Organic Molecular Materials Research Articles

25th Anniversary Article: Design of Polymethine Dyes for All‐Optical Switching Applications: Guidance from Theoretical and Computational Studies

All-optical switching--controlling light with light--has the potential to meet the ever-increasing demand for data transmission bandwidth. The development of organic π-conjugated molecular materials with the requisite properties for all-optical switching applications has long proven to be a significant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π-conjugated materials for all-optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.

Paper No 8.3: Nanoimprinted Organic Electrochemical Transistors

AbstractThe use of organic molecular and polymer materials in circuitry has received considerable attention during the last years. These multifunctional materials enable the low‐cost production of simple designed organic electronic devices and logic circuits that are driven at low voltages. We report on electrochemical transistors (ECTs) structured by nanoimprint lithography (NIL). With this technique it is possible to fabricate transistors with critical dimensions in the sub‐micrometer regime, which should lead to higher switching speed and improved overall transistor performance.

A new polymorph of bis(2-aminopyridinium) fumarate–fumaric acid (1/1) from powder X-ray diffraction

A new polymorph of bis(2-aminopyridinium) fumarate-fumaric acid (1/1), 2C₅H₇N₂⁺∙C₄H₂O₄²⁻-·C₄H₄O₄, was obtained and its crystal structure determined by powder X-ray diffraction. The new polymorph (form II) crystallizes in the triclinic system (space group P-1), while the previous reported polymorph [form I; Ballabh, Trivedi, Dastidar & Suresh (2002). CrystEngComm, 4, 135-142; Büyükgüngör, Odabaşoğlu, Albayrak & Lönnecke (2004). Acta Cryst. C60, o470-o472] is monoclinic (space group P2₁/c). In both forms I and II, the asymmetric unit consists of one 2-aminopyridinium cation, half a fumaric acid molecule and half a fumarate dianion. The fumarate dianion is involved in hydrogen bonding with two neighbouring 2-aminopyridinium cations to form a hydrogen-bonded trimer in both forms. In form II, the hydrogen-bonded trimers are interlinked across centres of inversion via pairs of N-H∙∙∙O hydrogen bonds, whereas such trimers are joined via single N-H∙∙∙O hydrogen bonds in form I, leading to different packing modes for forms I and II. The results demonstrate the relevance and application of the powder diffraction method in the study of polymorphism of organic molecular materials.

Exploring Cyclometalated Ir Complexes as Donor Materials for Organic Solar Cells

The performance of small molecular organic photovoltaic materials is typically limited by their low exciton diffusion lengths, poor solubility, and poor energy level alignment with fullerenes so that the design and synthesis of new materials remain a top priority. To overcome these limitations, we explored the use of an iridium complex as a donor material with the potential for compatibility with solution processing, long exciton diffusion length and easy molecular modification for tunable optical or electrical properties. A bilayer device with a cyclometalated iridium complex and C60 resulted in a power conversion efficiency as high as 2.8%. Furthermore, a VOC of 1 V was achieved in the bilayer device despite an estimated exciton energy of only 1.55 eV, and the device showed minimal temperature and light intensity dependence.

Experimental and DFT-D Studies of the Molecular Organic Energetic Material RDX

We have performed simulations utilizing the dispersion-corrected density functional theory method (DFT-D) as parametrized by Grimme on selected polymorphs of RDX (cyclotrimethylenetrinitramine). Additionally, we present the first experimental determination of the enthalpy of fusion (ΔHfus) of the highly metastable β-form of RDX. The characteristics of fusion for β-RDX were determined to be 186.7 ± 0.8 °C, 188.5 ± 0.4 °C, and 12.63 ± 0.28 kJ mol–1 for the onset temperature, peak temperature (or melting point), and ΔHfus, respectively. The difference in experimental ΔHfus for the α- and β-forms of RDX is 20.46 ± 0.92 kJ mol–1. Ambient-pressure lattice energies (EL) of the α- and β-forms of RDX have been calculated and are in excellent agreement with experiment. In addition the computationally predicted difference in EL (20.35 kJ mol–1) between the α- and β-forms is in excellent agreement with the experimental difference in ΔHfus. The response of the lattice parameters and unit-cell volumes to pressure for the α- and γ-forms have been investigated, in addition to the first high-pressure computational study of the ε-form of RDX—these results are in very good agreement with experimental data. Phonon calculations provide good agreement for vibrational frequencies obtained from Raman spectroscopy, and a predicted inelastic neutron scattering (INS) spectrum of α-RDX shows excellent agreement with experimental INS data determined in this study. The transition energies and intensities are reproduced, confirming that both the eigenvalues and the eigenvectors of the vibrations are correctly described by the DFT-D method. The results of the high-pressure phonon calculations have been used to show that the heat capacities of the α-, γ-, and ε-forms of RDX are only weakly affected by pressure.

Seamless and Accurate Modeling of Organic Molecular Materials.

The near endless possibilities for assembling molecular materials has long posed a difficult challenge for theory. All crystal-structure prediction methods acknowledge the crucial contribution of van der Waals or dispersion interactions, but few go beyond a pairwise additive description of dispersion, ignoring its many-body nature. Here we use two databases to show how a many-body approach to dispersion can seamlessly model both solid and gas-phase interactions within the coveted "chemical accuracy" benchmark, while the underlying pairwise approach fails for solid-state interactions due to the absence of many-body polarization and energy contributions. Our results show that recently developed methods that treat the truly collective nature of dispersion interactions are able to reach the accuracy required for predicting molecular materials, when coupled with nonempirical density functionals.

Organic sol–gel synthesis of microporous molecular networks containing spirobifluorene and tetraphenylmethane nodes

AbstractThe microporous molecular networks based on rigid tetrafunctional units are synthesized via organic sol–gel polymerization of 2,2′,7,7′‐tetraamino‐9,9′‐spirobifluorene (TASBF) and/or tetrakis(4‐aminophenyl)methane (TAPM) with a diisocyanate, hexamethylene diisocyanate (HDI), or p‐phenylene diisocyanate. This study is performed as an extension of our previous report on the first organic sol–gel method, which enabled the synthesis of microporous molecular networks via a two‐stage mechanism involving the formation of colloidal dispersions of the nanoparticulate molecular networks and their subsequent growth to monolithic networks on solvent evaporation. The sol–gel‐synthesized molecular networks obtained by incorporating TASBF as a network former show improved porosity, processability, and thermal stability than the TAPM‐based system. The improved porosity of TASBF‐based networks is attributed to higher rigidity of the spirobifluorene compared with the tetraphenylmethane units. We also demonstrate the synthesis of mixed organic molecular networks by sol–gel copolymerization of the two network formers, TASBF and TAPM, and a diisocyanate monomer. The sol–gel transformation of TASBF/TAPM/HDI occurred at longer reaction times with increasing the amount of TASBF in the TASBF/TAPM/HDI mixture. The results indicate that the organic sol–gel method can be further optimized by adjusting various synthesis parameters to create new functional organic molecular network materials. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Methods to produce B–C, B–P, B–N and B–S bonds in boron clusters

Boranes, heteroboranes and metallacarboranes, all named as boron clusters, offer an alternative to typical organic molecules or organic molecular materials. Carbon and boron share the important property of self-catenation thus these elements can produce individually large and sophisticated molecules. Boron clusters and organic molecules display electronic, physical, chemical and geometrical characteristics manifestly different. These differences highlight the complementarity of organic molecules and boron clusters, and therefore the feasibility or necessity to produce hybrid molecules incorporating both types of fragments. To join these two types of fragments, or alternatively these two types of molecular compounds, tools are needed. In this review the current methods of producing boron clusters with carbon, B-C, nitrogen, B-N, phosphorus, B-P and sulphur bonds, B-S, are indicated. As there are many existing borane clusters of different sizes, heteroboranes and metallacarboranes, the revision of methods to generate the B-C, B-P, B-S, and B-N bonds has been restricted to the most widely used boron clusters; [B12H12](2-), dianionic and an example of a borane, 1,2-C2B10H12, neutral and an example of a heteroborane, and [Co(C2B9H11)2](-), monoanionic and an example of a metallacarborane.

Morphological and mechanical characterization of composite calcite/SWCNT–COOH single crystals

A growing number of classes of organic (macro)molecular materials have been trapped into inorganic crystalline hosts, such as calcite single crystals, without significantly disrupting their crystalline lattices. Inclusion of an organic phase plays a key role in enhancing the mechanical properties of the crystals, which are believed to share structural features with biogenic minerals. Here we report the synthesis and mechanical characterization of composite calcite/SWCNT-COOH single crystals. Once entrapped into the crystals SWCNT-COOH appeared both as aggregates of entangled bundles and nanoropes. Their observation was possible only after crystal etching, fracture or FIB (focused ion beam) cross-sectioning. SWCNT-COOHs occupied a small volume fraction and were randomly distributed into the host crystal. They did not strongly affect the crystal morphology. However, although the Young's modulus of composite calcite/SWCNT-COOH single crystals was similar to that of pure calcite their hardness increased by about 20%. Thus, SWCNT-COOHs provide an obstacle against the dislocation-mediated propagation of plastic deformation in the crystalline slip systems, in analogy with the well-known hardness increase in fiber-reinforced composites.

A new MCM-41 supported HPF6 catalyst for the library synthesis of highly substituted 1,4-dihydropyridines and oxidation to pyridines: report of one-dimensional packing towards LMSOMs and studies on their photophysical properties

A new heterogeneous MCM-41 silica supported HPF6 catalyst has been synthesized and characterized using an array of sophisticated analytical techniques like BET, XRD, HRTEM, EDX, 29Si MAS NMR, TGA, FTIR, and pH measurement. The broad applicability of silica-HPF6 was probed through a library synthesis of highly substituted 1,4-dihydropyridines (1,4-DHPs). The 1,4-DHPs are further oxidized to pyridines with aqueous H2O2 as stoichiometric oxidant which is considered as an abundant and green oxidant. The pyridines spontaneously generate low molecular mass self-aggregated organic materials (LMSOMs). Their one-dimensional packing and interesting photophysical properties are reported. Synthesis using water as a solvent is of paramount importance for its sustainable green impact. Further, carbon dioxide and water were the only by-products, which added to its attractiveness. This silica-HPF6 catalyst retained its activity even after being exposed to ambient atmosphere for 10 days.

Effect of crystal density on sublimation properties of molecular organic semiconductors

Thermal evaporation is an essential process employed in the fabrication of optoelectronic devices based on small molecular organic materials. Knowing the evaporation properties (e.g. sublimation enthalpy, vapor pressure) of archetypal compounds and being able to predict these properties of new compounds is therefore important for the design of processes and deposition apparatus. To address this lack of reliable, easily reproducible information we used thermogravimetry to characterize the sublimation properties of pentacene; tris(8-hydroxyquinolino) aluminum (Alq3); 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA); metal-free phthalocyanine (H2Pc); boron subphthalocyanine chloride (SubPc); iron phthalocyanine (FePc); copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc). A linear relationship was found between enthalpy and vapor pressure, and crystal density, allowing for the estimation of thermophysical properties of untested compounds of a similar class using a simple model.

Modulation polarimetry of the topological effect in gold-organic nanocomposite films

The phenomenon of surface plasmon resonance in composite films consisting of gold nanoclusters in matrices of organic molecular materials calix[4]arene and poly(N-vinylcarbazole) has been investigated. The internal reflection coefficients Rs2 and Rp2 of s- and p-polarized light and their physical difference ρ = Rs2 − Rp2 have been measured according to the Kretschmann scheme as a function of the angle of light incidence θ at different wavelengths λ in the range 400–1000 nm. The angular characteristics reflect the cluster structure of the films, which is confirmed by electron microscopy. A topological size effect has been revealed. This effect is associated with the dependence of the excitation energy efficiency of surface plasmons on the azimuth of the linearly polarized light, the shape, and the distribution of nanoclusters in the coordinate space. The dependences ρ(λ) demonstrate that the local plasmon resonance is excited by both s- and p-polarized light, whereas the polariton resonance is excited by s-polarized light. The sign of the curvature of the dependence ρ(θ) determines the predominance of the excitation energy efficiency of electromagnetic modes with one of the two states of polarization of the excitation radiation.

ChemInform Abstract: Regioselective Synthesis of Tetraalkynylarenes by Consecutive Dual Sonogashira Coupling Reactions of the Bis(triflate) of 4,5‐Diiodobenzene‐1,2‐diol.

The regioselective synthesis of nonsymmetric tetraalkynylarenes has been readily achieved through consecutive sets of Sonagashira cross-coupling reactions of the bis(triflate) derivative of 4,5-diiodobenzene-1,2-diol. The initial coupling reactions proceeded with nearly complete selectivity for the reaction at the Ar–I linkages. Subsequent coupling reactions at the Ar–OTf linkages were efficiently conducted. The tetraalkynylarene products are of interest as components of organic molecular materials.

Synthesis, Structure, Properties, and Application of a Carbazole‐Based Diaza[7]helicene in a Deep‐Blue‐Emitting OLED

A carbazole-based diaza[7]helicene, 2,12-dihexyl-2,12-diaza[7]helicene (1), was synthesized by a photochemical synthesis and its use as a deep-blue dopant emitter in an organic light-emitting diode (OLED) was examined. Compound 1 exhibited good solubility and excellent thermal stability with a high decomposition temperature (T(d)=372.1 °C) and a high glass-transition temperature (T(g), up to 203.0 °C). Single-crystal structural analysis of the crystalline clathrate (1)(2)⋅cyclohexane along with a theoretical investigation revealed a non-planar-fused structure of compound 1, which prevented the close-packing of molecules in the solid state and kept the molecule in a good amorphous state, which allowed the optimization of the properties of the OLED. A device with a structure of ITO/NPB (50 nm)/CBP:5 % 1 (30 nm)/BCP (20 nm)/Mg:Ag (100 nm)/Ag (50 nm) showed saturated blue light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.10); the maximum luminance efficiency and brightness were 0.22 cd A(-1) (0.09 Lm W(-1)) and 2365 cd m(-2), respectively. This new class of helicenes, based on carbazole frameworks, not only opens new possibilities for utilizing helicene derivatives in deep-blue-emitting OLEDs but may also have potential applications in many other fields, such as molecular recognition and organic nonlinear optical materials.

Regioselective Synthesis of Tetraalkynylarenes by Consecutive Dual Sonogashira Coupling Reactions of the Bis(triflate) of 4,5‐Diiodobenzene‐1,2‐diol

AbstractThe regioselective synthesis of nonsymmetric tetraalkynylarenes has been readily achieved through consecutive sets of Sonagashira cross‐coupling reactions of the bis(triflate) derivative of 4,5‐diiodobenzene‐1,2‐diol. The initial coupling reactions proceeded with nearly complete selectivity for the reaction at the Ar–I linkages. Subsequent coupling reactions at the Ar–OTf linkages were efficiently conducted. The tetraalkynylarene products are of interest as components of organic molecular materials.

Porous organic molecular materials

Most nanoporous materials with molecular-scale pores are composed of directional covalent or coordination bonds, such as porous metal–organic frameworks and organic network polymers. By contrast, nanoporous materials comprised of discrete organic molecules, between which there are only weak non-covalent interactions, are seldom encountered. Indeed, the majority of organic molecules pack efficiently in the solid state to minimize the void volume, leading to nonporous materials. In recent years, a large number of nanoporous organic molecular materials (crystalline or amorphous) were discovered and their porosity was confirmed by gas adsorption. All of these materials were compiled in this highlight. In addition, advantages of porous organic molecular materials over porous networks are discussed.

Room temperature ferromagnetism in Teflon due to carbon dangling bonds

The ferromagnetism in many carbon nanostructures is attributed to carbon dangling bonds or vacancies. This provides opportunities to develop new functional materials, such as molecular and polymeric ferromagnets and organic spintronic materials, without magnetic elements (for example, 3d and 4f metals). Here we report the observation of room temperature ferromagnetism in Teflon tape (polytetrafluoroethylene) subjected to simple mechanical stretching, cutting or heating. First-principles calculations indicate that the room temperature ferromagnetism originates from carbon dangling bonds and strong ferromagnetic coupling between them. Room temperature ferromagnetism has also been successfully realized in another polymer, polyethylene, through cutting and stretching. Our findings suggest that ferromagnetism due to networks of carbon dangling bonds can arise in polymers and carbon-based molecular materials.

Zinc titanate nanopowder: an advanced nanotechnology based recyclable heterogeneous catalyst for the one-pot selective synthesis of self-aggregated low-molecular mass acceptor–donor–acceptor–acceptor systems and acceptor–donor–acceptor triads

Highly stable, environmentally benign ZnTiO3 nanopowder has been prepared via a sustainable sol–gel method. The nanopowder (90 nm) has been thoroughly characterized by SEM, XRD, EDS, Laser Raman, photoluminescence, UV and IR. The activity of the catalyst was probed through one-pot four-component reaction of aldehydes, ketones and two equivalent propanedinitriles in water without requiring any additives or anhydrous conditions. The reaction requires two different catalytic functions, i.e., an acidic one which is given by Ti(IV) ions and a basic one, given by the oxide ion incorporated within the ZnTiO3 metal oxide framework. The advantages of this method lie in its simplicity, cost effectiveness, environmental friendliness, and easier scaling up for large scale synthesis without using high pressure, temperature and toxic chemicals. As water was used as a reaction medium and since we are particularly interested in the isolation of a non-aromatic intermediate, the elimination of poisonous HCN was prevented by the Lewis acid character of Ti4+ up to a sufficiently high temperature. Thus, this process can be considered as a “green” process. Elimination of HCN at higher temperatures still maintains “green” attributes as HCN can be trapped by the basic catalyst under such conditions. Spontaneous generation of low molecular mass self-aggregated organic materials, their one-dimensional packing, and interesting photophysical properties are reported.

Application of hopping theory for the prediction of charge mobility in amorphous organic materials

ABSTRACTThe application of hopping theory for the prediction of charge (hole) mobility in amorphous organic molecular materials is studied in detail. Application is made to amorphous cells of N,N’-diphenyl-N,N’-bis-(3-methylphenylene)-1,1’-diphenyl-4,4’-diamine (TPD), N4,N4’-di(biphenyl-3-yl)-N4,N4’-diphenylbiphenyl-4,4’-diamine (mBPD), 1,1-bis-(4,4’-diethylaminophenyl)-4,4-diphenyl-1,3,butadinene (DEPB), N1,N4-di(naphthalen-1-yl)-N1,N4-diphenylbenzene-1,4-diamine (NNP), and N,N’-bis[9,9-dimethyl-2-fluorenyl]-N,N’-diphenyl-9,9-dimethylfluorene-2,7-diamine (pFFA). Detailed analysis of the computation of each of the parameters in the equations for hopping rate is presented, including studies of their convergence with respect to various numerical approximations. Based on these convergence studies, the most robust practical methodology is applied to investigate the dependence of mobility on such parameters as the monomer reorganization energy, the monomer-monomer coupling and the material density. The results give insight into what factors should be controlled to develop materials with higher (or lower) charge (hole) mobility, and what will be required to improve the accuracy of predictions of mobility in amorphous organic materials.

Solid-State Thermochromism and Phase Transitions of Charge Transfer 1,3-Diamino-4,6-dinitrobenzene Dyes

The lower 1,3-bis(hydroxyalkylamino) homologues of the strong intramolecular X-type charge transfer (CT) system 1,3-diamino-4,6-dinitrobenzene (DADNB) exhibit reversible color change in the solid state from yellow at room temperature (RT) to orange and red at high temperature (HT). To investigate the structural prerequisites for occurrence of this phenomenon, we prepared 10 new derivatives of DADNB where the hydroxyalkyl arms at the amino groups were replaced with substituents having different electronic and steric profiles. Two of the new materials exhibit sharp and reversible thermochromic change in the solid state: when heated, the bis(aminoethyl) derivative (DADNB-1) undergoes color change from orange-red to brown, while one of the three polymorphs of the bisphenyl product (DADNB-2) changes its color from red to yellow. The physicochemical analysis and the crystal structures of seven of these compounds, one of which is trimorphic, confirmed that both phenomena are due to solid-solid phase transitions. The brown high-temperature phase of DADNB-1 presents the first example where the absorption is shifted beyond the red region. Form C of DADNB-2 is the first material of this group that exhibits "negative" thermochromism, where the high-temperature phase absorbs at lower wavelength than the low-temperature one. The results demonstrate the potentials of these simple and easily accessible organic molecular materials for thermal switching of the optical properties by utility of intermolecular interactions to modulate the intramolecular CT.