Our research covers a broad spectrum of topics such as electrochemistry, charge-transfer, electroactive polymers, and novell ligand/metal-complex synthesis. Below you will find a few of the projects we are working on now. Just click the topic to learn more about what is going on in that project. Solid-State, Optically-Addressable Magnetic Sensor Materials Photophysics and Photochemistry of Cu(I)N4 Systems Conducting Polymer Films with Tunable Work Functions Cobalt Complex-Based Electron-Transfer Mediators in Dye-Sensitized Solar Cells

In studying the fundamental properties of a number of functionalized metal-complex dyes, our group has found a few molecules that may prove useful as optical sensor probes. Particularly the "triad" or "D-C-A" complexes (which contain electron donor and acceptor ligands) show promise as weak magnetic field probes. Figure 1 shows the transient absorption lifetimes of the charge-separated state in a typical triad complex as a function of applied magnetic field.

This data demonstrates this molecule’s ability to detect very weak magnetic fields up to 500 mTesla. However, trying to take advantage of this sensing characteristic is a challenge. First, this molecule is in solution when this measurement is made, so direct application to a sensing material is not possible. Second, this material is very sensitive and degrades rapidly in the presence of oxygen when illuminated. In fact, in taking the above measurements, the solutions are handled under darkroom conditions until they are rigorously degassed. These two factors represent a large challenge to realizing optically addressable magnetic sensing materials from D-C-A complexes.

Based on the large body of research on microemulsion, polymer encapsulation, micelles, and reverse micelles there remains a possibility of meeting the above challenges in order to obtain a material that combines the photophysics of a dye solution with the advantages of a solid-state material. Ideally a dye solution would be trapped in an oxygen impermeable solid material that can be addressed optically (no light scattering) and does not interfere with the sensing capabilities. Since our sensing dye is unstable under ambient conditions, a surrogate dye is used to first develop an appropriate material. The surrogate dye generally used is a tris(2,2'-bipyridine)ruthenium(II) (RUBPY) salt or a derivative thereof.

Our first approach was to try a solid polymer microemulsion of dye solution. This material was made by mixing various surfactants into neat styrene, followed by addition and vigorous stirring of tris(2,2'-bipyridine)ruthenium(II)chloride (RUBPYCL) in formamide. Styrene was chosen initially because a hydrophobic enough monomer was needed so that the partitioning of a polar solvent into the monomer would be minimized. In all cases the emulsion was stable only so long as the solution was stirred. The stability of the emulsion could be improved by raising the viscosity of the styrene with the addition of polystyrene. Polymerization of the remaining styrene was accomplished using 2 wt. % azo-bis(isobutyronitrile) (AIBN) by heating to 70 deg. C. While the resulting materials obtained from the above procedures showed an orange luminescence under UV light (evidence of the dye in a liquid solution), the polymer monoliths were quite opaque. This is due to light scattering by the rather large (mm to m m) phase separated domains of dye solution trapped in the polystyrene. To eliminate this, either the dye solution and polymer's refractive indices must be matched, or the size of the domains must be reduced below the scattering limit (into the nanometer regime). Because of material limitations, the second alternative is the only feasible approach.

Micelles and reverse micelles are formed by many surfactants under the appropriate conditions, and often have diameters on the order of a few nanometers, but are primarily studied in liquid/liquid system. However, Zhu et al. have made solid/liquid reverse micellar materials using styrene and divinylbenzene as the non-polar phase, sodium bis(2-ethylhexyl)sulfosuccinate (AOT) as the surfactant, and water as the polar phase. AOT as a surfactant is well documented for forming reverse micelles and it dominates the reverse micelle literature. AOT is an anionic surfactant that has a small polar head group with a large and branching greasy tail group. This makes for a three dimensional "wedge" shape, which favors the spherical micellar arrangement. Figure 2 shows both a commonly used cartoon and an accurate molecular model of AOT.

a) b)

We are investigating using this same approach to obtain optical quality polymer monoliths, which contain reverse micelles of dye in polar-aprotic solvents. Figure 3 shows a computer-generated depiction of the structure of such a material.

We are currently working on characterizing and optimizing these materials. Since these materials must exclude oxygen to be useful with the D-C-A complexes, synthesis is now being carried out in the inert environment of a glovebox. Oxygen permeation can then be determined (from the excited-state fluorescence lifetime of the entrapped dye) as a function of time under ambient conditions. If these materials allow oxygen permeation, then other monomers will be studied (switching to micelles in polar monomers is under consideration).

Since little work has been done using these types of materials, there is a great deal of opportunity for fundamental studies. Having reverse micelles trapped in solid-state materials allows for a wide variety of surface analysis techniques to be used, and we are currently working to obtain scanning probe microscopy (AFM) images of these structures. To our knowledge, this would represent the first time individual reverse micelles have been imaged directly (not a freeze fracture replica).

A series of tris(bipyridine)ruthenium(II) based monomers have been synthesized and examined. One such complex is shown below. This compound and its relatives are soluble in acetonitrile and can be drop coated or spin coated on a variety of substrates to yield glassy thin films. Thermal polymerization produces amorphous polymer films with interesting and potentially useful optical and electrochemical properties. It is possible to tune many properties of the complexes, and thereby resulting polymers, by chemically controlling the nature of the bipyridine based ligands.

With special electrochemical treatments, the polymer films can be rendered conductive. Briefly, the film is electrochemically reduced in the presence of an electrolyte containing a polymeric cation. As the film (composed originally of positively charged complexes) is reduced, anions are ejected in order to maintain charge neutrality. Past the point of zero charge, the film must uptake cations. Since this is impossible with the very large cation, the polymer can be "trapped" in the state of zero charge, with no counterions incorporated.

At this point, while the film macroscopically has no charge, microscopically a fairly large and equal number of oppositely charged complexes are present. These charged species are populated by thermal energy, as predicted directly from the Nernst equation. A strong analogy can be drawn here to a very narrow bandgap intrinsic semiconductor. While the orbitals in this case are surely much more localized than in an inorganic semiconductor, the very large number of charge carriers provides for good conduction. The result is that the specially reduced films have resistivities on the order of 10³ Ohm cm, which do indeed increase with lower temperatures, as would be expected. Finally, the films are stable in this state provided they do not contact oxidizing agents.

One property of particular interest in these conductive films is the work function. It is possible to predict work functions of the reduced polymers through electrochemical measurements. Work functions obtained via ultraviolet photoelectron spectroscopy agree quite well with the predicted values. Furthermore, work functions can be tailored by chemically modifying the structure of the monomer. To date, a series of reduced films have been produced with the work function varying from 3.6 eV to 2.9 eV. It should be possible to expand this window through future synthetic control over ligand systems.

The work functions of these films are relatively low; in reference to metals, the work function for calcium is 2.9 eV, while iron is 4.3 eV. For this reason, the films are being examined for use as cathode materials for organic light emitting devices (OLED’s). Very briefly, these devices operated much like traditional inorganic semiconductor LED’s, where electrons are injected from a cathode into the conduction band (CB) and holes from the anode into the valence band. In OLED’s, the nature of these electrode/organic emitter interfaces is of much interest and great importance to device performance. Currently, low work function metals such as calcium, aluminum, and magnesium are used as cathode materials in literature devices. However, replacing this metal/organic interface with a low work function polymer/organic interface may provide for improved device operation and/or lifetime. Such systems are currently under study, and a graphic depicting an actual preliminary OLED which has been constructed using aluminum quinolate as the emitter is shown below.

The common chromophore trisbypyridine ruthenium(II), while retaining a robust ³MLCT excited state, is costly to incorporate into economically-viable energy conversion materials as a result of the high cost and low natural abundance of the metal ruthenium. Cu(I)N₄ systems represent an alternative with a well-characterized brown-orange color attributed to a MLCT.

Scheme 1 delineates the incorporation of an acetal moiety in a bipyridine ligand system at the 3,3` carbons of the bipyridine ring. The 3,3`-bipyridine carboxaldehyde is obtained from a Korn-Blum oxidation on the commercially-available 3,3`-dimethylbipyridine. Acetals, stable in neutral and basic solution, present an opportunity to increase steric strain in system by enclosing the Cu(I) metal in a cage, decreasing efficiency of nonradiation pathway relaxation, increasing excited state lifetime to within the range of microsecond to milliseconds, consequently increasing luminescence quantum yield. Hydrolysis of acetal moeities on bipyridine ligand after ligand coordination to Cu(I) forms the rigid cage structure about Cu(I) (Scheme 1). Hexanentirile is required such that high reflux reaction temperatures may be achieved such that the 4-cyclohexanol acetal fragement can acquire sufficient thermal energy to wrap around and displace 1,4-transdimethanolcyclohexane, Equilibrium is pushed toward products using 4A mol sieves to remove water byproduct. To this end, mol sieves serve to preserve the acetal over the aldehyde by averting acid hydrolysis achieved by pyridine para-toluene sulfonic acid (PPTS). An argon atmosphere is required such that Cu(I) is not oxidized to Cu(II), which retains an undesired LMCT. Purification of the metal complex was challenging as complex contact with silica or water would cause its oxidation to the Cu(II).

Scheme 1. bis(3,3`-dimethanolcycloxaneddiacetalbipyridine) Cu(I) ClO₄- synthetic scheme

Such complexes retain brown-orange color under Ar atmosphere at ambient conditions for equal to or greater than three weeks. Electrochemical data (CV), UV-VIS data, and luminescent data given below were collected on the trial bipyridine complex.

Figure 1. UV-Vis spectra studies in hexanenitrile: Td to D₂d symmetry confirmed: absence of shoulder at longer wavelengths Lambda_max = 314 nm (brown-orange color)

Figure 2 below shows cyclic volammetric studies of the above complex. At oxidizing potentials the Cu(I)-Cu(II) redox process appears as a quasi- to irreversible wave. This electrochemistry is indicative of a strained copper complex. At reducing potentials there are redox processes attributed to Cu(1)-Cu(0) and ligand reduction. Most notable are the anodic stripping peaks indicating that metallic copper is plating on the electrode surface (and indicating the destruction of the complex).

Figure 2. Cyclic Voltammetry studies on glassy carbon, in 0.1M TBAH in hexanenitrile, and versus Ag-Ag⁺:

Currently, efforts focus on repeating the synthesis using the popular, more rigid 1,10-phenanthroline ligand. Literature substituent positions are common to the 2 and 9-position, but rarer at the 3 and 8-position, the intended substituent positions in the synthesis (Scheme 2).

Scheme 2. bis(2,9-dimethanolcyclohexanediacetalphenanthroline) Cu(I) ClO₄- synthetic scheme

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