Long wavelenth fluorescent probes based on ruthenium derivatives: Expectations and problems

摘要

With the aim of obtaining efficient near infra-red emitters, a family of ruthenium and polyruthenium complexes made from tetrapyridyl analogs and terpyridine ligands was synthetized. It was expected that the increase in size of the complex would result in the progressive shift of the absorption and emission spectra towards the infra red and above all that the stability and rigidity of these complexes would be associated to a high emission yield. If the expected shifts are obtained, with emission at 820 nm for a bisruthenium complex for instance, quantum yields are very low, usually between 10{sup}(-2) and 10{sup}(-3). More disturbing, these compounds display an "anormalous emission" at shorter wavelength. The latter does not appear to be due to impurities since it is different for all these molecules which are built from the same precursors treated in similar conditions. Preliminary studies point to the fact that the bite angles of the multidentate ligands are not adapted to the binding to ruthenium and that this mismatch gives rise to a tension which can be released only by having a ligand pop off one binding position. The resulting species is distorted, with a reduced conjugation or electronic communication between the metal centers, and displays the short wavelength emission. Rigidification of the tetrapyridyl ligand does not improve the emission yield, a fact which is in agreement with the above explanation. Absorption and emission spectra for all these compounds will be analyzed.