinear optical susceptibilities.

Bichalcophenes: A Concise Synthesis of Formyl Ester- and Cyano Ester-Substituted Bithiophenes, Bifurans, and Furanothiophenes

Syntheses of new formyl ester- and cyano ester-substituted bithiophenes, bifurans, and furanothiophenes in good yield are described. The key synthetic step uses Stille coupling of appropriately substituted bromo 5-ring heterocycles with stannyl-substituted 5-ring heterocycles.

Rh-catalyzed Negishi alkyl-aryl cross-coupling leading to alpha- or beta-phosphoryl-substituted alkylarenes

The catalytic cross-coupling between ArZnX and ICH(2)(CH(2))(n)P(O)(OEt)(2) (n = 0-3) has been investigated to determine the utility of the Rh catalyst during the alkyl-aryl cross-coupling and to develop a new synthetic method for phosphoryl-substituted alkylarenes. Rh-dppf exhibits an excellent catalytic activity for the reaction with the alkylphosphonate of n = 1, whereas for the reaction with those of n = 2 or 3, beta-hydride elimination mainly takes place. As for the reaction with an alkylphosphonate of n = 0, a polarity inversion of the coupling components is necessary in order to provide the coupling products; the phosphoryl analogue of the Reformatsky reagent and ArI give the cross-coupling products in good yields through the catalysis by Rh-dppf.

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.