Gregory D. Phelan

From molecules to opto-chips: organic electro-optic materials

Recent advances in polymeric electro-optic materials and device fabrication techniques have significantly increased the potential for incorporation of these materials and devices into modern high bandwidth (fiber and wireless) telecommunication, information processing, and radar systems. Charge transfer π-electron chromophores characterized by molecular first hyperpolarizability (second order optical non-linearity) values approaching 3000×10 n –30 n esu have been synthesized. Elucidation of the role of intermolecular electrostatic interactions in inhibiting the efficient translation of molecular optical non-linearity to macroscopic electro-optic activity has permitted systematic modification of materials to achieve electro-optic coefficients approaching 100 pm V n –1 n. Improvements in the optical loss of polymeric materials at wavelengths of 1.3 and 1.55 µm have been effected. Mode matching of passive transmission and active electro-optic waveguides has been addressed, permitting a dramatic reduction in insertion loss. The putative ability of polymeric electro-optic materials to be efficiently integrated with very large scale integration semiconductor electronic circuitry and with passive optical circuitry has been demonstrated. Several devices of varying degrees of complexity have been fabricated and evaluated to operational frequencies as high as 150 GHz. The operational stability of polymeric devices is very competitive with devices fabricated from lithium niobate and gallium arsenide.

Europium beta-diketonate temperature sensors: Effects of ligands, matrix, and concentration

Europium beta diketonates are easily synthesized highly luminescent complexes with high temperature sensitivity. We report on the temperature dependence of the luminescence of recently synthesized europium complexes originally prepared for use as light emitting diodes. It has been discovered that when incorporated in a polymer matrix, their decay lifetime can provide accurate measurement of temperature. Their lifetime as a function of temperature depends on three factors: (i) the type and number of ligands in the complex, (ii) the particular polymer used for the matrix, and (iii) the europium chelate to polymer matrix concentration ratio. Various tris and tetrakis europium chelates are used to study ligand effects, while the polymers FIB, polycarbonate, and Teflon© are used to analyze matrix effects. In all cases studied, higher concentrations give rise to shorter lifetimes and higher temperature sensitivities, with sensitivity defined as ΔI/(IrefΔT). We propose to explain this phenomenon by using the fol...

Barometric sensitive coatings based upon osmium complexes dissolved in a fluoroacrylic polymer.

Pressure sensitive paints (PSP) that measure the changes in air pressure have proved to be useful in the design of aircraft and other vehicles. In this study we incorporate highly luminescent divalent osmium complexes into PSP. The divalent osmium complexes were heptafluorobutyrate salts of [Os(N-N)2(L-L)]2+ or [Os(L-L)2(N-N)]2+, where N-N is a derivative of 1,10-phenanthroline, and L-L is a diphosphine or diarsine ligand. The complexes were dissolved into poly(1,1,1,3,3,3-hexafluoroisopropylmethacrylate-co-1H,1H-dihydroperflurobutylmethacrylate) (FIB) at a concentration of 0.002 g of complex to 1.000 g of polymer. The luminescence of the coatings was tested for pressure sensitivity, temperature dependence, and photodegradation. The paints featured strong pressure response, and the temperature dependence of the luminescence was measured as low as -0.11% degrees C(-1). Several of the complexes exhibited little photodegradation upon prolonged exposure to 400 nm light. These attributes make the complexes very desirable luminescent dyes for PSP.

Organic light-emitting diodes containing fluorinated asymmetrical europium cored beta-diketone complexes

Novel luminescent materials based on europium-cored complexes have been synthesized and incorporated into light emitting diodes using poly (N-vinyl-carbazole) and poly (vinyl naphthalene) blends as doping hosts. The complexes consists of fluorinated β-diketone ligands chelated to europium. Excitation of the ligands and efficient transfer of energy from the excited ligands to the metal core results in the emission of optically pure red light. The ligands were designed such that they include a polycyclic aromatic compound, phenanthrene, and a second substituent to improve processibility. Phenanthrene is used to so that the ligand energy will match with the energy of the metal center. Partially fluorinated substituents were also used to help improve the efficiency and charge transfer capability of the resulting metal complex. The complex consisted of one equivalent of europium and three equivalents of the ligand. One equivalent of either 1,10-phenanthroline or 4,7-diphenyl-1,10-phenanthroline was also chelated to enhance the stability of the complex. Double and triple layer devices were synthesized with the configuration of ITO/BTPD-PFCB/Europium complex in a polymer blend/Ca/Ag for the double layer device and ITO/BTPD-PFCB/Europium complex in a polymer blend/PBD/Ca/Ag for the triple layer device. The double layer devices made with a polymer blend of PVN outperformed the devices made from PVK as the emission bands of the PVN better match the absorption bands of the ligands. A maximum brightness of 178 cd/m2 with a maximum external quantum efficiency of 0.45% was measured for the double layer device.

Tuned emission from organic light-emitting devices based upon divalent osmium complexes

Electrophosphorescence tuned from the green to red (522 nm - 650 nm) was achieved from double-layer light emittng devices using osmium (Os) complexes doped blend of either poly(vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD), or poly(vinyl naphthalene) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVN:PBD) as the emitting layer. Blending PVN with PBD greatly suppresses the electromer emission of PVN. The PVN:PBD blend emanates a short wavelength EL emission peaking at around 375 nm, which well overlaps with the absorption spectra of the Os complexes and ensures very efficient energy transfer to the Os complex dopants. PVK:PBD has an EL emission around 450 nm which does not overlap the absorption bands of the osmium complexes and also produces devices of lower efficiency, but PVK is a better transport layer and therefore produces brighter devices. The best external quantum efficiency, of the double-layer devices was 2.2%, with a photometric efficiency of 1.9 cd/A. The brightest device achieved was 1,600 cd/m2.

Organic light-emitting diodes incorporating nanometer thick films of europium-cored complexes

Europium cored complexes may be used as a source of red emission in light emitting diodes. Novel europium cored complexes have been synthesized and incorporated into organic light emitting diodes (OLEDs). These complexes emit red light at 615 nm with a full width half maximum (FWHM) of less than 5 nm. The europium complexes consist of one equivalent of europium chelated to three equivalents of a nonsymmetrical β-diketone ligand. The Claissen condensation of a polycyclic aromatic sensitizer and an ester of a fluorinated carboxylic acid create the ligands. The use of a sensitizer such as phenanthrene results in a ligand that has an emission band that directly overlaps with the absorption band of europium. The use of fluorinated chains improves the overall processibility as well as the charge transfer capability of the resulting metal cored complex. The europium core is further encapsulated by the inclusion of an additional polycyclic aromatic compound such as 4, 7 diphenyl - 1, 10 phenanthroline. Emission of 615 nm light is accomplished through excitation of the ligand and efficient Forrester energy transfer to the europium complex. A multiple layer device consisting of a substrate of indium tin oxide, followed by thin layers of BTPD-PFCB (with a thickness of 20nm), a polymer blend containing the europium complex (30 nm), followed by a layer of calcium (50nm) and finally a protective layer of silver (120 nm). The polymer blends were either poly(n-vinyl carbazole)(PVK) or poly vinyl naphthalene (PVN). The device performance was further improved by the incorporation of another lanthanide metal complex. These complexes were based upon similar ligands surrounding gadolinium. In these devices, there is a Dexter energy transfer as well as the Forster energy transfer. For the devices that are based on a PVN:PBD as a polymer host, the lowest turn on voltage was 12.0 volts. The devices that use PVK:TPD devices was 178 cd/m2 with an external quantum efficiency of 0.61%.For PVK:TKD the brightness was 116 cd/m2 with an external quantum efficiency of 0.048%. Devices that incorporate the gadolinium complexes have the turn on voltage of 5.6 volts. We report a maximum brightness of 201 cd/m2 with an external quantum efficiency of 1.0%.

Organic light emitting devices based upon divalent osmium complexes. Part 1: design, synthesis, and characterization of osmium complexes

In this work we present the synthesis and characterization of several novel osmium complexes of the form [Os(N-N)2L-L]2+ 2X- designed for organic light emitting device (OLED) applications. In the complex notation N-N represents a derivative of 2,2-bipyridine or 1,10-phenanthroline and L-L represents a strong π-acid arsine or phosphine ligand. The complexes feature 3MLCT emission that ranges from 611 - 650 nm, which makes them suitable as an emission source for red OLEDs. Phosphorescent quantum yields as high as 45% and emission lifetimes as short as 400 nanoseconds have been reached.

Synthesis and design of organic light-emitting devices containing lanthanide-cored complexes

There is a considerable interest in the use of metal centered materials as a light source in the growing field of organic light emitting devices (OLEDs). In these devices, a polymeric host matrix containing either a carbazole type polymer or polyfluorene derivatives is used to help facilitate energy transfer to the luminophore. We have shown that by using a gadolinium complex that consist of three equivalents of a chelated dibenzoylmethane b-diketone ligand and one equivalent of a phenanthroline type ligand as a component in the host matrix, the performance of a double layer type OLED is improved. We have studied OLED systems that contain tris chelated europium compounds that contain three equivalents of partially fluorinated β-diketone type ligands and an equivalent of a phenanthroline type ligand. In these devices, the external efficiency has shown a 30-fold increase. We have also shown there is an increase for Osmium based OLEDs that use the gadolinium complex as part of the polymer matrix. In these devices, the maximum quantum efficiency increased from 2.1% to a value of 3.8%.