One-electron oxidation of an aromatic donor (D) generates the paramagnetic cation-radical which spontaneously associates with its neutral counterpart to form a stabilized dimeric cation-radical (eq. 3).
Stable DIMERIC AROMATIC CATION RADICALS (D2+) are of fundamental importance to organic materials science, since they constitute the smallest intermolecular units that carry a delocalized positive charge and thus provide the basis for the (photo)conductivity, ferromagnetism, and non-linear optical response in various organic materials. For example, we recently isolated π-dimer or mixed-valence cation radical of octamethylbiphenylene, OMB (as the SbCl6- salt) as crystalline solid at low temperatures and determined its crystal structure (see Figure 4). The dimeric cation radicals can also be characterized spectroscopically by the observation of new charge-resonance bands in the near IR region (see Figure 4 below).
Molecular Wires. By definition, a molecular wire is composed of two redox sites that are linked by a bridging spacer. The most important criterion in designing a functional molecular wire is to transport energy or an electron (or hole) from a donor to an acceptor moiety via a bridging spacer. Through-bond charge resonance in these wire-like materials is readily characterized spectrally by the appearance of new bands with strong absorptions in the near IR region, which are analogous to those observed in intermolecular aromatic interactions in mixed-valence aromatic cation radicals (see, Figure 4). We have prepared a variety of molecules in which intramolecular electron exchange in D-spacer-D molecules with two identical redox centers (D) of different oxidation states (i.e. intervalence systems), where the driving force for electron transfer DGET ~ 0, the electronic coupling critically depends on the distance and the structural and electronic nature of the spacer between the bridging redox moieties (D).
For example, strong D/D coupling leads to a significant shift of E2ox in the case of 1a (n = 1) whereas a 2e- oxidation occurs at the same potential in the case of 2c (n = 3), which indicates a complete lack of electronic coupling. Another very useful indicator of electronic coupling is the presence of charge-resonance or intervalence absorption bands in the near-infrared (NIR) region characteristic of D+-spacer-D ? D-spacer-D+ transformation (see Figure 5).
We exploit a variety of spectroscopic techniques (including time-resolved laser spectroscopy) as well as X-ray crystallography to probe the nature of through-space and through-bond electronic interaction between various redox centers in supramolecular and macrocyclic assemblies. We are also utilizing these molecular assemblies for the detailed study of long-range electron transfer process that control the critical solid state properties such as conductivity, ferromagnetism and non-linear optical response.
We are presently exploring the synthesis of a variety of electro-active organic molecules and supramolecular assemblies bearing multiple redox-active chromophores for the preparation of molecular wires and mixed-valence materials that will find widespread applications in molecular electronics and in the emerging field of nanotechnology.