Latest News in Organic and Molecular Electronics for Summer-Autumn 2006
10.06.06 Organic Magnets Minireview We used to think of magnetic materials as something heavy, metallic, solid. And it is difficult to notice in our everyday life that new types of magnetic materials emerged in scientific and industrial labs and even in some commercial products. These materials may be flexible and light as plastics, even transparent as glass. They may change their magnetic properties in unusual, even weird way. They are organic molecular magnets. Here, we summarize some of the very recent scientific advances in the development of these materials, but before, please visit our dictionary page to recall some necessary knowledge about molecular magnetism. Molecular magnetism and mass media Recently, the History channel offered a great presentation of magnetism and magnetic materials in their historical retrospective and modern developments. One of the most important points of the presentation was interview-introduction of organic magnetism with Arthur J. Epstein form the Ohio State University, who is one of the pioneers of the field. Interview was accompanied by demonstration of samples of organic magnetic materials and their properties. The other event was a recent publication "Putting a spin on electronics" about magnetic materials for semiconductor spintronics by M. Jacoby in Chemical & Engineering News, 2006, august 28, page 30. As we also mentioned before, spintronics is a new branch of electronic science and technology that employs both electric current and magnetism of electron spins to perform logic functions. Spintronics use two main relatively independent approaches to achieve these goals: magnetic approach and semiconductor approach. The 'magnetic' approach manipulates with microscopic magnetic domains to achieve logic response. Two types of it: Magnetic domain wall, and Quantum-Dots Cellular Automata we have recently discussed. The 'semiconductor approach' employs new type of materials - magnetic semiconductors to achieve logic functions. This approach is often dismissed due to great difficulty of the synthesis of magnetic materials that are semiconductors at the same time. The C&EN's article, however, provides facts of recent progress in this field that demonstrate it to be quite promising. Although the article covers only inorganic magnetic semiconductors, it states that the principal works and it is real. If it is possible to create inorganic magnetic semiconductors, it will be possible the same with the organic ones. The biggest problem of existing inorganic and organic magnetic semiconductors is low Curie temperature which is usually only cryogenic. It is clear, that electronic materials can be much more useful if they operate at room temperature. The C&EN's article reviews few methods, recently developed, that may increase Tc of inorganic molecular magnets. What about the organic ones? Yes, few are already known, for example TCNE salts, such as V(TCNE)2. We expect more coming very soon.
Why we need organic magnets and what is the difference between organic and conventional metallic magnets? Except spintronics, organic magnets may be used in a variety of applications, including those well known and long time used. They may possess certain advantages over inorganic magnets, and here we provide comparative analysis of organic and inorganic magnetic materials: Advantages:
Drawbacks. 1. High temperature applications: organic materials possess low thermal stability as well as Curie temperatures are also usually low and intrinsically may not be higher than thermal decomposition temperatures. 2. Strength and durability. Although some organic materials recently emerged that possess strength close to that of metals (such as carbon fiber plastics), it is still unlikely for organic magnets to reach that level of strength in the nearest future. 3. Cost. Although it is just a matter of time for any of organic compound to become cheap and available, right now the cost of known organic molecular magnets vary from high to very high.
Recent scientific reviews on organic magnetic materials.
Several recent reviews on organic magnets show significant progress in the field that has been achieved during the last few years. Thus, the review of J. S. Miller (mentioned above) summarizes progress in his and other groups in the developments of TCNE based magnets. One of the objects of recent research of the Miller's group are ruthenium dinuclear complexes that possess unusual "paddle-wheel" structure and spin number of S = 3/2. Charge compensated with chromium counterpart, these complexes with basic structure of [Ru2(O2CMe)4]3[Cr(CN)6] exhibit high coercivity and anomalous hysteresis behavior. A recent entire issue of J. Mater. Chem., 2006, number 16 is dedicated to molecular magnetism and related materials. The issue's editorial introduction: "Trends and challenges in molecular-based magnetic materials" (page 2513) by E. Coronado and D. Gatteschi briefly summarizes modern situation, main directions and challenges that exist in the field. Useful for beginners to get orientation in the field. Although the issue contains mainly reviews on inorganic types of molecular magnets, few of them are dedicated to the organic ones. The following topics on organic magnetism were reviewed: Spin crossover phenomena. In the introduction part, the authors summarize major structural types of spin-crossover (SCO) magnetic compounds and stress importance of the phenomena in terms of possibility of fine control of spin states by temperature, pressure, or light irradiation (LIESST effect). The control of spin states, in turn, allows for the fine-tuning of magnetic behavior and other physical properties such as optical (color). Further, the authors focus on iron(II) mononuclear and dinuclear complexes. Why iron(II)? Iron(II) complexes are convenient objects for the study of SCO because iron(II) atoms possess zero spin in a low spin state (LS) and therefore fully diamagnetic (see scheme below). Whereas high spin state (HS) of iron(II) nuclei S = 2. Using thorough selection of ligands by field strength it is possible to manipulate with spin-crossover properties. Thus a mononuclear complex [Fe(bpym)(py)2(NCS)2] possesses three different ligands with different field strength: chelating bpym (structure see below) very high field strength, py - intermediate, and NCS- - low field strength. Combination of these ligands affords [LS]↔[HS] transition at 115K. Dinuclear complexes may exhibit two [LS-LS]↔[LS-HS] or even three transition states: [LS-LS]↔[LS-HS]↔[HS-HS], that strongly expands possibility of a logic magnetic response of a molecule. Thus, the first type of response is characteristic of complex: {[Fe(NCBH3)(4-phpy)]2(m-bpypz)2}, where ligands 4-phpy and bpypz are shown on the scheme below.
Organic radicals. In the introduction part, the authors stress importance of both magnetic and electroconducting properties of thiazyl radicals and provide brief theoretical background for electrical conductivity of thiazyl radicals and organic compounds in general. All isolecetronic members of fairly large family of thiazyl radicals were depicted (as many as nine members). However, only two, the most studied members: dithiazolyl (RCSNSCR) and dithiadiazolyl (RCNSSN) (1 and 2 on the scheme below) were reviewed in detail. Very interesting work written in a simple and clear fashion. Among recent original research, few other new interesting types of organic electroconducting radicals have been reported. Thus, a JACS 2006, 128, 1418 communication paper by J. Huang and M. Kertesz from Georgetown University, USA, stresses high room-temperature 1D-conductivity (0.3 S cm-1) and semiconductor behavior of boron-containing spiro-radical 3. The paper is entitled: "One-dimensional metallic conducting pathway of cyclohexyl-substituted spiro-biphenalenyl neutral radical molecular crystal". Specific electroconductance along with temperature independent Pauli paramagnetism (characteristic of metals) make this compound a promising candidate for organic semiconductor spintronic applications (see this review above). The other original research by E. Fukuzaki and H. Nishide from Waseda University, Japan, published in JACS 2006, 128, 996, describes synthesis of a high-spin polyradical of a nanometer size. The work is entitled: "Room-temperature high-spin organic single molecule: nanometer sized hyperbranched poly[1,2,(4)-phenylenevinyleneanisyl aminium]". The goal of the research was to 'stuff' a single polymer molecule with as many radicals as possible and as dense as possible to achieve high spin state. High temperature stability and possible ferromagnetic coupling of the radicals were also desirable. Ultimately, ferromagnetic coupling and ordering at room temperature. The authors partially succeeded in achieving of this goal through the synthesis of triarylaminium polyradical 4, where n = 11-14, spin density ~0.65 spin/unit, and spin quantum number S = 7/2, even at 70 oC. A single molecule of the compound was ~15 nm of size and was observable as a globule by atomic and magnetic force microscopes. The authors mention also that this high-spin state stable organic molecule at the temperatures over ambient is reported for the first time. The other interesting paper reports the synthesis and properties of paramagnetic radicals which also possess chirality and liquid crystalline properties at the same time. The paper: "Ferroelectric properties of paramagnetic, all-organic, chiral nitroxyl-radical liquid crystals" by N. Ikuma, R. Tamura, S. Shimono and coworkers from Japan is published in Advanced Materials 2006, 18, 477. Such a combination of the features in one molecule (5 and analogs on the scheme below) was expected to afford paramagnetic ferroelectric liquid crystals (FLCs), where orientation would be controlled by weak magnetic fields. Indeed, the authors succeeded to obtain thermally stable chiral and paramagnetic smectic phases (SmC*) with 5, where side alkyl chains were C-11 or longer. The authors stress such a success to be reported for the first time.
Chiral magnets and single molecule magnets (SMMs): The authors tested such chiral hydroxyacid ligands as (R)-mandelic acid (6 on the scheme below), methoxymandelic acid, and (R)-malic acid (7), and found that bimetallic complexes of Mn(II)-Co(II), and Mn(II)-Ni(II) with 7 possess long-range ferromagnetic ordering characteristic of chiral magnets. Polynuclear complexes of 3d metals have been known as single-molecule magnets (SMMs) for quite a while. SMMs are magnetic materials, where each molecule in its isolated state represents a microscopic magnet, with all attributes of a magnet such as two poles. This may be possible when 'giant' spin of S = 10 or bigger is achievable in a single molecule. The unique feature of SMMs compared to macroscopic magnets is in so-called quantum tunneling of magnetization (QTM), which is also known as 'resonance tunneling'. Dramatically simplified, this effect may be explained as extremely facilitated changing of SMM's poles (magnetization reversal) under the action of crossing magnetic fields of a particular strength (Breson).
In order to achieve a huge spin number, many magnetic 3d-nuclei have to be introduced in a single molecule. Thus, dodecanuclear complex of Mn(III), and Mn(IV) ions: [Mn12O12(OAc)16(H2O)4] was one of the first molecular magnets discovered. Recently however, it was found that a huge spin number may be achieved with fewer nuclei if heavy multielectron 4f-elements are introduced in a complex. Such complexes sometimes exhibit SMM's behavior with as few as just a single 4f-atom. Thus a recent communication paper in J. Am. Chem. Soc. 2006, 128, 1440 by F. Mori, T. Ishida, H. Nojiri and coworkers from Japan describes use of a 4f-element dysprosium(III) in a trinuclear Dy-Cu-Dy complex as a single molecule magnet. To stuck the nuclei together, the authors use a special bridging ligand di-2-pyridyl ketoximate (dpk-, 8 on the scheme below). Along with coordinating hfac ligands (9), the entire complex is [{Dy(hfac)3}2{Cu(dpk)2}]. The SMM behavior of the complex was established by measurements of magnetization using a pulsed magnetic field at subcryogenic temperatures.
In conclusion, we wish to introduce the other review that contains large quantity of scientific detail from the one side, and fabulous systematization of the material accumulated in the entire field from the other side. The review: "Calorimetric investigation of phase transitions occurring in molecule-based magnets" by M. Sorai, M. Nakano, and Y. Miyazaki from Osaka University, Japan is published in a recent issue of Chemical Reviews 2006, 106, 976. The work summarizes use of calorimetric methods in the investigation of phase transition points and dimensionality of molecular magnets. The authors classify existing methods of measuring of heat-capacity and especially emphasize some newest approaches: relaxation calorimetry and differential scanning calorimetry (DSC). Relaxation calorimetry uses short pulses of energy to heat a sample followed by measurement of the "resulting temperature rise and fall" and corresponding computing of the results. It is suitable for small quantity samples from 10 to 100 mg. Suitability of calorimetric methods for measurements of small quantities is highly important in study of molecular magnets because research samples are often costly. The DSC method allows for use of even smaller quantities of few milligrams. It measures sample's behavior under very precise, programmed temperature change followed by computing of the sample properties. It requires special precise apparatus and computing which became commercially available recently. In addition to detailed description of use of the calorimetric methods, the review is highly valuable in terms of classification of 1. molecular magnets by structure and 2. molecular magnets by properties. We also found this review written in a logic and simple fashion, that allows for easy understanding of the material by chemists and physicists who are not narrowly specialized in the field.
Year 2006 C&EN's Analysis of Flat Display Market Trends
Exactly one year has passed since previous C&EN's analysis of the situations and trends in the development of flat displays and television market. What is the difference between previous and current analyses, how the trends have changed after one year? The current report is divided in two main articles: "Riding on flat panels" by J.-F. Tremblay, 2006, June 26, page 13, and "Second wind for OLEDs" by A. H. Tullo, page 20. In contrast to the previous year, the competition between traditional cathode-ray sets and liquid crystalline displays (LCDs) is no longer in the center of the articles focus. It seems that in spite of their structural complexity, LCDs gain sufficient price-quality ratio in order to compete successfully with both cathode rays and their main 'flat rivals' - plasma displays as well. This is probably due to well-established technology, reliability, and other attractive features of LCDs. TFT-LCD technology is mentioned to become a prevailing LCD technology to be used by major LCD-producers. This year report analyses OLED technology in a separate article. Although the entire report is quite optimistic, some 'cooling water' is also added. According to interviews with executives of some major developers of OLED technology such as Cambridge Display Technology (CDT) and DuPont, significant challenges in OLED production have to be overcame in order to improve OLED market potential. Problems with the backplanes and lifetime of colors of early OLED products (which mostly were small auxiliary displays in various electronic devices) are also mentioned. Market forecasts are more moderate now and consider longer development times, though still very optimistic in general. Every new technology meets some small and big obstacles on the early stages of the development - the author stresses.
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