Chemical reduction of π-expanded functionalized pentacene: cooperation of side group in alkali metal binding

Chemical reduction of a vertically expanded pentacene, TIPS-peri-pentacenopentacene (TIPS-PPP), with sodium metal in THF readily afforded a doubly-reduced product isolated as [{Na+(THF)3}2(TIPS-PPP2-)]. Single-crystal X-ray diffraction revealed the formation of a π-complex of TIPS-PPP2- with two {Na+(THF)3} moieties. The sodium ion is coordinated to three C-atoms at the most negatively charged edge sites and at the lateral ethynyl group. The delocalisation of the charge on the ethynyl function is accompanied by its notable elongation (Δ = 0.015 Å) coupled with the contraction of the adjacent single C-C bond (Δ = 0.033 Å). Bond length analysis supported by Harmonic Aromaticity Oscillator Model (HOMA) shows that the dianion can be written with four aromatic sextets in agreement with the Clar representation. Although TIPS-PPP2- has 36 π-electrons (4n), it does not show a global magnetic paratropic response. The rings with the most negative charge density have a very low paratropic response, while the other rings have a low diatropic response. Overall, the dianion is a non-aromatic molecule.

Machine Learning-Based Quantitative Structure-Property Relationships for the Electronic Properties of Cyano Polycyclic Aromatic Hydrocarbons

In this study, quantitative structure-property relationships (QSPR) based on a machine learning (ML) methodology and the truncated degree of π-orbital overlap (DPO) to predict the electronic properties, namely, the bandgaps, electron affinities, and ionization potentials of the cyano polycyclic aromatic hydrocarbon (CN-PAH) chemical class were developed. The level of theory B3LYP/6-31+G(d) of density functional theory (DFT) was used to calculate a total of 926 data points for the development of the QSPR model. To include the substituents effects, a new descriptor was added to the DPO model. Consequently, the new ML-DPO model yields excellent linear correlations to predict the desired electronic properties with high accuracy to within 0.2 eV for all multi-CN-substituted PAHs and 0.1 eV for the mono-CN-substituted PAH subclass.

Charge transport modulation in pseudorotaxane 1D stacks of acene and azaacene derivatives

Acenes have received a lot of attention because of their inherent and tunable absorbing, emissive, and charge transport properties for electronic, photovoltaic, and singlet fission applications, among others. Such properties are directly governed by molecular packing, and therefore, controlling their arrangement in the solid state is of utmost importance in order to increase their performance. Herein, we describe a new solid-state ordering strategy that allows obtaining 1D mixed π-stacks of acene and azaacene derivatives. In addition, we illustrate that charge transport can be modulated by the electronic nature of the encapsulated phenazine, opening new perspectives in the design, preparation and development of supramolecular organic semiconductors.

Dynamics of singlet fission and electron injection in self-assembled acene monolayers on titanium dioxide

We employ a combination of linear spectroscopy, electrochemistry, and transient absorption spectroscopy to characterize the interplay between electron transfer and singlet fission dynamics in polyacene-based dyes attached to nanostructured TiO2. For triisopropyl silylethynyl (TIPS)-pentacene, we find that the singlet fission time constant increases to 6.5 ps on a nanostructured TiO2 surface relative to a thin film time constant of 150 fs, and that triplets do not dissociate after they are formed. In contrast, TIPS-tetracene singlets quickly dissociate in 2 ps at the molecule/TiO2 interface, and this dissociation outcompetes the relatively slow singlet fission process. The addition of an alumina layer slows down electron injection, allowing the formation of triplets from singlet fission in 40 ps. However, the triplets do not inject electrons, which is likely due to a lack of sufficient driving force for triplet dissociation. These results point to the critical balance required between efficient singlet fission and appropriate energetics for interfacial charge transfer.

Synthesis and Properties of a Twisted and Stable Tetracyano-Substituted Tetrabenzoheptacene

An approach for the synthesis of pyrene-fused acenes that allows the introduction of electron-withdrawing cyano groups in key positions that simultaneously (i) induce twists in the aromatic framework and (ii) stabilize the LUMO level is reported. This combination of steric and electronic features provide a twisted, stable, and n-type tetrabenzoheptacene as confirmed by a combination of theoretical calculations and optical, electrochemical, thermal, and electrical characterization.

Synthesis, physical properties, and chemistry of donor–acceptor-substituted pentacenes

Pentacenes bearing electron-donating and (or) -withdrawing groups, namely methoxy-, dialkylamino-, and nitroaryl moieties, are synthesized to afford polarized pentacenes. The optical, electrochemical, and chemical properties of these derivatives are explored. The cycloaddition reaction of selected derivatives with tetracyanoethylene (TCNE) is explored, and the experimental results are rationalized on the basis calculations using density functional theory (DFT). X-ray crystallographic data provides insight into the molecular structure and intermolecular interactions present in the solid state.

The conversion of donor to acceptor and rational design for diketopyrrolopyrrole-containing small molecule acceptors by introducing nitrogen-atoms for organic solar cells

Following the introduction of nitrogen, a donor could convert into an acceptor, the conversion mechanism of which, acquired from theoretical calculations, will promote the development of acceptors.

Photo-induced characteristic study of the smallest fullerene fragment, 1,6,7,10-tetramethylfluoranthene as an acceptor

In this study, the use of the novel 1,6,7,10-tetramethylfluoranthene as an acceptor in the organic solar cell has been demonstrated.

Molecular electron acceptors for efficient fullerene-free organic solar cells

Small molecule electron acceptors pairing with wide bandgap or narrow bandgap electron donors are reviewed and discussed for fullerene-free organic solar cells.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Roll coated large area ITO- and vacuum-free all organic solar cells from diketopyrrolopyrrole based non-fullerene acceptors with molecular geometry effects

We investigate three diketopyrrolopyrrole based small molecular non-fullerene acceptors with molecular geometry effect on mainly their PV performance of both spin-coated ITO based and roll coated large area, ITO- and vacuum-free organic solar cells.

Bisthiadiazole-Fused Tetraazapentacenequinone: An Air-Stable Solution-Processable n-Type Organic Semiconductor

The synthesis and characterization of a tetraazapentacenequinone fused to two thiadiazoles is reported. This linear derivative constituting seven fused rings shows a very low LUMO level (-4.46 eV) and a low HOMO-LUMO gap (1.77 eV). Its high solubility, endowed by four triisopropylsilyl groups, allows the fabrication of air-stable field-effect transistors by liquid deposition methods that show electron mobilities up to 2.42 × 10(-3) cm(2) V(-1) s(-1) without any device optimization.

Theoretical study of the fluorination effect on charge transport properties in fused thiophene derivatives

A strategy for improving electron mobility of fused thiophenes by fluorination.

Synthetic applications of hypophosphite derivatives in reduction

The purpose of this review is to collect the applications in fine synthesis of hypophosphite derivatives as reducing agents.

Aryl substitution of pentacenes

A series of 11 new pentacene derivatives has been synthesized, with unsymmetrical substitution based on a trialkylsilylethynyl group at the 6-position and various aryl groups appended to the 13-position. The electronic and physical properties of the new pentacene chromophores have been analyzed by UV–vis spectroscopy (solution and thin films), thermoanalytical methods (DSC and TGA), cyclic voltammetry, as well as X-ray crystallography (for 8 derivatives). X-ray crystallography has been specifically used to study the influence of unsymmetrical substitution on the solid-state packing of the pentacene derivatives. The obtained results add to our ability to better predict substitution patterns that might be helpful for designing new semiconductors for use in solid-state devices.

Small molecule semiconductors for high-efficiency organic photovoltaics

Organic photovoltaic cells (OPVs) are a promising cost-effective alternative to silicon-based solar cells, and possess light-weight, low-cost, and flexibility advantages. Significant progress has been achieved in the development of novel photovoltaic materials and device structures in the last decade. Nowadays small molecular semiconductors for OPVs have attracted considerable attention, due to their advantages over their polymer counterparts, including well-defined molecular structure, definite molecular weight, and high purity without batch to batch variations. The highest power conversion efficiencies of OPVs based on small molecular donor/fullerene acceptors or polymeric donor/fullerene acceptors are up to 6.7% and 8.3%, respectively, and meanwhile nonfullerene acceptors have also exhibited some promising results. In this review we summarize the developments in small molecular donors, acceptors (fullerene derivatives and nonfullerene molecules), and donor-acceptor dyad systems for high-performance multilayer, bulk heterojunction, and single-component OPVs. We focus on correlations of molecular chemical structures with properties, such as absorption, energy levels, charge mobilities, and photovoltaic performances. This structure-property relationship analysis may guide rational structural design and evaluation of photovoltaic materials (253 references).

The influence of terminal push-pull substitution on the electronic structure and optical properties of pentacenes

The synthesis of 2,3-R(2)-9,10-(OMe)(2)-substituted pentacenes (R=OMe, F, Br, CN; 1-4) from 2,3-R(2)-9,10-dimethoxy-6,13-dihydro-6,13-ethanopentacene-15,16-diones (α-diketone-bridged precursors) by photochemically induced bis-decarbonylation (Strating-Zwanenburg reaction) is described. Under matrix-isolation conditions (solid Ar, 10 K) the S(1) transitions of 1 and 2 undergo hypsochromic and those of 3 and 4 bathochromic shifts compared to parent pentacene. The S(1) transition wavelengths correlate well with the difference of substituent parameters σ(p). A computational analysis of the excited states at the CAM-B3LYP/6-311+G** level of theory provides an assignment of the electronic transitions. Photolysis in solution at room temperature yields red [R=OMe (1)], blue [R=Br (3), F (2)], and green [R=CN (4)] pentacenes. The compounds are oxygen-sensitive and have low solubility, but their formation can be monitored by UV/Vis and, in the case of R=CN, also by (1)H NMR spectroscopy. The S(1) transition in 4 does not show the typical pentacene fine structure in the electronic absorption spectrum. Photogeneration in the presence of oxygen leads to a number of photoproducts that could be identified by monitoring the reaction by (1)H NMR spectroscopy for R=OMe.

Synthesis, reactivity, and electronic properties of 6,6-dicyanofulvenes

A series of 6,6-dicyanofulvene derivatives are synthesized starting from masked, dimeric, or monomeric cyclopentadienones. The reactivities of 6,6-dicyanofulvenes relative to their parent cyclopentadienones are discussed. 6,6-Dicyanofulvenes are capable of undergoing two consecutive, reversible, one-electron reductions and are presented as potential n-type small molecules.

n-Type organic semiconductors in organic electronics

Organic semiconductors have been the subject of intensive academic and commercial interest over the past two decades, and successful commercial devices incorporating them are slowly beginning to enter the market. Much of the focus has been on the development of hole transporting, or p-type, semiconductors that have seen a dramatic rise in performance over the last decade. Much less attention has been devoted to electron transporting, or so called n-type, materials, and in this paper we focus upon recent developments in several classes of n-type materials and the design guidelines used to develop them.

Oligomers and Polymers Based on Pentacene Building Blocks

Functionalized pentacene derivatives continue to provide unique materials for organic semiconductor applications. Although oligomers and polymers based on pentacene building blocks remain quite rare, recent synthetic achievements have provided a number of examples with varied structural motifs. This review highlights recent work in this area and, when possible, contrasts the properties of defined-length pentacene oligomers to those of mono- and polymeric systems.