Synthetic Principles for Bandgap Control in Linear pi-Conjugated Systems

Exploring the Photocyclization Pathways of Styrylthiophenes in the Synthesis of Thiahelicenes: When the Theory and Experiment Meet

The introduction of thiophene rings to the helical structure of carbohelicenes has electronic effects that may be used advantageously in organic electronics. The performance of these devices is highly dependent on the sulfur atom topology, so a precise knowledge of the synthetic routes that may afford isomeric structures is necessary. We have studied the photocyclization pathway of both 2- and 3-styrylthiophenes on their way to thiahelicenes by experiment and theory. To begin with, the synthesis of stereochemically well-defined 2- and 3-styrylthiophenes allowed us to register first, and simulate later, the UV-vis electronic spectra of these precursors. This information gave us access through time-dependent density functional theory calculations to the very nature of the excited states involved in the photocyclization step and from there to the regio- and stereochemical outcome of the reaction. For the widely known case of a 2-styrylthiophene derivative, the expected naphtho[2,1-b]thiophene type of ring fusion was predicted and experimentally observed by synthesis. On the contrary, 3-styrylthiophene derivatives have been seldom used in synthetic photocyclizations. Among the two possible structural outcomes, only the naphtho[1,2-b]thiophene type of ring fusion was found to be mechanistically sound, and this was actually the only compound observed by synthesis.

Synthesis of Ring-Locked Tetracyclic Dithienocyclopentapyrans and Dibenzocyclopentapyran via 1,5-Hydride Shift and Copper-Catalyzed C-O Bond Formation for Nonfullerene Acceptors

We discovered a unique synthetic route to construct 2H-pyran-containing tetracyclic dithienocyclopentapyran (DTCP) and dibenzocyclopentapyran (DBCP) architectures. The synthesis involves an acid-induced dehydration cyclization followed by a [1,5] hydride-shift isomerization to form a cyclopentanone moiety which was converted to the pyran-embedded tetracyclic products by a CuI-catalyzed intramolecular C-O bond formation in good yield. DTCP was used as a building block to prepare an acceptor-donor-acceptor (A-D-A) type n-type material DTCP-BC leading to a solar cell efficiency of 9.32%.

Fused Dithienopicenocarbazole Enabling High Mobility Dopant-Free Hole-Transporting Polymers for Efficient and Stable Perovskite Solar Cells

As a critical component in perovskite solar cells (PSCs), hole-transporting materials (HTMs) have been extensively explored. To develop efficient dopant-free HTMs for PSCs, a decent hole mobility (>10^-3 cm² V^-1 s^-1) is critically essential, which is, however, seldom reported. In this work, we introduce two novel donor-acceptor (D-A) type conjugated polymers (PDTPC-1 and PDTPC-2) with narrow bandgap unit, i.e., fused dithienopicenocarbazole (DTPC), as the donor building block and benzo[c][1,2,5]thiadiazole derivatives as the acceptors. The highly planar and strong electron-donating DTPC endows the polymers with superior hole mobility up to ∼4 × 10^-3 cm² V^-1 s^-1. Because of the better energy alignment with perovskite and excellent film-forming property, PSCs with PDTPC-1 as HTM show an appreciably enhanced PCE of ∼17% in dopant-free PSCs along with improved device stability as opposed to PDTPC-2. Our work revealed for the first time that the introduction of narrow bandgap DTPC in D-A polymers could achieve remarkably high hole mobility in the pristine form, favoring the application in dopant-free PSCs.

Optoelectronic properties of diketopyrrolopyrrole homopolymers compared to donor-acceptor copolymers

Diketopyrrolopyrrole (DPP) is a component of a large number of materials used for optoelectronic applications. As it is exclusively used in combination with aromatic donors, the properties of its homopolymers are unknown. Because donor-acceptor character has been shown for other systems to reduce bandwidths, DPP homopolymers should have even larger conduction bands and better n-type conductivity than the thiophene-flanked systems, which have exceptional n-type conductivity and ambipolar character. Therefore, a theoretical study was carried out to elucidate the properties of the unknown DPP homopolymer. Calculations were done with density functional theory and with the complete active space self-consistent field method plus n-electron valence state perturbation theory for the dynamic correlation. Poly-DPP is predicted to have radical character and an extremely wide low-lying conduction band. If it were possible to produce this material, it should have unprecedented n-type conductivity and might be a synthetic metal. A comparison with various unknown donor-acceptor systems containing vinyl groups and thienyl rings with a higher concentration of DPP than the known copolymers reveals how donor-acceptor substitution reduces bandwidths and decreases electron affinities.

Near-infrared dyes for two-photon absorption in the short-wavelength infrared: strategies towards optical power limiting

The recent advances in the field of two-photon absorbing chromophores in the short-wavelength infrared spectral range (SWIR 1100–2500 nm) are summarized, highlighting the development of optical power limiting devices in this spectral range.

Quinoidal conjugated polymers with open-shell character

Quinoidal π-Conjugated polymers with open shell character represent an intriguing class of macromolecules in terms of both fundamental research and practical applications.

Boron dipyrromethene (BODIPY) in polymer chemistry

The present review provides both a summary and outlook on the exciting field of BODIPYs in polymer chemistry.

An attempt to synthesize a terthienyl-based analog of indacenedithiophene (IDT): unexpected synthesis of a naphtho[2,3-b]thiophene derivative

We report herein our attempt to synthesize an analog of indacenedithiophene (IDT) based on a tetraphenylhexyl substituted, covalently bridged syn-terthienyl unit. Instead of the expected compound the adopted synthetic route led to the formation of an unexpected, new naphtho[2,3-b]thiophene derivative. The structure of this compound was fully characterized by NMR and HRMS as well as single crystal X-ray diffraction and its electronic properties have been analyzed by UV-vis absorption spectroscopy and cyclic voltammetry. A possible mechanism for the formation of this compound is also proposed on the basis of detailed theoretical investigations.

Serendipitous synthesis of a novel naphtho[2,3-b]thiophene derivative. Detailed investigation of the structure and properties as well as a possible mechanism for its formation.

Regioselective C-H Alkenylation and Unsymmetrical Bis-olefination of Heteroarene Carboxylic Acids with Ruthenium Catalysis in Water

An efficient weak carboxylate-assisted oxidative cross-dehydrogenative C-H/C-H coupling (CDC) of heteroarenes with readily available olefins has been devised employing water as green solvent under ruthenium(II) catalysis. The reaction is operationally simple, accommodates a large variety of heteroaromatic carboxylic acids as well as olefins, and facilitates a diverse array of high-value olefin-tethered heteroarenes in high yields (up to 87%). The potential of this ortho-C-H bond activation strategy has also been exploited toward tunable synthesis of densely functionalized heteroarenes through challenging unsymmetrical bis-olefination process in a one-pot sequential fashion. Mechanistic investigation demonstrates a reversible ruthenation process and C-H metalation step might not be involved in the rate-determining step.

A General Strategy to Design Highly Fluorogenic Far-Red and Near-Infrared Tetrazine Bioorthogonal Probes

Highly fluorogenic tetrazine bioorthogonal probes emitting at near-infrared wavelengths are in strong demand for biomedical imaging applications. Herein, we have developed a strategy for forming a palette of novel Huaxi-Fluor probes in situ, whose fluorescence increases hundreds of times upon forming the bioorthogonal reaction product, pyridazine. The resulting probes show large Stokes shifts and high quantum yields. Manipulating the conjugate length and pull-push strength in the fluorophore skeleton allows the emission wavelength to be fine-tuned from 556 to 728 nm. The highly photo-stable and biocompatible probes are suitable for visualizing organelles in live cells without a washing step and for imaging of tumors in live small animals to depths of 500 μm by two-photon excitation.

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.

Abdullah S, Brza MA. Conducting Polymers for Optoelectronic Devices and Organic Solar Cells: A Review

In this review paper, we present a comprehensive summary of the different organic solar cell (OSC) families. Pure and doped conjugated polymers are described. The band structure, electronic properties, and charge separation process in conjugated polymers are briefly described. Various techniques for the preparation of conjugated polymers are presented in detail. The applications of conductive polymers for organic light emitting diodes (OLEDs), organic field effect transistors (OFETs), and organic photovoltaics (OPVs) are explained thoroughly. The architecture of organic polymer solar cells including single layer, bilayer planar heterojunction, and bulk heterojunction (BHJ) are described. Moreover, designing conjugated polymers for photovoltaic applications and optimizations of highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy levels are discussed. Principles of bulk heterojunction polymer solar cells are addressed. Finally, strategies for band gap tuning and characteristics of solar cell are presented. In this article, several processing parameters such as the choice of solvent(s) for spin casting film, thermal and solvent annealing, solvent additive, and blend composition that affect the nano-morphology of the photoactive layer are reviewed.

Static- and frequency-dependent NLO properties of dithienothiophene and thienothiophene bridges — A computational investigation

We have explored detailed linear and nonlinear optical properties of push-pull systems bearing thienothiophene and dithienothiophene spacers. By using density functional theory (DFT), frequency-dependent strategies were applied to examine the polarizability ([Formula: see text] and hyperpolarizability ([Formula: see text] and [Formula: see text]. The set of global and range-separated hybrid functionals with different Hartree–Fock (HF) exchange percentage at two basis sets cc-pVDZ and cc-pVTZ were used to evaluate the nonlinear optical (NLO) properties. The observed trends in the absorption maxima supported by perturbation potential analysis; as the absorption maxima increases, the respective amplitude potential decreases. For the investigated compounds, [Formula: see text]-conjugation along with the type of substituted acceptor plays a crucial role in the enhancement of NLO properties. The presence of acceptor group and length of conjugation increase between the D and A group; the first- and second-order intrinsic hyperpolarizability increases, leads to enhanced first- as well as second-order hyperpolarizability. Bond length alternation (BLA)/bond order alteration (BOA) exploration suggested that compounds attain cyanine limit. The trends in NLO properties for investigated compounds are supported by chemical reactivity descriptors, hardness and hyperhardness analysis. The polarizability is linearly correlated with the hyperpolarizability parameters ([Formula: see text] and [Formula: see text] and shows a good regression coefficient by figures of merit analysis.

Tailoring topological order and π-conjugation to engineer quasi-metallic polymers

Topological band theory predicts that a topological electronic phase transition between two insulators must proceed via closure of the electronic gap. Here, we use this transition to circumvent the instability of metallic phases in π-conjugated one-dimensional (1D) polymers. By means of density functional theory, tight-binding and GW calculations, we predict polymers near the topological transition from a trivial to a non-trivial quantum phase. We then use on-surface synthesis with custom-designed precursors to make polymers consisting of 1D linearly bridged acene moieties, which feature narrow bandgaps and in-gap zero-energy edge states when in the topologically non-trivial phase close to the topological transition point. We also reveal the fundamental connection between topological classes and resonant forms of 1D π-conjugated polymers.

Nitro-Perylenediimide: An Emerging Building Block for the Synthesis of Functional Organic Materials

Perylenediimide (PDI) is one of the most important classes of dyes and is intensively explored in the field of functional organic materials. The functionalization of this electron-deficient aromatic core is well-known to tune the outstanding optoelectronic properties of PDI derivatives. In this respect, the functionalization has been mostly addressed in bay-positions to halogenated derivatives through nucleophilic substitutions or metal-catalyzed coupling reactions. Being aware of the synthetic difficulties of obtaining the key intermediate 1-bromoPDI, we will present as an alternative in this review the potential of 1-nitroPDI: a powerful building block to access a large variety of PDI-based materials.

Synthesis of side-chain regioregular and main-chain alternating poly(bichalcogenophene)s and an ABC-type periodic poly(terchalcogenophene)

Three unsymmetrical diiodobichalcogenophenes SSeI2, STeI2, and SeTeI2 and a diiodoterchalcogenophene SSeTeI2 were prepared. Grignard metathesis of SSeI2, STeI2, SeTeI2, and SSeTeI2 occurred regioselectively at the lighter chalcogenophene site because of its relatively lower electron density and less steric bulk. Nickel-catalyzed Kumada catalyst-transfer polycondensation of these Mg species provided a new class of side-chain regioregular and main-chain AB-type alternating poly(bichalcogenophene)s-PSSe, PSTe, and PSeTe-through a chain-growth mechanism. The ring-walking of the Ni catalyst from the lighter to the heavier chalcogenophene facilitated subsequent oxidative addition, thereby suppressing the possibility of chain-transfer or chain-termination. More significantly, the Ni catalyst could walk over the distance of three rings (ca. 1 nm)-from a thiophene unit via a selenophene unit to a tellurophene unit-to form PSSeTe, the first ABC-type regioregular and periodic poly(terchalcogenophene) comprising three different types of 3-hexylchalcogenophenes.

Elaboration of low-band-gap π-conjugated systems based on thieno[3,4-b]pyrazines

In order to understand the electronic contribution of the thienopyrazine building block on the electronic properties of π-conjugated systems, small molecules containing thiophene and thienopyrazine moieties are synthesized and jointly studied by theoretical (DFT) and experimental methods (UV-Vis, UPS). Taking advantages of these preliminar attractive results, four low band gap extended structures have been elaborated on the base of Donor-Acceptor-Donor sequences (DAD); these elongated π-conjugated molecules exhibit noticeable electronic and absorption properties spreading from the near UV to NIR regions.

Relation between the Electronic Properties of Regioregular Donor-Acceptor Terpolymers and Their Binary Copolymers

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C₇₁-butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations.

Distortion-Controlled Redshift of Organic Dye Molecules

It is shown, quantum chemically, how structural distortion of an aromatic dye molecule can be leveraged to rationally tune its optoelectronic properties. By using a quantitative Kohn-Sham molecular orbital (KS-MO) approach, in combination with time-dependent DFT (TD-DFT), the influence of various structural and electronic tuning parameters on the HOMO-LUMO gap of a benzenoid model dye have been investigated. These parameters include 1) out-of-plane bending of the aromatic core, 2) bending of the bridge with respect to the core, 3) the nature of the bridge itself, and 4) π-π stacking. The study reveals the coupling of multiple structural distortions as a function of bridge length and number of bridges in benzene to be chiefly responsible for a decreased HOMO-LUMO gap, and consequently, red-shifting of the absorption wavelength associated with the lowest singlet excitation (λ≈560 nm) in the model cyclophane systems. These physical insights together with a rational approach for tuning the oscillator strength were leveraged for the proof-of-concept design of an intense near-infrared (NIR) absorbing cyclophane dye at λ=785 nm. This design may contribute to a new class of distortion-controlled NIR absorbing organic dye molecules.

Comprehensive theoretical and experimental study of near infrared absorbing copolymers based on dithienosilole

A new conjugated copolymer with alternating dithienosilole and thienoisoindigo units displays improved near-infrared absorption compared to previously reported dithienosilole-based copolymers.

Non-planar core-modified dibenzi expanded isophlorin

The role of aromatic subunits is crucial to the structural and electronic properties of isophlorin and its expanded derivatives. Herein we describe the effect of benzene ring substitution in a 32[Formula: see text] expanded isophlorin core, which is at the crossroads of hexaphyrin and octaphyrin. Synthesis, structural characterization and electronic properties of a non-planar 32[Formula: see text] core-modified expanded isophlorin resembles that of a hexaphyrin like macrocycle. Dibenzi hexaphyrin is known to retain a planar structure; however, the dibenzi expanded isophlorin reported here loses its planarity and is found to adopt a unique V-shaped structure. The non-antiaromatic nature observed from spectroscopic analysis is ably supported by computational studies. Electronic absorption studies and mass spectrometry support a two-electron ring oxidation as expected of antiaromatic macrocycles.

Narrow bandgap difluorobenzochalcogenadiazole-based polymers for high-performance organic thin-film transistors and polymer solar cells

A series of difluorobenzochalcogenadiazole-bithiophene copolymers are developed for high-performance organic semiconductors.

Recent developments in the synthesis of regioregular thiophene-based conjugated polymers for electronic and optoelectronic applications using nickel and palladium-based catalytic systems

Thiophene-based conjugated polymers are important conjugated polymers due to their exceptional optical and conductive properties, over the past few decades many researchers have designed novel strategies to reach more efficient materials for electronic applications.

Synthesis, photophysical and electropolymerization properties of thiophene-substituted 2,3-diphenylbuta-1,3-dienes

Electropolymerizable diphenylbuta-1,3-diene derivatives with AIE or AEE properties were synthesized allowing low bandgap polymers to be obtained through electropolymerization processes.

Concise synthesis of 3-alkylthieno[3,2-b]thiophenes; building blocks for organic electronic and optoelectronic materials

Four step synthesis of 3-alkylthieno[3,2-b]thiophenes in the literature was reduced to two steps in good yields, through the preparation of the mono ketone, i.e. 1-(thiophene-3-ylthio)alkan-2-one, from 3-bromothiophene and ring formation reaction. This convenient method provides an easy access with good yields to the preparation of 3-alkylthieno[3,2-b]thiophenes, which are important materials for organic electronic and optoelectronic applications. SEM, AFM and contact angle (CA) analyses of their electropolymers on indium tin oxide (ITO) indicated that as the alkyl chains became longer, the polymers provide a more hydrophobic layer with CA up to 107°.

Four step synthesis of 3-alkylthieno[3,2-b]thiophenes in the literature was reduced to two steps in good yields, through the preparation of the mono ketone, i.e. 1-(thiophene-3-ylthio)alkan-2-one, from 3-bromothiophene and ring formation reaction.

Computational study on optoelectronic and charge transport properties of diketopyrrolopyrrole-based A-D-A-D-A structure molecules for organic solar cells

Eight novel diketopyrrolopyrrole (DPP)-based A-D-A-D-A structure molecules were designed for organic solar cells (OSCs) applications. In these molecules, the electron-deficient DPP and dicyanovinyl groups were used as the acceptor groups and different planar electron-rich groups were employed as the donor π-bridges. Applying the B3LYP/6-31G (d,p) and TD-B3LYP/6-31G (d,p) methods, the optoelectronic and charge transport properties were investigated. It turned out that the different π-bridges can tune effectively the frontier molecular orbital energy levels, band gap, and absorption spectra. Furthermore, the different π-bridges also affect the charge transport properties of the designed molecules. Our results suggest that the investigated molecules can serve as donor materials. Additionally, some investigated molecules can also be used as hole and/or electron transport materials for OSCs. Graphical abstract A series of novel A-D-A-D-A molecules are investigated systematically. The optical and electronic properties can be tuned effectively by the π-bridges. All derivatives can be used as donor materials for OSCs. Some designed molecules can be used as hole and/or electron transport materials. The different π-bridges do not significantly affect the stability of the molecules.

Synthesis and electropolymerization of N-heterocyclic carbene complexes of Pd(ii) and Pt(ii) from an emissive imidazolium salt with a terthiophene backbone

The synthesis of an emissive terthiophene-based imidazolium iodide: 1,3-dimethyl-4,6-di(thiophen-2-yl)-thienoimidazol-3-ium iodide (4), the corresponding N-heterocyclic carbene (NHC) preligand, and Pd and Pt-NHC complexes (5a, 5b) are reported. A related Pd-NHC complex (8) with a mono-thiophene backbone has also been synthesized. The solid-state structures were determined by single crystal X-ray diffraction studies. Compounds 4, 5a and 5b were characterized by absorption/emission spectroscopy and an emission-quench phenomenon of 5a was observed. The Pd complexes 5a and 8 were shown to catalyze Heck type C-C bond forming reactions. The compounds with terthiophene backbones (4, 5a, 5b) were electropolymerized onto indium tin oxide (ITO)-coated glass electrodes to yield thin films of conducting metallopolymers (CMPs) (P1-3). UV-vis-NIR spectroelectrochemistry data of P1-3 showed reversible color changes upon oxidation due to the generation of charge carrier species along the polythiophene backbones.