Current Research Projects


Molecular Machines for Solar Energy Conversion
Porphyrin Electrochemistry
Double Bonds in Group 14 Compounds

Molecular Machines for Solar Energy Conversion.

I like to think of my research as simply an extension of one of my favorite childhood activities-playing with Tinker ToysTM (the precursor of LegosTM). These were those wonderful sticks and discs with holes in them that could be used to construct real or imagined objects-bridges, airplanes, buildings, etc.
What does this have to do with chemistry? One of the most sought-after goals of chemistry during the past half century has been the construction of a chemical system that has the ability to absorb light from the sun and convert it to a useful (and storable) form. One way in which the sun's energy can be stored is in the form of chemical bonds. Plants do this rather efficiently during photosynthesis. In terms of a successful artificial system, there are certain requirements that must be met:
* the light must be captured efficiently (leads to an "excited state")
* the absorption of the light must result in migration of charge
* the flow of charge must be directional
* the excited state must last long enough for chemistry to occur
From a chemical point of view, this means that an electron is transferred when the system absorbs light energy (this is the energy capture part). The electron is used to form a new bond or change the oxidation number of some substance (this is the energy storage part). Light energy is changed to chemical energy.
Constructing a chemical entity possessing the characteristics listed above is very challenging. No one molecule possesses all of the characteristics. The approach that I and my collaborators (Professor Richard Eisenberg at the University of Rochester and Professor William Connick at the University of Cincinnati) are taking involves a "division of labor." The idea is to string together a number of molecules each suited well to perform a specific task. For example, molecule one is good at absorbing light (the "chromophore"). It has special properties that result in a large change in the distribution of electrons when it absorbs the light. Molecule two is a good connecting molecule. It is able to join two of the working units together and transfer charge to molecule three. Molecule three has the ability to catalyze a reaction that enables the energy to be stored in the form of a bond. (This is a pretty simple description of a rather complicated process). A schematic representation is shown below.
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Of course we have not achieved this goal-no one has yet. But we have made progress toward it. We have a number of different molecules that can act as the chromophore (light absorber). One such type of complex is described in a paper with a Geneseo undergraduate co-author (Donald Lipa '98). This molecule has very good charge-transfer characteristics. As the figure below shows, electron density (charge) shifts from the left side of the molecule to the right side when a photon of light is absorbed to form an excited state. The red and blue areas represent regions of high electron density.
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Receptors of the electrons (where the catalysis takes place) include a number of different possibilities, such as nickel complexes or colloidal platinum or gold. Our current work is focused on the synthesis of molecules capable of linking the units together (so-called linker ligands).

Porphyrin Electrochemistry.

Name the most significant biological process carried out by plants. Do the same for animals. If you thought about it for a few moments, you'd probably answer photosynthesis and respiration, respectively. Both are of paramount importance if life is to be sustained. Without photosynthesis, there could be no life as we know it (with the possible exception of the recently discovered hydrogen sulfide metabolizing marine organisms that thrive only in the vicinity of submerged vulcanic vents).
These two processes are dependent on the presence of coordinated metal ions (in particular, of iron, magnesium, manganese, and copper) which are involved in electron transfer and oxygen transport and storage. Elsewhere, coordinated metal ions catalyze reactions resulting in the solubilization of toxins (so they can be expelled) and the disproportionation of hydrogen peroxide. In some enzymes, metal ions serve as "structural supports" and hold the protein or substrate in some specific geometry.
In the systems which carry out the processes of photosynthesis or respiration in higher animals, at least one metal ion is coordinated to a macrocycle consisting of four covalently linked pyrrole units which form a planar, tetradentate unit. Note that the metal ion has its axial coordination sites open and so may take on one or two additional monodentate ligands. This fact is of great importance for a number of reasons. Consider hemoglobin, which contains four heme units--iron(II)porphyrins--and is found in red blood cells where it transports oxygen from the lungs, and myoglobin, found in muscle tissue where it serves as an oxygen storage depot. Each heme uses an axial coordination site to bind oxygen (see the figure below).
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Another aspect of axial ligation concerns the variation of redox potentials found in the cytochromes (which are employed in a number of electron transfer processes). By changing various features of the axial ligands, nature modulates the redox potential and electron transfer is facilitated. Cyclic voltammetry is an excellent method for exploring the effect of variations in the axial ligands or changes in the porphyrin periphery on the reduction potential of the iron center.
Our efforts are focused on two areas:
1. Determination of binding constants for various axial ligands.
2. Examination of how subtle changes in the porphyrin structure affect the iron redox potential.
For our studies, we use model compounds--compounds that are similar in structure to the heme group but without the rest of the protein present. Our iron(III) porphyins are synthesized in a two step procedure that is represented below for the tetraphenylporphyrin complex.
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Changing the aldehyde used allows us to modify the electronic structure of the porphyrin. For example, using the p-fluorobenzaldehyde would result in a less basic porphyrin and using p-tolualdehyde would give a more basic porphyrin.

The Double Bond in Heavier Group 14 Elements.

Molecular orbital calculations are becoming increasingly important for the understanding of bonding and structure relationships in all areas of chemistry. Until recently, calculations involving transition metal complexes were uncommon because of the CPU time and memory requirements associated systems with a large number of electrons, the inclusion of d orbitals and problems associated with partially filled orbitals. However, with commercially available software and fast, cheap computing power available today, compounds for which theoretical studies were prohibitively expensive are now accessible at the undergraduate level. We are exploring the electronic structure of ethene, disilene (H2Si=SiH2), and digermene (H2Ge=GeH2) using the density functional method available in Spartan'02.
Although the concept of multiple bonds is well-established for compounds with second period elements, multiple bonding between heavier elements has only recently been established. The first isolable species in which a multiple bond was postulated between two heavier main group elements was [Sn{CH(SiMe3)2}2]2 reported in 1973 and the structure was confirmed in 1976. Since then, numerous examples have been isolated and structurally characterized.
Contrary to expectations based on a familiarity with ethene, the heavier analogues of alkenes are often not planar (1) and exhibit so-called trans-bent geometry (2). The degree of deformation is measured in terms of the pyramidalization angle, q. Potentially, a comparison of q values for a series of molecules can give insight into the differences in chemical reactivity exhibited by these compounds.

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Although ethene is relatively stable, the heavier analogues are not. One of the goals of our studies is to find an explanation of the differences in chemical reactivity of these compounds. We have chosen the hydrogen substituted analogues to study to keep the complexity of the system at a minimum and, hence, the computation time down.
Our results show that the pyramidalization angle increases markedly down the group. An examination of the frontier orbitals is instructive. The HOMO and LUMO orbitals of the silicon and germanium analogues clearly show a weakening of the p interaction and increasing localization of electron density (i.e., more nonbonding character). The increase in s-p mixing is consistent with the dative, bent bond description of p bonding for these molecules. The HOMO-LUMO gap decreases dramatically from ethene to disilene with a more modest decrease from disilene to digermene. This is consistent with the increasing reactivity of the heavier congeners of alkenes.
Bond energies determined using DFT calculations at the level employed in this work show very good agreement with experimentally determined values. The E=E bond energy is defined as the energy difference between H2E=EH2 and two EH2 fragments. The trend toward a decrease in bond energy down the group is clearly observed. This is consistent with the observed behavior of heavier Group 14 alkene analogues: all known distannene or diplumbenes dissociate in solution at room temperature.

I hope this gives you some idea of my research interests. I am happy to discuss them in further detail.
Dave Geiger