The Omary Research Group

Research Description:

1. Photophysical and Photochemical Properties of d⁸ and d¹⁰ Complexes

 The following types of complexes are being studied: Class 1. Thiocyanato complexes of d8 Pt(II); Pd(II); Ir(I). Class 2. Isonitrile complexes of d10 Au(I); Ag(I). Class 3. Adducts of Classes 1 and 2 with Tl+ and Pb2+. Class 4. Adducts of anionic complexes in Class 1 with lanthanide ions. Class 5. Cun clusters (n=4-8) with thiolate and amine ligands. Photoluminescence studies of these types of complexes are being conducted in order to assess their potential use in electroluminescence display devices (see below).The luminescence in several of these systems is attributed to the formation of metal-metal bonded excited-state complexes (exciplexes).Such species are known to be intermediates in photochemical reactions involving polynuclear organic compounds such as pyrene.However, no such studies are known for coordination compounds to relate their photochemical reactivity to the formation of metal-metal bonded exciplexes.Therefore, the compounds among the above whose luminescence spectra suggest exciplex formation will be subject to photochemical studies.The photoproducts will likely include colloidal particles of metals such as platinum, gold, and silver.Such colloids have been receiving increasing attention recently in the field of nanotechnology due to their large number of applications in materials science and biochemistry.

2. New Classes of Phosphorescent LEDs

 Electroluminescence (EL) is the phenomenon used in video display of electronic equipment. The conventional electronic display devices are cathode ray tubes (CRTs), which have excellent quality. However, CRTs are not practical in mobile devices such as laptops, in which liquid crystal displays (LCDs) are used because of their lighter weight and higher shock resistance. A competing technology is the use of "organic" light emitting diodes (OLEDs) for emissive flat panel display (FPD). This is a promising new technology that may potentially replace the LCD technology because of its lower cost and its independence of viewing angle. One or more emissive organic or metal-organic films are typically used in OLEDs. Recent advances have lead to the development of luminescent materials for OLEDs with emission energies that cover the entire visible spectrum. We envision a dramatic improvement in the performance of molecular LEDs by using luminescent materials based on coordination compounds of heavy transition metals with d8 and d10 configurations, instead of organic LEDs. Typical OLED's use a fluorescent organic compound or polymer. These materials only emit fluorescent light with negligible phosphorescence. From simple spin statistics, the maximum theoretical quantum efficiency (QE) for organic fluorescent materials is 25%, compared to 100% QE for the coordination compounds proposed that are capable of emitting both fluorescence and phosphorescence (due to spin-orbit effects in heavy metals). In reality, even the 25% limit remains elusive as the best devices available today typically have less than 10% QE. Meanwhile, only very recently have scientists in this field started to seriously investigate phosphorescent heavy metal complexes with a few examples starting to appear since 1998 reporting such devices possessing significant QE (greater than 5% at 100cd/m2). Organic luminophores are luminescent as isolated molecules; thus their photoluminescence efficiencies are higher in solution than as solids. In contrast, molecular aggregation increases the quantum efficiency in d8 and d10 compounds if the emission is due to intermolecular metal-metal bonding. Classes 1 and 2 (above) form chain structures with weak metal-metal interactions, but photoexcitation leads to luminescence from metal-metal bonded excimers and exciplexes. Adducts of d8 and d10 complexes also form strongly luminescent exciplexes with Tl+ and Pb2+.We also propose to make multicolor devices displaying sensitized emissions due to lanthanide ions with the sensitizers being the anionic complexes proposed above (Class 1). Finally, Cu(I) forms strongly luminescent clusters (and is also relatively inexpensive), but these clusters have not yet been studied for LED applications.

3. New Solar Cells Using Dye Complexes Tethered on Wide Band Gap Semiconductors

 Increased utilization of solar energy is a logical and timely strategy for increasing our energy capacity beyond the current technology. There is a striking little attention to research that capitalizes on solar radiation. Although efficient solar energy conversion (10-20%) has been achieved by solid-state silicon cells, alternatives are sought to overcome their undesirable features of high manufacturing costs due to the requirement of superior crystallinity for Si, and their tendency to corrode. We aim to develop electrochemical solar cells of wide band gap semiconductors that will potentially outperform, in terms of both yield and economy, the silicon cell technology. Semiconductors such as TiO2 have band gaps that do not overlap with the desired solar spectral range in the visible- near infrared (vis-NIR) region with a maximum at ~1200nm. To overcome this problem, we propose to attach a dye molecule to the surface of the semiconductor, which will absorb strongly across the vis-NIR region. Dyed semiconductors can be used as colloids in the suggested devices, which renders them more economical than silicon cells. Although d6 Ru(II) complexes have been used in such devices, their absorption cut-offs are less than 800nm. We envision a significant improvement in performance when dyes of d8 and d10 complexes are used. Such complexes exhibit strong electronic vis-NIR absorptions. Our focus is on Pt(II) and Pd(II) dyes with pyridyl and thiolate ligands that exhibit intense charge-transfer (CT) that approach and even surpass 800 nm absorptions. The target complexes contain substituents (e.g., carboxylates; phosphonates) that can be used to "anchor" the dye molecules to the semiconductor surface. "Fine tuning" of the CT energies will be accomplished by placing electron-donor and electron-acceptor groups on the pyridyl and thiolate ligands, respectively. "Coarse tuning" will be achieved by synthesizing extended-chain supramolecules from synthetic strategies we are currently pursuing. The reactions of d8 and d10 complexes with specific electron acceptors (e.g., TCNQ) and cations (e.g., Tl+; Pb2+) lead to molecules with red-shifted absorption bands with strong absorbances in the NIR region where the solar radiation is strongest. In summary, we believe that this approach to preparing dye-modified semiconductors that makes use of solar energy will lead to large improvements in the energy storage capacity of silicon technology as we enter an era of increased demand for efficient, safe and clean energy sources.

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