Ethynyl Spacer Research Articles

A Novel BODIPY‐Based Low‐Band‐Gap Small‐Molecule Acceptor for Efficient Non‐fullerene Polymer Solar Cells

We report herein an efficient A1‐C≡C‐A2‐C≡C‐A1 type small‐molecule 4,4'‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐ indacene (BODIPY) acceptor (A1=BODIPY and A2=diketopyrrolopyrrole (DPP)) by following the A‐to‐A excited electron delocalization via the BODIPY meso‐position, the inherent directionality for the excited electron delocalization. The lowest unoccupied molecular orbital (LUMO) delocalizes across over whole the two flanking A1 and the central A2, and the highest occupied molecular orbital (HOMO) localizes dominantly on the ‐C≡C‐DPP‐C≡C‐ segment. The excited electron upon light excitation of the DPP segment delocalizes over both the BODIPY and DPP segments. The acceptor in chloroform shows an unprecedented plateau‐like broad absorption between 550 and 700 nm with a large FWHM value of 195 nm. Upon transition into solid film, the acceptor shows absorption in the whole near ultraviolet‐visible‐near infrared wavelength region (300‐830 nm) with a low band gap of 1.5 eV and a maximum absorptivity of 0.85×105 cm‐1. Introduction of the ethynyl spacer between the A1 and A2 and the close BODIPY‐to‐DPP LUMO energy levels are crucial for the excited π−electron delocalization across over whole the conjugation backbone. A power conversion efficiency of 6.60% was obtained from the ternary non‐fullerene solar cell with PTB7‐Th:p‐DTS(FBTTh2)2 (0.5 : 0.5) as the donor materials, which is the highest value among the non‐fullerene organic solar cells with BODIPY as the electron acceptor material.

Spectroscopic characterization of free-base hydroxy(arylethynyl)porphyrins in acidic and basic media

The addition of arylethynyl groups to the porphyrin macrocycle represents an effective strategy with which to enhance the light-harvesting properties of porphyrins. We now extend this modification to arylethynyl porphyrins with two or four [Formula: see text]-hydroxyphenyl substituents. Arylethynyl porphyrins bearing four, but not two, [Formula: see text]-hydroxyphenyl substituents show evidence of aggregation under acidic conditions. Under basic conditions, deprotonation of the peripheral hydroxyphenyl substituents results in substantially red-shifted spectral features and enhanced absorption in the Q-band region. When the hydroxyphenyl groups are appended to the porphyrin macrocylce via the ethynyl spacers, the spectral shifts observed upon deprotonation are significantly enhanced relative to those observed for hydroxyphenylporphyrins, highlighting the role of expanded conjugation in altering porphyrin photophysics.

Tetraalkoxyphenanthrene-Fused Thiadiazoloquinoxalines: Synthesis, Electronic, Optical, and Electrochemical Properties, and Self-Assembly.

π-Extended thiadiazoloquinoxaline (TQ) derivatives 1a,b-3a,b, in which a tetraalkoxyphenanthrene moiety is annulated with the TQ core and benzene rings are incorporated via the ethynylene spacer, were synthesized. They display absorption bands reaching into 750 nm and possess the electron-affinity comparable to [60]fullerene. The CF3- and OMe-substituents on the benzene rings have moderate effects on modulation of the HOMO and LUMO levels. Tetraalkoxyphenanthrene-fused TQs 1a,b-3a,b aggregate in the solid state and assemble in solution through π-π stacking interactions. The self-assembly of 1a,b-3a,b into 1D superstructures was confirmed, and the difference in the alkoxy groups and the solvents for self-assembly proved to change their morphology. Comparison of the properties of 1a and those of reference compounds 4 and 5 clarified the effects of both the fusion of the phenanthrene moiety and the introduction of ethynylene spacers on the properties.

Electronic structure and optical properties calculation of Zn-porphyrin with N-annulated perylene adsorbed on TiO2 model for dye-sensitized solar cell applications: A DFT/TD-DFT study

The current work scrutinizes the chemical behavior of a set of promissory dyes, the Zn-porphyrins with N-annulated Perylene, WW3m-WW8m, within the most important steps in the solar cell: photoexcitation, electron injection and dye regeneration. The photoexcitation step was studied through TD-DFT framework, finding that the most intense band in WW4m, WW6m-WW8mcorresponds to the electronic transition of the frontier orbitals HOMO-LUMO. Among these, WW6mis highlighted, because the electronic density of the LUMO is localized over the anchoring group. Therefore, the presence of two ethynylene spacers in the WW6mporphyrin originate an enhancement in the light absorption. On the other hand, we analyze the electron injection modelling two (mono and bidentate) adsorbing modes in WW3m@TiO2-WW8m@TiO2 using DFT (B3LYP+D3) calculations. Adsorption energies show that WW3m@TiO2-WW8m@TiO2 are coordinated in bidentate mode. In this sense, to analyze the density of states (DOS) we found that WW6m@TiO2 present the narrowest band gap (1.53eV), promoting an easy electron injection, which could explain the origin of the highest overall efficiency of the solar cell for this porphyrin. Finally, dye regeneration, was studied through the free energy associated, ΔGregen, the values are between −6.61 and −6.93eVfor WW3m@TiO2-WW8m@TiO2 showing that this step is spontaneous and similar in all porphyrins.

Aryl-triazole foldamers with ethynyl spacers as effective receptors for halides and oxyanions

A new aryl-triazole receptor incorporating ethynyl spacer was readily constructed. By using UV–Vis spectroscopic titration, the foldamers were proved to be capable of binding various shaped anions (nBu4N+A−, A=Cl, Br, I, NO3, HSO4, H2PO4 and AcO) with high affinities but a low selectivity.

Tröger's Base‐Containing Aromatic Polymers with Unexpected Properties Endowed by the Insertion of Alkyne Spacers

Novel Tröger's base‐containing linear polymers are synthesized by the Pd‐catalyzed polycondensation of 2,8‐disubstituted Tröger's base monomers. Despite the nonplanarity of Tröger's base structure, there is a solubility problem for the resulting polymers. Careful selection of the comonomers and π‐spacers eventually produce high molecular weight polymers. All the polymers show high thermal stabilities with decomposition temperatures (Td5%) exceeding 300 °C. The optical and electrochemical properties of Tröger's base‐containing polymers are, to the best of our knowledge, investigated for the first time. They absorb UV light and emit a blue fluorescence in CH2Cl2 solutions with the quantum yield reaching 0.72. The polymers with ethynylene spacers display a remarkable light‐green fluorescence in thin films as a result of excellent intermolecular interactions. Also, the redox activities depend on the chemical structures of the comonomers and π‐spacers. Remarkably, Tröger's base‐fluorene copolymer does not show any oxidation peaks, whereas the ethynylene‐linked copolymer exhibits a well‐defined oxidation at ≈1.1 V (vs Ag/Ag+). The use of triphenylamine comonomers further enhances the redox activities. A facilitated oxidation occurs at the benzidine subunits, and a faster redox response is revealed by spectroelectrochemical measurements. image

Tunable emission of hydrazine-containing bipyrrole fluorine–boron complexes by linear extension

The π-conjugate system of a bipyrrole fluorine–boron-based fluorophore was linearly extended by coupling methodology, and the resulting complexes were fully characterized. Various substituents linked to the framework of the bipyrrole fluorine–boron complexes by ethynyl spacers not only extended the π-conjugate system but also caused the emission maxima to red-shift by ∼10 nm increments. The emission maxima of these complexes spanned 49 nm, extending from 487 nm (blue-green) to 536 nm (green-yellow). These complexes were highly emissive both in solution and as a solid powder. Triple bonds, along with the substituent effect, can be regarded as ideal for fluorescence modulation.

Synthesis of rigid ethynyl-bridged polytopic picolinate ligands for MOF applications

Segmented homopolytopic ligands that consist of a rigid central arylene platform, ethynylene spacers, and terminal chelating picolinate subunits have been synthesized in good yields in a two-step procedure involving a Sonogashira-type cross coupling reaction between the ester methyl 5-bromopyridine-2-carboxylate and several arylacetylenes, followed by hydrolysis of the resulting methyl picolinates. A similar strategy has been employed for the preparation of heteroditopic ligands containing picolinate and a second non-chelating pyridine or benzoate unit. The compounds are potential candidates for organic linkers in metal–organic frameworks (MOFs).

Series of Carbazole-Pyrimidine Conjugates: Syntheses and Electronic, Photophysical, and Electrochemical Properties.

A series of carbazole-pyrimidine conjugates 1-17 were synthesized by Pd-catalyzed cross-coupling, oxidation, and nucleophilic aromatic substitution reactions. In 1-17, the carbazole moieties are connected at the 4,6-positions of the pyrimidine ring either directly or via ethynylene or vinylene spacers, and various electron-donating or electron-withdrawing substituents are introduced at the 2-position of the pyrimidine ring. The effects of structural variations on the electronic, photophysical, and electrochemical properties of 1-17 were comprehensively investigated. Compounds 1-17 exhibit intramolecular charge-transfer (ICT) states, which essentially lead to moderate-to-strong fluorescence emission with large Stokes shifts depending on the solvent polarity. These compounds tend to show significant changes in optical and fluorescence properties upon addition of trifluoroacetic acid. The electron-accepting ability of these compounds can be tuned by both substituents on the pyrimidine moiety and spacers. The ethynylene spacer lowers both the HOMO and LUMO levels, while the vinylene spacer elevates the HOMO level and lowers the LUMO level. The X-ray crystal structures of 3, 6, 11, and 14 are also disclosed.

Electron transfer pathways in mixed-valence paracyclophane-bridged bis-triarylamine radical cations.

A series of paracyclophane (PC) bridged mixed-valence (MV) bis-triarylamine radical cations with different ([2.2], [3.3], [4.4]) linkers, with and without additional ethynyl spacers, have been studied by quantum-chemical calculations (BLYP35-D3/TZVP/COSMO) of ground-state structures, thermal electron-transfer barriers, hyperfine couplings, and lowest-lying excited states. Such PC-bridged MV systems are important intra-molecular model systems for inter-molecular electron transfer (ET) via π-stacked aromatics, since they allow enforcement of a more or less well-defined geometrical arrangement. Closely comparable ET barriers and electronic couplings for all [2.2] and [3.3] bridges are found for these class-II MV systems, irrespective of the use of pseudo-para and pseudo-meta connections. While the latter observation contradicts notions of quantum interference for off-resonant conduction through molecular wires, it agrees with the less intricate nodal structures of the highest occupied molecular orbitals. The ET in such MV systems may be more closely connected with hole conduction in the resonant regime. Computations on model cations, in which the [2.2] linkers have been truncated, confirm predominant through-space π-π electronic coupling. Systems with [4.4] PC bridges exhibit far more structural flexibility and concomitantly weaker electronic interactions between the redox centers.

High-conductance surface-anchoring of a mechanically flexible platform-based porphyrin complex

The conductances of molecular model junctions comprising a triazatriangulenium platform with or without an ethynyl spacer and an upright Zn-porphyrin are probed with a low-temperature scanning probe microscope. The platform alone is found to be highly conductive. The ethynyl-linked Zn-porphyrin moiety reduces the conductance by three orders of magnitude and leads to an unexpected, non-monotonous variation of the force that was measured simultaneously at the tip of the microscope. Density functional theory calculations show that this variation results from an induced tilting of the porphyrin.

The position effect of an ethynyl spacer on the carrier mobility of anthracene derivatives

The naphthalenyl ethynyl substitution at the 1- and 2-positions of anthracene exhibits totally different film forming ability and hole mobility.

The influence of an ethynyl spacer on the electronic properties in 2,5-ferrocenyl-substituted heterocycles

A series of binuclear complexes based on five-membered heterocycles featuring alkynyl ferrocenyl groups, 2,5-(FcCC)2-cC4H2E (E=NPh, 3a; S, 3b; O, 3c), has been synthesized using the Sonogashira C,C cross-coupling protocol. The molecular structures of 3a and 3c in the solid state are discussed. The influence of the ethynyl unit on the electronic and (spectro)electrochemical properties of these compounds is shown by cyclic voltammetry (CV), square wave voltammetry (SWV) and in situ UV–Vis–NIR and IR spectroscopy. The high ΔEp values (140–204mV) of the single reversible redox events indicate that two individual oxidation processes take place in a close potential range. Nevertheless, the NIR measurements indicate weak and broad intervalence charge transfer (IVCT) absorptions for the mixed-valent monocationic species 3a–c+. Compared to 2,5-Fc2-cC4H2E (E=NPh, S, O) the absorptions for the IVCT transitions are shifted hypsochromically, showing that the charge transfer process is promoted at higher excitation energies, due to larger metal–metal distances in 3a–c. The occurrence of IVCT absorptions demonstrates that the 2,5-diethynylferrocenyl heterocycles are class II systems according to Robin and Day.

N-Annulated Perylene as An Efficient Electron Donor for Porphyrin-Based Dyes: Enhanced Light-Harvesting Ability and High-Efficiency Co(II/III)-Based Dye-Sensitized Solar Cells

Porphyrin-based dyes recently have become good candidates for dye-sensitized solar cells (DSCs). However, the bottleneck is how to further improve their light-harvesting ability. In this work, N-annulated perylene (NP) was used to functionalize the Zn-porphyrin, and four "push-pull"-type NP-substituted and fused porphyrin dyes with intense absorption in the visible and even in the near-infrared (NIR) region were synthesized. Co(II/III)-based DSC device characterizations revealed that dyes WW-5 and WW-6, in which an ethynylene spacer is incorporated between the NP and porphyrin core, showed pantochromatic photon-to-current conversion efficiency action spectra in the visible and NIR region, with a further red-shift of about 90 and 60 nm, respectively, compared to the benchmark molecule YD2-o-C8. As a result, the short-circuit current density was largely increased, and the devices displayed power conversion efficiencies as high as 10.3% and 10.5%, respectively, which is comparable to that of the YD2-o-C8 cell (η = 10.5%) under the same conditions. On the other hand, the dye WW-3 in which the NP unit is directly attached to the porphyrin core showed a moderate power conversion efficiency (η = 5.6%) due to the inefficient π-conjugation, and the NP-fused dye WW-4 exhibited even poorer performance due to its low-lying LUMO energy level and nondisjointed HOMO/LUMO profile. Our detailed physical measurements (optical and electrochemical), density functional theory calculations, and photovoltaic characterizations disclosed that the energy level alignment, the molecular orbital profile, and dye aggregation all played very important roles on the interface electron transfer and charge recombination kinetics.

Diarylethene Photoswitches Featuring Tetrathiafulvalene-Containing Aryl Units

The modulation of the electron-donor properties of tetrathiafulvalene (TTF) with light is attempted by two molecular designs: the integration of the TTF core as aryl unit(s) into the hexatriene system of diarylethene (DAE) derivatives and the attachment of terminal TTF unit(s) to a conventional 1,2-dithienylcyclopentene chromophore via conjugated ethynylene spacers. While photochromism is inhibited in the first approach, presumably by a twisted intramolecular charge transfer mechanism, the second approach yields photoswitchable TTF derivatives. Remarkably, the extension of the conjugated π-system upon ring closure affects the first oxidation potential values of the TTF–DAEs only slightly. Nevertheless, the electronic changes in the photochromic core are foreseen to play a crucial role in the intramolecular electron transfer process in donor–DAE–acceptor architectures.

Synthesis and Characterization of Redox-Active Mononuclear Fe(κ2-dppe)(η5-C5Me5)-Terminated π-Conjugated Wires

Several new redox-active Fe(κ2-dppe)(η5-C5Me5) arylacetylide complexes featuring pendant ethynyl (Fe(κ2-dppe)(η5-C5Me5)[{C≡C(1,4-C6H4)}nC≡CH] (1b–d; n = 1–3), Fe(κ2-dppe)(η5-C5Me5)[C≡C(1,3-C6H4)C≡CH] (2)) or ethenyl (Fe(κ2-dppe)(η5-C5Me5)[C≡C(1,4-C6H4)CH═CH2] (3)) groups have been synthesized and characterized under their Fe(II) and Fe(III) states. In contrast to the known ethynyl Fe(III) complex [Fe(κ2-dppe)(η5-C5Me5)(C≡CH)][PF6] (1a[PF6]), most of the new Fe(III) derivatives turned out to be kinetically stable in solution. A consistent picture of the electronic structure of the latter complexes in both redox states emerged from experimental data and DFT calculations. This study revealed that beyond the first 1,4-phenylene ring, modification or extension of the carbon-rich linker using (4-phenylene)ethynylene spacers will have only a minor influence on their electronic properties in their ground state, while still maintaining some (weak) electronic interaction along the carbon-rich backbone.

Synthesis and Characterization of Self-Assembled Nanoscopic Metallarectangles Capable of Binding Fullerenes with Size-Selective Responses

Two new metallarectangles, 4 and 5, were obtained from the self-assembly of areneruthenium-based molecular clips 2 and 3 with a new dipyridyl donor ligand 1 containing a diamide core and ethynyl spacers. The metallarectangles were characterized by multinuclear NMR, electrospray ionization mass spectrometry, and UV-vis spectroscopy, and the molecular structure of 4 was unambiguously determined by single-crystal X-ray diffraction analysis. Because of the presence of an extended π-electron aromatic surface, the tetracene-containing molecular rectangle 5 was capable of binding C60 and C70 fullerenes as quantified by UV-vis, emission, and (1)H NMR experiments, providing an example of a supramolecular host capable of recognizing large guest molecules.

Columnar Self‐Assembly in Electron‐Deficient Heterotriangulenes

A series of soluble carbonyl-bridged heterotriangulenes, in which flexible n-dodecyl chains are attached through different spacers to the planar nitrogen-centered polycyclic core, have been synthesized. The introduction of triisopropylsilylethynyl moieties enabled, for the first time, the characterization of single-crystal columnar packing of a substituted heterotriangulene by X-ray crystallography. Electrochemical studies disclosed the carbonyl-bridged heterotriangulene core as a reasonably strong acceptor for a reversible two-electron transfer. The tendency of substituted heterotriangulenes to self-assemble in solution, on surfaces, and in the bulk appeared to sensitively depend on the nature of the lateral substituents, their steric demand, and the applied solution processing conditions. It can be concluded that 1) additional phenylene moieties between the heterotriangulene core and the n-dodecyl chains facilitate self-assembly by extending the π-conjugated polycyclic disc, 2) the rod-like ethynylene spacers introduce some additional flexibility and hence lower the overall aggregation tendency, and 3) the combination of both features in the phenylene-ethynylene moieties induces thermotropic liquid crystallinity.

An Acetylene-Containing Perylene Diimide Copolymer for High Mobility n-Channel Transistor in Air

A new solution-processable conjugated copolymer (P1) of perylene diimide (PDI) and dithienothiophene (DTT) incorporating acetylene spacers was synthesized by palladium(0)-catalyzed Sonogashira coupling reaction. Theory calculation reveals that introduction of rod-like ethynylene spacer in the polymer main chain promotes planarity and π-conjugation of the polymer main chain. Relative to the conjugated copolymer (P2) of PDI and DTT without acetylene spacers, polymer P1 exhibits 0.1 eV down shift of LUMO level and 89 nm red shift of low-energy absorption band. Polymer P1 in top-contact bottom-gate organic field-effect transistors (OFETs) exhibits a saturation electron mobility of 0.06 cm2 V–1 s–1 in air, while P2 does not function in the same device in air. Additionally, a saturation electron mobility of 0.075 cm2 V–1 s–1 in air, and after six days storage in air, an electron mobility of 0.034 cm2 V–1 s–1 were observed for P1 in bottom-contact bottom-gate OFETs; while a saturation electron mobility of 0.038 cm2 V–1 s–1 in air, and after six days storage in air, an electron mobility of 0.013 cm2 V–1 s–1 were observed for P2 in the same device. The better air stability and higher electron mobility of P1 are attributed to densely ordered packing of the polymer chains excluding oxygen or water and the lower LUMO level of P1.

Photophysical properties of isoelectronic oligomers with vinylene, imine, azine and ethynylene spacers bearing triphenylamine and carbazole end-groups

Ten symmetrical oligomers containing either triphenylamine or carbazole substituents as end-groups and 1,4-phenylenevinylene, 1,4-phenylene imine, azine and 1,4-phenylene ethynylene as π-spacers, have been synthesized by polycondensation reactions between aromatic aldehydes (4-formyltriphenylamine and 3-formyl N-hexylcarbazole) or iodides (4-iodotriphenylamine and 3-iodo N-hexylcarbazole) with bisphosphonate derivative, 1,4-diaminobenzene, hydrazine or 1,4-diethynylbenzene, respectively. These oligomers are models for the corresponding conducting polymers, have a well-defined molecular structure, can be highly purified using common methods and processed as thin films by vacuum evaporation, dip or spin coating. The oligomers preserve all of the properties and potential applications of the corresponding polymers. The structural characterization of these oligomers was performed using usual spectroscopic methods (1H-RMN, FT-IR and DSC) and their photophysical properties were analyzed by UV–Vis and fluorescence spectroscopy. Their redox properties were studied by cyclic voltammetry.