Case study on a rare effect: the experimental and theoretical analysis of a manganese(III) spin-crossover system.

The six-coordinated mononuclear manganese(III) complex [Mn(5-Br-sal-N-1,5,8,12)]ClO(4) has been synthesized and isolated in crystalline form. Magnetic measurements and variable-temperature single-crystal X-ray crystallography corroborated with theoretical analysis provided firm evidence for the spin-crossover effects of this system. The monomeric complex cations are made by a hexadentate mixed-donor Schiff base ligand imposing a distorted octahedral geometry and subtle structural effects determining the manifestation of the variable spin properties of the manganese(III) centers. The spin crossover in [Mn(5-Br-sal-N-1,5,8,12)]ClO(4) has resulted in an unprecedented crystallographic observation of the coexistence of high-spin (HS; S = 2) and low-spin (LS; S = 1) manganese(III) complex cations in equal proportions around 100 K. At room temperature, the two crystallographically distinct manganese centers are both HS. Only one of the two slightly different units undergoes spin crossover in the temperature range ∼250-50 K, whereas the other remains in the HS state down to 50 K. The density functional theory calculations, performed as relevant numerical experiments designed to identify the role of orbital and interelectron effects, revealed unedited aspects of the manganese(III) spin-conversion mechanisms, developed in the conceptual frame of ligand-field models.

Carbazole endcapped heterofluorenes as host materials: theoretical study of their structural, electronic, and optical properties.

By mimicking the molecular structure of 4,4'-bis(N-carbazolyl)-2,2'-biphenyl (CBP), which is a widely used host material, a new series of host molecules (carbazole-endcapped heterofluorenes, CzHFs) were designed by linking the hole-transporting carbazole to the core heterofluorene molecules in either meta or para positions of the heterofluorene. The aromatic cores considered in this study are biphenyl, fluorene, silafluorenes, germafluorenes, carbazole, phosphafluorene, oxygafluorene, and sulfurafluorene. To reveal their molecular structures, optoelectronic properties and structure-property relationships of the proposed host materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of these designed molecules have been studied with emphasis on the highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), energy gaps (E(g)), triplet energy gaps ((3)E(g)), as well as some other electronic properties including ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ), triplet exciton generation fraction (χ(T)), spin density distributions (SD), and absorption spectra. These photoelectronic properties can be tuned by chemical modifications of the heteroatom and the carbazole substitution at different positions. This study provides theoretical insights into the nature of host molecules, and shows that the designed CzHFs can meet the requirements of the host materials for triplet emitters.

Theoretical studies of the structural, electronic, and optical properties of phosphafluorenes.

Phosphafluorenes have drawn increasing attention recently in the applications of organic electronic devices due to their particular optoelectronic properties. To reveal their molecular structures, optoelectronic properties, and structure-property relationships of the newly emerged functional materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The optimized geometric and electronic structures in both ground and exited states, the mobility of the hole and electron, the absorption and emission spectra, and the singlet exciton generation fraction of these novel phosphors-containing materials have been studied by density functional theory (DFT), single excitation configuration interaction (CIS), time-dependent density functional theory (TDDFT) methods, and the polarizable continuum model (PCM). The results show that the highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), triplet energies ((3)E(g)), energy gaps (E(g)), as well as some other electronic properties including ionization potentials (IPs), electron affinities (EAs), reorganization energies (lambda), the singlet exciton generation fraction, radiative lifetime, and absorption and emission spectra can be easily tuned by chemical modifications of the phosphorus atom via methyl, phenyl, oxygen, sulfur, or selenium substitution, indicating that the phosphafluorenes are interesting optoelectronic functional materials, which have great potential in the applications of OLEDs, organic solar cells, organic storage, and sensors.

Effects of Temperature and Solvent on the Energy Transfer and ß-Phase Formation in the Iridium(III) Complex-Containing Polyfluorene in Solutions and as Suspended Nano-Particles.

The effects of temperature and solvent on the ß-phase formation and energy transfer in an Ir(III) complex-containing polyfluorene were investigated. Efficient energy transfer from polyfluorenes host to Ir complexes guest can be realized at low temperature. The formation of ß-phase was observed both in THF solution at low temperature and as suspended nano-particles at room temperature. In addition, phosphorescent polymer nanoparticles were prepared successfully and exhibited efficient phosphorescent emission.

Tristable electrical conductivity switching in a polyfluorene-diphenylpyridine copolymer with pendant carbazole groups.

Tristable electrical conductivity switching and non-volatile memory effects are demonstrated in a conjugated copolymer of poly(2,6-diphenyl-4-((9-ethyl)-9H-carbazole)-pyridinyl-alt-2,7-(9,9-didodecyl)-9H-fluorenyl) (PPCzPF). The indium-tin oxide (ITO)/PPCzPF/Al device can be switched from the low-conductivity (off) state to the first high-conductivity (on-1) state at 1.8 V, with an on/off current ratio of approximately 100. The device can be further switched from the on-1 state to the next higher conductivity (on-2) state at 2.4 V, with an on-2/on-1 current ratio of approximately 20. All the three conductivity states are accessible, stable and non-erasable. The tri-level conductance switching can be explained in terms of field-induced conformational ordering of the polymer chains and enhanced charge-transfer interaction at the PPCzPF/ITO interface.

Electrical Conductance Tuning and Bistable Switching in Poly(N-vinylcarbazole)-Carbon Nanotube Composite Films.

By varying the carbon nanotube (CNT) content in poly(N-vinylcarbazole) (PVK) composite thin films, the electrical conductance behavior of an indium-tin oxide/PVK-CNT/aluminum (ITO/PVK-CNT/Al) sandwich structure can be tuned in a controlled manner. Distinctly different electrical conductance behaviors, such as (i) insulator behavior, (ii) bistable electrical conductance switching effects (write-once read-many-times (WORM) memory effect and rewritable memory effect), and (iii) conductor behavior, are discernible from the current density-voltage characteristics of the composite films. The turn-on voltage of the two bistable conductance switching devices decreases and the ON/OFF state current ratio of the WORM device increases with the increase in CNT content of the composite film. Both the WORM and rewritable devices are stable under a constant voltage stress or a continuous pulse voltage stress, with an ON/OFF state current ratio in excess of 10(3). The conductance switching effects of the composite films have been attributed to electron trapping in the CNTs of the electron-donating/hole-transporting PVK matrix.

Conductivity switching and electronic memory effect in polymers with pendant azobenzene chromophores.

Electronic memory devices having the indium-tin oxide/polymer/Al sandwich structure were fabricated from polymers containing pendant azobenzene chromophores in donor-acceptor structures. The reversibility, or rewritability, of the high-conductivity (ON) state was found to be dependent on the terminal moiety of the azobenzene chromophore. While the polymers with electron-accepting terminal moieties (-Br or -NO2) in the pendant azobenzene exhibit write-once, read-many-times (WORM) type memory behavior, those with electron-donating terminal moieties (-OCH3) exhibit rewritable (FLASH) memory behavior. The WORM memory devices have low switching ("write") voltages below -2 V and high ON/OFF current ratios of about 10(4)-10(6). The polarity of the "write" voltage can be reversed by using an electrode with a higher work function than Al, thus excluding metallic filamentary conduction as a cause of the bistable switching phenomenon. The FLASH memory devices have low "write" and "erase" voltages of about -1.7 to -1.8 V and 2.0 to 2.2 V, respectively, and ON/OFF current ratios of about 10(3)-10(4). The electrical bistability observed can be attributed to charge trapping at the azobenzene chromophores, resulting in the charge-separated, high-conductivity state. The proposed mechanism is supported experimentally by a red shift and peak broadening in the UV-visible absorption spectra of the polymer films resulting from the OFF-to-ON electrical transition.

Nonvolatile polymer memory device based on bistable electrical switching in a thin film of poly(N-vinylcarbazole) with covalently bonded C60.

A functional polymer (PVK-C60), containing carbazole moieties (electron donors) and fullerene moieties (electron-acceptors) in a molar ratio of about 100:1, was synthesized via covalent tethering of C60 to poly(N-vinylcarbazole) (PVK). The molecular structure and composition of PVK-C60 were characterized by FTIR, Raman, and UV-vis absorption spectroscopy, gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CyV). The C60-modified PVK exhibited an enhanced glass-transition temperature (Tg = 226 degrees C) and good solubility in organic solvents such as toluene, tetrahydrofuran, chloroform, and N,N-dimethylformamide (DMF). It could be cast into transparent films from solutions. For a thin film of PVK-C60 sandwiched between an indium tin oxide (ITO) electrode and an Al electrode (ITO/PVK-C60/Al), the device behaved as nonvolatile flash (rewritable) memory with accessible electronic states that could be written, read, and erased. The polymer memory exhibited an ON/OFF current ratio of more than 105 and write/erase voltages around -2.8 V/+3.0 V. Both the ON and OFF states were stable under a constant voltage stress of -1 V for 12 h and survived up to 108 read cycles at -1 V under ambient conditions.

Bistable electrical switching and memory effects in a thin film of copolymer containing electron donor-acceptor moieties and europium complexes.

A nonconjugated methacrylate copolymer (PCzOxEu) containing carbazole moieties (electron donors), 1,3,4-oxadiazole moieties (electron acceptors), and europium complexes in the pendant groups was synthesized via free radical copolymerization of methacrylate monomers containing the respective functional groups. The molecular structure and composition of PCzOxEu was characterized by elemental analysis, FT-IR, 1H NMR, 13C NMR, UV-vis absorption and fluorescence spectroscopies, gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CyV). The resulting copolymer exhibited a relatively high glass transition temperature (Tg approximately 125 degrees C) and good solubility in common organic solvents. It could be cast into transparent films from solutions. For a thin film of PCzOxEu sandwiched between an indium-tin oxide (ITO) electrode and an Al electrode (ITO/PCzOxEu/Al), the structure behaved as a nonvolatile flash (rewritable) memory with accessible electronic states that could be written, read, and erased. The polymer memory exhibited an ON/OFF current ratio up to 10(5), switching response time of approximately 1.5 micros, more than 10(6) read cycles, retention time of more than 8 h, and write/erase voltages of about 4.4 V/-2.8 V under ambient conditions. The roles of oxadiazole moieties in improving the response time and retention time of the memory device were elucidated from the molecular simulation results.

[Study on spectral properties of tri-functional polymers containing terbium complex].

In the present paper, a kind of tri-functional polymer containing terbium complex was synthesized. The spectral properties were studied by means of FTIR, UV-Vis, and TOF-SIMS. The results indicated that the polymer contained the oxadiazole group, carbazole group and terbium complex. The fluorescence property was investigated in different states. In solution, the spectra consisted of the emission band from the pi-->pi* transition of the oxadiazole and carbazole group and the emission peaks from the transitions between the 4f states of terbium. In solid state, the emission from the oxadiazole and carbazole group was suddenly suppressed, and the result also indicated that the efficiency of energy transfer in the solid state is much higher than that in solution. The polymer emits pure green light under the UV light in solid state.

Synthesis and dynamic random access memory behavior of a functional polyimide.

The device under testing was a plastic dynamic random access memory based on a donor-functionalized polyimide (TP6F-PI), which exhibited the ability to write, read, erase, and refresh the electrical states. The device had an ON/OFF current ratio up to 105, promising minimal misreading error. Both the on and off states were stable under a constant voltage stress of 1 V and survived up to 108 read cycles at 1 V.