Randolph P. Thummel

Our main interest is the design and synthesis of novel ligand systems. Ligands, in the broadest sense, are species which interact with other species through the formation of non-covalent bonds: metal-ligand complexes are one good example. In that context, we have prepared a wide variety of derivatives of 2,2'-bipyridine, 2,2';6,2"-terpyridine and 1,10-phenanthroline (see 3D x-rays on homepage). In many cases we have used polymethylene bridging to control the conformation of the ligand as in the diphenanthrolines 1. The related terphenanthroline forms a helical trinuclear complex with Cu(I) which resembles the coiling of DNA (Figure 1). Aza-aromatic rings can be arranged in a parallel fashion to provide rigid syn-orthocyclophanes that complex two metals in a layered orientation. We have prepared chiral steroidal phenanthrolines such as 2 as well as a trans-chelating bipyridine analogue 3.

Cavity shaped molecules possessing indole subunits can accept neutral organic guests by taking advantage of a well organized hydrogen-bonding assembly as shown by structure 4. Hosts of this type show excellent potential for binding a variety of biologically important molecules such as amides and ureas. Bridged bithiazolium and biimidazolium salts undergo the reversible transfer of one or two electrons to provide radical cations or neutral species such as 5. The redox properties and donor-acceptor abilities of these species can be tuned by varying the length of their polymethylene bridges.

More recently we have become interested in using ligand design for the chemical utilization of solar energy. We are developing a combinatorial approach to the synthesis of new photosensitizers for semi-conductor based solar cells. We have also synthesized a new family of ruthenium catalysts that very efficiently convert water into dioxygen. Attempts are underway to make this process photocatalytic, leading to a practical system for solar water decomposition. We have found that some of our Ru(II) and Cu(I) complexes have a high affinity for DNA. Irradiation of the DNA-bound complex subsequently may lead to strand scission or cross-linking and these complexes are being evaluated as candidates for photodynamic therapy (PDT). We have active collaborations with a number of other research groups around the world.

Figure 1. Trinuclear Cu(I)
complex of terphenanthroline.
Figure 2. A dinuclear Ru(II) catalyst
which can efficiently oxidize water.