SEMINAR: Bayliss Seminar Series

Some of the most elementary questions that may be asked about a conjugated molecule is the degree to which its electronic structure can be considered as ‘localised’ or ‘delocalised’. Mixed-valence compounds have proven to be useful platforms through which to explore the spectroscopic signatures of ‘localised’ or ‘delocalised’ structures. The Creutz-Taube ion1 is perhaps the most prototypical mixed-valence compound, however the discussion of its electronic structure has spanned almost 50 years! The interest in the problems raised by the Creutz-Taube ion have inspired the wide-spread examination of other bimetallic complexes in which two redox active end-caps are spanned by a seemingly never-ending array of bridging ligands. It is common for these complexes to be described in terms of the Robin-Day classification system2 (Class I: localised electronic structures with non-interacting end-caps; Class II: localised electronic structures with electron-exchange between the end-caps; Class III: delocalised electronic structures), with the ‘class’ assigned on the basis of key spectroscopic features in the NIR and IR spectra as well as related methods such as ESR, Mössbauer and Stark spectroscopy.

However, each of these methods operates over a different timescale, and a compound that appears localised on the time-scale of one method may be delocalised on another. Recent work3-4 has shown that such categorisations can be further blurred by dynamic changes in molecular conformation and hence electronic structure.

Despite the interest in the classical bi-metallic mixed-valence compounds there has been little work on systems in which branched or cross-conjugated ligands bridge more then 2 redox-active moieties and which offer multiple sites for electron localisation or distribution, and hence multiple pathways for electron transfer.

Such systems would potentially have applications in optoelectronic devices, as transistor-like components in single molecule electronics as well as offering new challenges to the conventional spectroscopic methods when searching for a description of electronic structure.