Push-pull Dyes Research Articles

Sensitive 1,1-dicyanovinyl push-pull dye for primary amine sensing in solution by fluorescence

1,1-dicyanovinyl push-pull dye PTTCN undergoes rapid and reversible aza-Michael addition with volatile primary aliphatic amines in solution in DMSO, yielding highly fluorescent imine derivatives. This reaction is accompanied by a dramatic colour change as well as a strong increase in fluorescence (1200-fold increase) allowing sensitive turn-on detection of primary amine in solution. In the solid state, PTTCN is emissive and reaction with primary amine vapours results in a shift in fluorescence and a possible ratiometric detection of primary amines. The selectivity is excellent for primary amines over secondary, tertiary and aromatic amines as well as over thiols and fluorescence spectroscopy allows to discriminate between primary amines and other nitrogen nucleophiles such as hydrazine and ammonia. Finally, we show as proof-of-concept that the fluorescence turn-on reaction can be used in response to the vapours of volatile amines released during food spoilage.

Y-shaped alkynylimidazoles as effective push-pull fluorescent dyes for luminescent solar concentrators (LSCs)

The synthesis of three 2-arylalkynyl-4,5-diarylimidazoles endcapped with typical electron-donating (ED) and electron-withdrawing (EW) groups (push-pull system) and their evaluation as fluorescent dyes for thin-film luminescent solar concentrators (LSCs) is described. These novel Y-shaped imidazole-based fluorophores were easily assembled in good chemical yields through a three-step synthetic sequence involving a double Suzuki arylation, followed by a dehydrogenative alkynylation, and a Knoevenagel condensation. Their optical and spectroscopic properties were analyzed both in solution and in poly(methyl methacrylate) (PMMA) films. All the molecules in PMMA films showed an intense, red-shifted emission between 540 and 575 nm with large Stokes shift (>120 nm), moderate-to-good fluorescence quantum yields (QY) and good device efficiencies (ηdev), especially for compound 3.

Morphology and Intramolecular Interactions in P(VDF-TrFE) Electrospun Nanofibers Doped with Disperse Orange 3 Dye: A Joint Infrared Spectroscopy and Electron Microscopy Study.

In this study, we describe a host–guest system consisting of a push–pull dye, the 4-amino-4′-nitroazobenzene (Disperse Orange 3, DO3), mixed with the copolymer poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] as a potential candidate for nonlinear optics (NLO) applications. We developed electrospun nanofibers of the polymer/dye blend, showing a highly anisotropic molecular structure, where DO3 molecules are mostly oriented parallel to the polymer chain, running in the fiber axis direction. The technique opens a way for obtaining non-centrosymmetric ordering of the NLO chromophore without requiring further poling. The supramolecular architecture is deeply investigated through infrared vibrational spectroscopy, which allows detecting a new phase involving DO3 molecules linked together by strong directional H-bonds. Electron microscopies highlight peculiar nanofiber morphologies with a preferred localization of DO3 at the surface layers.

Push-Pull Heterocyclic Dyes Based on Pyrrole and Thiophene: Synthesis and Evaluation of Their Optical, Redox and Photovoltaic Properties

Three heterocyclic dyes were synthesized having in mind the changes in the photovoltaic, optical and redox properties by functionalization of 5-aryl-thieno[3,2-b]thiophene, 5-arylthiophene and bis-methylpyrrolylthiophene π-bridges with different donor, acceptor/anchoring groups. Knoevenagel condensation of the aldehyde precursors with 2-cyanoacetic acid was used to prepare the donor-acceptor functionalized heterocyclic molecules. These organic metal-free dyes are constituted by thieno[3,2-b]thiophene, arylthiophene, bis-methylpyrrolylthiophene, spacers and one or two cyanoacetic acid acceptor groups and different electron donor groups (alkoxyl, and pyrrole electron-rich heterocycle). The evaluation of the redox, optical and photovoltaic properties of these compounds indicate that 5-aryl-thieno[3,2-b]thiophene-based dye functionalized with an ethoxyl electron donor and a cyanoacetic acid electron acceptor group/anchoring moiety displays as sensitizer for DSSCs the best conversion efficiency (2.21%). It is mainly assigned to the higher molar extinction coefficient, long π-conjugation of the heterocyclic system, higher oxidation potential and strong electron donating capacity of the ethoxyl group compared to the pirrolyl moiety.

Imidazole-Functionalized Y-Shaped Push-Pull Dye for Nerve Agent Sensing as well as a Catalyst for Their Detoxification.

A Y-shaped push-pull dye (1) with N,N-dimethylanilino donors and a benzonitrile acceptor connected via an imidazole-based π-conjugated spacer was designed. It showed a dark yellow color in solution due to facile intramolecular charge-transfer interaction, but no fluorescence was detected, presumably due to the photo-induced electron transfer effect of the imidazole moiety. However, addition of nerve agents such as diethyl chlorophosphate (DCP, sarin mimic) and diethyl cyanophosphate (DCNP, Tabun mimic) resulted in a blue-colored fluorescence with fading of the native dark yellow color. Mechanistic studies indicated nucleophilic attack of imidazole at the phosphorus of DCP or DCNP, leading to the formation of a phosphorylated intermediate, which undergoes time-dependent hydrolysis (∼24 h) in aqueous medium. This process recovers the free probe (enzyme-like behavior) and releases a less-toxic organophosphate compound as the byproduct. The phosphorylated derivative of 1, formed during such interaction, shows a different electronic behavior, which reduces the extent of charge-transfer interaction as well as nonradiative decay and supports emissive properties. Considering the high sensitivity of 1 towards DCP and DCNP with LOD 35 and 42 ppb, we prepared easy test strips for on-site vapor-phase detection of nerve agents.

Synthesis, optical and electrochemical properties of a series of push-pull dyes based on the 4,4-bis(4-methoxy phenyl)butadienyl donor

A series of twelve dyes based on the 4,4-bis(4-methoxyphenyl)butadienyl donor and differing by the electron acceptors have been designed and synthesized. The different dyes were characterized by UV–visible absorption spectroscopy as well as cyclic voltammetry. By fine tuning the electron-accepting ability of the fourteen selected acceptors, dyes absorbing between 400 and 650 nm could be obtained with this π-extended donor. To get a deeper insight into the optical properties, solvatochromism was investigated in 23 different solvents and remarkable linear correlations could be obtained using the Taft and Catalan solvatochromism scales. A comparison with four dyes used as reference compounds and differing from the series of twelve dyes by the structure of the electron-donating groups was also established so that the electron releasing ability of the 4,4-bis(4-methoxyphenyl)butadienyl donor could be compared with the reference ones. To prepare these four references compounds, only two electron acceptors previously used for the design of the twelve dyes have been employed. Interestingly, variation of the absorption maxima of the twelve dyes was determined as being more influenced by the polarizability of the solvent rather than by its polarity. To support the experimental results, theoretical calculations were carried out.

Carbazole-Cyanobenzene Dyes Electrografted to Carbon or Indium-Doped Tin Oxide Supports for Visible Light-Driven Photoanodes and Olefin Isomerizations.

The organic carbazole-cyanobenzene push-pull dye 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene was derivatized and attached to carbon or indium-doped tin oxide (ITO) electrodes by simple diazonium electrografting. The surface-bound dye is active and stable for the visible light photosynthetic isomerization of a wide range of functionalized stilbene and cinnamic acid derivatives. Up to 87,000 net turnovers were obtained for the isomerization of trans-stilbene. The isomerizations can be carried out in air with a 33% reduction in the rate. The ITO photoelectrodes are also active and stable toward photo-oxidations under basic and acidic conditions.

High-performance sunlight induced polymerization using novel push-pull dyes with high light absorption properties

In this article, a series of three-component photoinitiating systems (PISs) based on push–pull dyes are proposed. The systems are composed of the newly synthesized push-pull dyes and different co-initiators, namely an iodonium salt and a tertiary amine (ethyl dimethylaminobenzoate EDB). Photoinitiation abilities and photochemical properties of the different investigated photoinitiating systems were studied in detail. Notably, the mechanisms related to the polymerization efficiencies were determined using different techniques such as UV–visible absorption and fluorescence spectroscopy. Moreover, the photoinitiation abilities of the three-component systems for the sunlight-induced polymerization processes were also examined, indicating that sunlight can act as an effective light source for photopolymerization. Finally, 3D patterns with a spatial resolution were successfully produced by the direct laser writing (DLW) approach, even with photosensitive resins containing silica fillers.

N-ethyl carbazole-1-allylidene-based push-pull dyes as efficient light harvesting photoinitiators for sunlight induced polymerization

In this article, the free radical polymerization of acrylates was successfully achieved with a series of twelve push-pull chromophores comprising the N-ethyl carbazole-1-allylidene group as the electron-donating group. Using this strong donor, the resulting dyes, never synthesized before, could efficiently initiate the free radical photopolymerization of acrylates upon mild irradiation conditions e.g. using sunlight. These novel multi-component systems comprising the above mentioned push-pull dyes, a tertiary amine (ethyl dimethylaminobenzoate EDB) and an iodonium salt could act as efficient photoinitiating systems (PISs) in free radical polymerization (FRP), leading to the formation of 3D patterns with precise shapes under the direct laser write (DLW) approach. Among these new PISs, dyes 3, 6, 7 and 8 were selected due to their efficient photoinitiation performances for chemical mechanism studies including steady state photolysis, fluorescence quenching measurements, electron spin resonance (ESR) spin-trapping experiments and cyclic voltammetry. Markedly, their excellent photochemical reactivities prompted us to investigate the performance of these multi-component photoinitiating systems upon sunlight as an ecofriendly approach. As a result, this research proved that sunlight could be used as a potential light source that can advantageously replace LEDs when highly reactive push-pull dyes are used as photosensitizers for the free radical photopolymerization of acrylate resins. Finally, 3D patterns could be prepared with the new photocomposites, and silica fillers could be even incorporated within the photosensitive resins so that a gradient of resolution could be successfully demonstrated.

Synthesis and fluorescence properties of red-to-near-infrared-emitting push-pull dyes based on benzodioxazole scaffolds.

Fluorescence imaging with high temporal and spatial resolution has emerged as one of the most promising techniques to monitor biomolecules and biological processes in living systems. Among many kinds of small molecular fluorescent dyes, 2,1,3-benzoxadiazole (BD) derivatives have been widely applied in many chemical and biological applications due to their excellent photophysical properties. However, only a limited number of BD dyes with long emission wavelengths were reported. Herein, we have reported a new class of red-to near-infrared-emitting small molecular dyes 2a-3a based on benzodioxazole scaffolds, which are named VBDfluors. To bathochromically shift both absorption and emission, the conjugation system was extended by introducing electron-withdrawing group-substituted vinyl groups at position 7 via a Knoevenagel condensation reaction. The basic photophysical properties of VBDfluors were detected and summarized. The VBDfluors display excellent photophysical properties, including emission in the red-to-NIR region, large Stokes shifts, good stability/photostability and cell permeability. The geometry of the molecules was optimized by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Bioimaging results indicated that 2a and 3a exhibited excellent cell permeability and could be utilized for visualization of lipid droplets in living cells.

Computational and photophysical characterization of a Laurdan malononitrile derivative.

The malononitrile group is considered one of the strongest natural electron-withdrawing groups in a chemist's arsenal. However, surprisingly little is known about how this group functions in push-pull fluorophores. In a recent computational study, we reported that replacing the ketone group of the traditional push-pull dye Laurdan with a malononitrile group significantly improves the optical properties while retaining the membrane behavior of the parent molecule Laurdan. Motivated by these results, we report here the synthesis and photophysical characterization of the said compound, 6-(1-undecyl-2,2-dicyanovinyl)-N,N-dimethyl-2-naphthylamine (CN-Laurdan). To our surprise, this new CN-Laurdan probe is found to be much less bright than the parent Laurdan due to a large drop in the fluorescence quantum yield. Using computational methods, we determine that the origin of this low quantum yield is related to the existence of a non-radiative decay pathway related to a rotation of the malononitrile moiety, suggesting that the molecule could nonetheless function very well as a molecular rotor. We confirm experimentally that CN-Laurdan functions as a molecular rotor by measuring the quantum yield in methanol/glycerol mixtures of increasing viscosity. Specifically, we found a consistent increase in the quantum yield across the entire range of tested viscosities.

Structural, spectroscopic and electrical investigations of novel organic thin films bearing push-pull azo – Phenol dye for UV photodetection applications

In the current study, novel thin films of a phenol-based push-pull azo dye, 2-acetyl-4-(4-chloro-phenylazo) phenol (ACAP), with tunable optical and electronic properties were designed, synthesized and characterized for UV photodetection applications. The crystalline structure and morphological features of the thermally evaporated ACAP thin films are investigated. The fabricated thin films exhibit an amorphous-like structure with low-intensity crystalline regions of average crystallite size of about 29.51 nm and a smooth surface with nanostructured sheets formation. The optical transmittance, reflectance, and absorption of ACAP thin films are measured in the spectral range UV–vis-NIR. A significant high UV absorption extending from 190 nm to 385 nm is observed with semi-transparency nature in the visible region. Furthermore, a good agreement is obtained between the estimated value of the direct energy gap that is obtained experimentally (3.62 eV) and that calculated from the theoretical DFT approach (3.74 eV). The dispersion behavior is analyzed in terms of the single oscillator model and is employed to estimate the dispersion parameters. Finally, an organic/inorganic heterojunction device based on Au/ACAP/n-Si/Al for UV photodetection is successfully fabricated. The current–voltage relations of the manufactured photodetector showed significant stability and sensitivity to the incident UV illumination. The fabricated UV photodetector exhibits responsivity ~25.7 mA/W, specific detectivity ~2 × 109 Jones, efficiency ~16.74%, a fast and reproducible ON/OFF switching behavior with 480 ms and 218 ms rise and fall time, respectively.

Ultrabright Silica-Coated Organic Nanocrystals for Two-Photon In Vivo Imaging

Herein, we demonstrate the potential of ultrabright fluorescent silica-coated organic nanocrystals for two-photon in vivo imaging. These unique nanoparticles (NPs) containing a crystalline core of small push-pull dipolar dye are specifically designed to exhibit two-photon absorption for 2 fluorescence imaging. The NPs can be easily functionalized using click chemistry in pure water, with preservation of the organic core. A novel small-volume DLS technique was used to evaluate the effect of PEGylation on the colloidal stability of the NPs in complex media containing salts and proteins, mimicking the composition of blood serum. The potential of these bright red-emitting NPs for two-photon fluorescence imaging is demonstrated both in vitro and in vivo.

Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production

The light-induced processes occurring in two dye-catalyst assemblies for light-driven hydrogen production were investigated by ultrafast transient absorption spectroscopy. These dyads consist of a push-pull organic dye based on a cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) bridge, covalently linked to two different H2-evolving cobalt catalysts. Whatever the nature of the latter, photoinduced intramolecular electron transfer from the excited state of the dye to the catalytic center was never observed. Instead, and in sharp contrast to the reference dye, a fast intersystem crossing (ISC) populates a long-lived triplet excited state, which in turn non-radiatively decays to the ground state. This study thus shows how the interplay of different structures in a dye-catalyst assembly can lead to unexpected excited state behavior and might open up new possibilities in the area of organic triplet sensitizers. More importantly, a reductive quenching mechanism with an external electron donor must be considered to drive hydrogen production with these dye-catalyst assemblies.

Styryl-based new organic chromophores bearing free amino and azomethine groups: synthesis, photophysical, NLO, and thermal properties.

Herein we report the synthesis and characterization of a new series of styryl-based push-pull dyes containing a free amino group and their Schiff base derivatives. The dyes include the dicyanomethylene group as an acceptor and different para-substituted alkylamines as donors. Morever as a proton-sensitive group a pyridin-2-yl substituent was attached to the para-position of the phenyl moiety in both series of compounds. The photophysical properties of the dyes were examined in various solvents with different polarities and showed absorption in the visible region and green-red emission with low quantum yields. The absorption and the emission maxima were shifted bathocromically by increasing the solvent’s polarity. However, there was no correlation with the polarity parameters of the solvents. The pH-sensitive properties of all prepared Schiff bases were examined against TBAOH in DMSO, via deprotonation of the OH group in the salicylidene moiety and their reverse protonation was also investigated using TFA. The Schiff bases exhibited a bathochromic shift upon the addition of TBAOH to their solutions in DMSO. Therefore, they showed potential to be utilized as colorimetric and luminescence pH sensors. The second-order nonlinear optical (NLO) responses of the dyes were measured by the electric field-induced second harmonic (EFISH) generation method. The highest μβ values were obtained for the dyes bearing the julolidine donor as 1430 × 10−48 esu (for free amino derivative) and 1950 × 10−48 esu (for Schiff base derivative), respectively. The structural and electronic properties of the dyes as well as their NLO properties were further studied using DFT calculations. The thermal stabilities of all dyes were evaluated by thermogravimetric analysis (TGA). The TGA data showed that all dyes were thermally stable up to 250 °C.

Synthesis, optical and electrochemical properties of a series of push-pull dyes based on the 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) acceptor

A series of chromophores was designed and synthesized using 2-(3-cyano-4,5,5-trimethylfuran-2 (5H) -ylidene) malononitrile (TCF) as the electron acceptor and differing from each other by the use of thirteen different electron donors. The different dyes were characterized for their optical and electrochemical properties and theoretical calculations were also carried out to support the experimental results. By changing the electron donor in the thirteen dyes, chromophores absorbing between 430 nm and 700 nm could be synthesized. Solvatochromism of the different dyes was analyzed in 23 solvents of different polarity and a positive solvatochromism was determined for all chromophores using the semi-empirical solvent polarity scales based on the Kamlet-Taft parameters (π *) or the Catalan parameters (SdP and SPP).

Golden Age of Fluorenylidene Phosphaalkenes-Synthesis, Structures, and Optical Properties of Heteroaromatic Derivatives and Their Gold Complexes.

The substitution of 2,7-dibromo-9-fluorenyl phosphaalkenes with heteroaromatic substituents (bithiophene, benzothiophene, pyridine) offers access to interesting push–pull dye molecules. Steric shielding due to the bulky P-substituent gives marked different reactivities at the 2- and 7-positions, allowing the synthesis of mixed/asymmetric derivatives. Further functionalization via gold(I) coordination was demonstrated and increased the acceptor character, concomitant with a red-shifted absorption.

Exploration on the Combination of Push-Pull Porphyrin Dyes and Copper(I/II) Redox Shuttles toward High-performance Dye-sensitized Solar Cells

The effects of push-pull porphyrin dyes as well as copper(I/II) redox shuttles on photovoltaic properties have been examined toward high-performance dye-sensitized solar cells (DSSCs). DSSC with a combination of porphyrin dye LG4 and bis(4,4′,6,6′-tetramethyl-2,2′-bipyridyl)copper(I/II) exhibited the highest power conversion efficiency of 5.07% ever reported for porphyrin-based DSSCs using copper(I/II) redox shuttles.

Indole based push-pull dyes bearing azo and dimethine: Synthesis, spectroscopic, NLO, anion affinity properties and thermal characterization

A series of novel indole based push-pull dyes bearing azo and dimethine chromophores in same structure were synthesized and characterized by Fourier Transform Infrared (FT-IR), 1H/13C nuclear magnetic resonance (NMR), and mass spectrometry. The photophysical properties of the dyes were determined in three solvents in UV–vis and theoretical studies. Their chemosensor and nonlinear optic (NLO) properties were also investigated. Anion selectivity and sensitivities of dyes were studied with some selected anions in ACN and interaction mechanisms were confirmed by spectrophotometric and 1H NMR titration techniques. While the dyes showed affinity to F−, AcO−, CN−, and H2PO4− in the organic solvents such as DMSO and ACN and they showed selective behaviour only to CN- in partial aqueous solution. The thermal stabilities obtaining with thermogravimetric analysis (TGA) showed that the dyes have a good thermal stability up to around 300 °C.

Molecular trapping in two-dimensional chiral organic Kagom\xe9 nanoarchitectures composed of Baravelle spiral triangle enantiomers

The supramolecular self-assembly of a push-pull dye is investigated using scanning tunneling microscopy (STM) at the liquid–solid interface. The molecule has an indandione head, a bithiophene backbone and a triphenylamine–bithiophene moiety functionalized with two carboxylic acid groups as a tail. The STM images show that the molecules adopt an “L” shape on the surface and form chiral Baravelle spiral triangular trimers at low solution concentrations. The assembly of these triangular chiral trimers on the graphite surface results in the formation of two types of chiral Kagomé nanoarchitectures. The Kagomé-α structure is composed of only one trimer enantiomer, whereas the Kagomé-β structure results from the arrangement of two trimer enantiomers in a 1:1 ratio. These Kagomé lattices are stabilized by intermolecular O-H···O hydrogen bonds between carboxylic acid groups. These observations reveal that the complex structure of the push-pull dye molecule leads to the formation of sophisticated two-dimensional chiral Kagomé nanoarchitectures. The subsequent deposition of coronene molecules leads to the disappearance of the Kagomé-β structure, whereas the Kagomé-α structure acts as the host template to trap the coronene molecules.