Organic Molecular Materials Research Articles

Electronic structures of organic molecular materials for organic electroluminescent devices studied by ultraviolet photoemission spectroscopy

Electronic structures of evaporated films of five organic light-emitting and carrier-injecting materials for organic electroluminescent devices were studied by ultraviolet photoemission spectroscopy. The compounds examined were (i) light-emitting materials tris(8-hydroxyquinolino) aluminum (Alq3) and 1,2,3,4,5-pentaphenylcyclopentadiene, (ii) a hole-injecting material N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine, and (iii) electron-injecting materials N,N′-diphenyl-1,4,5,8-naphthyletracarboxyldiimide and 1,3,5-tris(5-phenyl-1,3,4-oxadiazol-2-yl)benzene. The spectral features corresponding to the top parts of the valence states, which dominate the electric properties of the materials, were assigned by the comparison with the simulated density of states obtained from PM3 molecular orbital calculations. Using these calculations, the evolution of the electronic structure of each molecule from those of constituent parts was discussed. The characters of the unoccupied states obtained by these calculations were also presented. Using these data, the correlation between the ionization threshold energies determined by ultraviolet photoemission spectroscopy and the carrier-injecting and light-emitting properties were discussed.

Crystal Potential Formula for the Calculation of Crystal Lattice Sums

A new formula is derived for the determination of the potential energy of the central unit cell of a finite crystal; this formula is called the crystal potential formula. The crystal potential formula is based on a two-center Cartesian multipole expansion. The key feature of the crystal potential formula is that it achieves a separation of the lattice geometry and multipole moments. This feature allows one to compute in a highly effective way the crystal energies as a function of the molecules contained in the unit cell. Numerical implementation is discussed, and a numerical example is given. The example mimics an organic crystal composed of dipolar molecules. It is found that the crystal environment may create local minima and that the effects of the environment might be well-approximated by consideration of the proximate unit cells only. Both of these effects have important implications for the crystal engineering of molecular organic materials.

Role of vibrational softening in fast dynamics of an amorphous polyimide far below Tg

We report quasielastic neutron scattering results on an amorphous polyimide (AURUM) in a temperature range from 10 to 300 K below the glass transition temperature Tg(=250 °C). As temperature increases from 10 K, anharmonic excess scattering intensity appears at around 200 K or about 300 K below Tg. This excess scattering is very similar to the so-called fast process in picosecond order observed for glass-forming polymers as well as organic and inorganic low molecular weight glass-forming materials. We first analyzed the spectra in a simple way assuming validity of the Bose scaling for the vibrational density of states and a Lorentzian for the additional relaxational process, and found that the excess scattering intensity appears in the frequency range below the Boson peak and the characteristic time is independent of temperature which are common features for the fast processes reported previously. The second analysis using the recent vibration-relaxation (VR) model, which is more realistic, indicated that softening of the vibrational modes may play an important role for the change of spectra from inelastic-like to quasielastic-like. Nevertheless, relaxational process is still necessary to describe the excess scattering although the temperature range examined is very far below Tg.

Two-time clock NMR observation of the ferroelectric transition in the organic compound cyclohexane-1,1′-diacetic acid

The ferroelectric transition in the molecular organic ferroelectric material cyclohexane-${1,1}^{\ensuremath{'}}$-diacetic acid has been studied with a ${}^{1}H$ NMR two-time clock experiment. On the spin-lattice relaxation time scale ${(10}^{8}\mathrm{Hz})$ the molecular motions are observed as slow in both the paraelectric and the ferroelectric phase so that the instantaneous value of the crystal symmetry is observed in both phases and no symmetry breaking effectively takes place at ${T}_{c}.$ On the kHz time scale of the NMR line shape and the dipolar relaxation time experiments, the phase transition is observed and confirms the model of a dynamically disordered structure in the paraphase.

Recent developments in molecular organic electroluminescent materials

AbstractA review of recent developments in the design and use of small molecular organic electroluminescent materials for display applications is presented. The material issues pertaining to transport properties, color, emission efficiencies, and operational stability are described.

Computationally Assisted Structure Determination for Molecular Materials from X-ray Powder Diffraction Data

For organic molecular materials, the key step in solving crystal structures from high-resolution powder diffraction data is the generation of reliable trial structures for final refinement. In this paper we demonstrate an efficient new methodology for generating trial structures that predicts the correct molecular packing in a crystal lattice given the unit cell dimensions and space group. The method uses a systematic search of intermolecular, atom−atom interactions to examine and rank all possible packing arrangements. The approach is demonstrated for a number of systems with increasing degrees of search complexity including indigo, 6,13-dichlorotriphendioxazine, phenanthrene, paracetamol, and benzophenone. The final example is of particular importance as it illustrates the first application of this type of methodology to a conformationally flexible system.

High-spin organic molecular materials

High-spin organic molecular materials are recent subjects of active studies. In diradicals and radical pairs, the ground states are usually singlet due to the overlap of the orbitals containing the unpaired electrons. When the singly occupied orbitals are orthogonal to each other, triplet states become ground states. Thanks to the promotion of our understanding of how to stabilize the triplet relative to the singlet, a number of examples of purely organic assemblies of free radicals and oligo/polyradicals have been accumulated, in which more than two electron spins are aligned in parallel. Approaches toward high T C and tunable organic molecule-based magnets are in steady progress.

Muon studies of organic ferromagnets and conductors

Muon spin rotation (μSR) experiments can be especially helpful in the study of various organic molecular materials. The technique has been used to study the magnetization in nitronyl nitroxide organic ferromagnets, the spin-density-wave ground state in various charge transfer salts, the dynamics of carriers in organic conductors and the vortex structures in organic superconductors. I outline the motivation for studying these new materials and review and discuss the results of recent work.

The birefringence of the optically nonlinear crystal 4-amino benzophenone

A precise method is presented for measuring the refractive index of an anisotropic prism with light polarized orthogonal to the prism axis which is also a dielectric axis. Prisms have been cut from crystals of the optically nonlinear 4-amino benzophenone with exceptionally low defect concentration as identified by synchrotron white light topography. The linear refractive indices of these have been measured and fitted to four-parameter Sellmeier equations to an accuracy of 7×10−6–3×10−3. The values are accurately verified by observation of the external angle of incidence to a (001) plane required to meet the conditions of type-I phase matching with a fundamental wavelength of 1064 nm. The phase matching is of Hobden class six for this wavelength. In common with similar organic molecular materials, the molecular charge transfer axis defines the most polarizable direction in the crystal, with the C=O direction being much less significant.

Predicting the crystal structure of organic molecular materials

This paper describes a novel method for predicting the crystal structure of organic molecular materials which employs a series of successive approximations to focus on structures of high probability, without resorting to a brute force search and energy minimization of all possible structures. The problem of multiple local minima is overcome by assuming that the crystal structure is closely packed, thereby eliminating 217 of the 230 possible space groups. Configurations within the 13 remaining space groups are searched by rotating the reference molecule about Cartesian axes in rotational increments of 15°. Initial energy minimization is performed using (6–12) Lennard–Jones pair potentials to produce a set of closely packed structures. The structures are then refined with the introduction of a Coulombic potential calculated using molecular multipole moments. This method has successfully located local minima which correspond to the observed crystal structures of several saturated and unsaturated hydro-C atoms with no a priori information provided. For large polycyclic aromatic hydrocarbons, additional refinements of the energy calculations are required to distinguish the experimental structure from a small number of closely packed structures. Our methodology for a priori crystal structure prediction represents the most efficient algorithm presented to date, in a field where the first successes have only been described within the past year and have been few and far between. Since our algorithm is capable of locating a large number of reasonable structures with similar energy in a short period of time, and is more likely to locate a minimum corresponding to the experimental structure, our program provides a superior framework to determine the level of theory required to calculate the intermolecular potential. For all but highly asymmetric hydrocarbons, however, distinguishing the observed structure from a large number of highly probable structures requires more rigorously calculated intermolecular interactions than pair potentials, plus an ad hoc electrostatic potential, and is thus beyond the scope of this paper. All calculations were performed on the Ohio Supercomputer Center's Cray Y-MP.

Electronic properties of metal-poly(pyrrole) junctions†

Abstract In recent years there has been a growing interest in the use of molecular organic materials in microelectronic devices, such as polymer diodes, transistors and gas sensors. Conducting polymers form an important class of organic materials that are being investigated as a substitute for conventional inorganic semiconductor materials. Electrodeposited conducting polymers, such as the pyrrole-based family, have some distinct advantages in their ease of microdeposition and in situ doping, 'tunable’ band-gap and high electronic mobility. In this paper we report on the fabrication and properties of novel poly(pyrrole)-tosylate/metal PPy(pTSA)/metal junctions. Thin (∼0-5 μ,m) PPy(pTSA) films were electrochemically deposited onto small gold electrodes on silicon microelectronic substrates. The PPy(pTSA)/Au junction always forms an ohmic junction. Low work function metal contacts (e.g. Cu and Ag) were either physically evaporated onto the poly(pyrrole) or electrochemically deposited to form the junction. Poor reproducibility was observed in both types of junction with about half the devices exhibiting weakly rectifying junctions (ideality factor of about eight). There was no discernible difference in the characteristics of the evaporated and electrodeposited junctions. The electronic characteristics of the junctions were not consistent with conventional Schottky theory; for example, PPy(pTSA)/Ag always showed an ohmic contact. Instead, the poor stability and general behaviour seems consistent with the formation of an MOS junction following reaction at the interface. Our observations suggest that the development of electrodeposited pyrrole-based microelectronic devices is inherently difficult.

Spiroconjugation: An added dimensi in the design of organic molecular materials

Molecules with predictable electrical, optical, and magnetic properties are required for molecular devices and new materials. One approach to overcoming the disadvantages associated with the quasi‐one‐dimensional π‐conjugated acceptors and donors that have played a big part so far is to use spiroconjugated molecules that are conjugated but not planar. The concepts behind acceptors such as that shown in the Figure are explained. magnified image

Nonlinear Optical Materials

The current state of materials development in nonlinear optics is summarized, and the promise of these materials is critically evaluated. Properties and important materials constants of current commercial materials and of new, promising, inorganic and organic molecular and polymeric materials with potential in second- and third-order nonlinear optical applications are presented.

Molecular Electronics

Molecular electronics in its broadest definition encompasses all actual and potential applications of organic molecular materials to electronics and opto-electronics in which the molecular material plays an active role. The macroscopic properties of organic materials are already utilised in xerography and liquid crystal displays. Advances in understanding of electrical and nonlinear optical properties offer prospects for new applications in the near future. Molecular scale electronics has been frequently discussed but in the absence of experimental tests remains speculative. Progress in physics, chemistry, engineering and biology is now providing the means to conduct experiments at the molecular scale. This has opened up a number of alternative routes which may lead to the realisation of molecular scale electronics.

Molecular Microanalysis of Tissues

Fourier transform infrared (FT-IR) microspectroscopy is the most direct and facile technology available for detecting, identifying and mapping molecular species. Biologists have a need to relate molecular chemistry with a tissue's microstructure. While electron microbeam analyses provide elemental maps and both SIMS and micro-Raman spectroscopy demonstrated limited molecular mapping capabilities, complex inorganic and organic materials are not readily amenable to these techniques. Infrared spectroscopic analyses of micro-domains have gained wide-spread acceptance since high sensitivity FT-IR spectrometers and research quality, reflecting optical microscopes were united.Morphologically distinct parts of plant tissue can vary in their chemical composition and these variations can effect the plant's value. Plant biologists and agronomists have been endeavoring to localize the chemical species to individual botanical parts. While microscopy has been useful in establishing plant microstucture, chemical information on cellular components has been difficult to obtain. Molecular microspectral mapping renders these chemical analyses more tractable.The microstructure of the outer portion of a cereal grain consists of four distinct regions, see Figure 1.

Physical limits of integration and information processing in molecular systems

A review of the evolution of electronic information processing from small-scale integration to ultra-large-scale integration and beyond is presented recalling the milestones achieved in the development of electronic hardware. Physical principles which limit the size and performances of electronic circuits and the rate of information processing are discussed. In order to explore the new courses offered by organic molecular materials towards atomic-scale integration, the elementary computational capabilities of molecules are discussed by casting the quantum dynamic equations in the framework of system theory. Bond dynamics in polymeric networks is considered, thus showing the possibility of obtaining molecular probabilistic circuits.

Materials for nonlinear optics‐orientationally ordered polymer films

AbstractEmerging guided‐wave integrated optical technologies require suitable active devices and optical interconnects. Certain organic molecular materials possess the required optical nonlinearities for application in active devices. Glassy polymers, such as poly(methyl methacrylate), have been used to fabricate high optical quality passive structures. For application in integrated optics, requirements include high optical quality (low scattering and absorption losses), low dielectric constant, and suitable fabrication techniques.We describe the preparation and optical properties of a new material, class, poled polymer glasses, which combines the attractive properties of the optical nonlinearities of organic molecules and the optical quality of polymer glasses. These materials are formed by incorporating organic compounds possessing large optical nonlinearities into a host material lacking long‐range order. The orientational order is imparted to the composite system by applying a strong electric field at a temperature above the glass transition temperature, where molecular motion is enhanced, thereby aligning the optically nonlinear dopant molecules. By cooling the material through the phase transition with the field still applied, the orientational order is frozen in, and the composite material possesses second‐order nonlinear optical properties.

Effects of Order on Nonlinear Optical Processes in Organic Molecular Materials

Abstract This paper establishes a general framework relating nonlinear optical processes to the molecular orientational order within the material. The orientational order is described as an ensemble average of orthogonal functions transforming from the molecular to the macroscopic frames using a distribution function appropriate for the particular material class. These molecular additivity concepts can be applied to many materials including crystals, liquid crystals, polymer liquid crystals, Langmuir-Blodgctt films, and polymer glasses. The literature is reviewed in the context of this description. Limitations, including inhomogeneity, defects, and quadrupole contributions are also discussed.

UV Photoelectron Spectroscopy of Organic Molecular Materials

An overview is given on the recent progress in UPS studies of organic molecular materials taking the examples mainly from the results obtained in the authors' laboratory. We discuss molecular solids with and without strong intermolecular interactions, molecular complexes and polymers.

Spiral-wound new thin film composite membrane for a single-stage seawater desalination by reverse osmosis

New thin film composite membrane system, designated PEC-1000, formed by the acid catalyzed polymerization on the surface of a reinforced-porous supporting membrane, make it possible to produce potable water from seawater by reverse osmosis in a single-stage with a high recovery operation. TDS rejection over 99.9% and stable water fluxes of 0.20–0.30 m 3/m 2-day (5.0–7.4 gal/ft 2-day) have been attained with 3.5% synthetic seawater at an applied pressure of 56Kg/cm 2(800psi). For brackish water, sodium chloride rejections of 99.6–99.9% and fluxes of 0.61–0.81m 3/m 2-day(15.0-20.0 gal/ft 2-day) have been attained with 5000 ppm sodium chloride feed at an applied pressure of 40Kg/cm 2 (571psi). TDS rejection of 99.8% and water flux of 0.30 m 3/m 2-day (7.4 gal/ft 2-day) have been attained with two- or four-inch diameter PEC-1000 composite membrane elements at an applied pressure of 56Kg/cm 2(800psi) in a single-stage synthetic seawater desalination test. This performance is kept for more than 1500 hours in PEC-1000 thin film composite membrane and two-inch diameter element. 280 ppm in TDS and water flux of 0.11 m 3/m 2 day (2.7 gal/ft 2-day) are observed at an applied pressure of 56Kg/cm 2-40% water recovery with one four-inch diameter spiral-wound PEC-1000 composite membrane element in a single-stage seawater desalination. This membrane shows high selectivity for low molecular weight valuable organic materials such as ϵ-caprolactam, dimethylsulfoxide, dimethylformamide. The thickness of ultrathin salt barrier of the composite membrane is found to be 300Å by the Electron Microscopy with special ultrathin section techniques.