Time-resolved photoluminescence excitation characterisation of lanthanide and group III tris-(8-hydroxyquinoline) molecules

Time resolved photoluminescence measurements of lanthanide and group III metal chelates of 8-hydroxyquinoline (Q) have been performed as a function of temperature and excitation wavelength. For the lanthanide complexes it has been shown that either singlet or triplet luminescence can be observed depending on the excitation wavelength. Life-time measurements of these emissions show that competing non-radiative paths are very important in the performance of these molecules. For ErQ we have shown that it is the singlet state that couples most efficiently to the ion. Radiative lifetime measurements of the ion emission show relatively short lifetimes that are indicative of quenching mechanisms. For the group III metal chelates at room temperature the luminescence is dominated by the singlet emission but at 80 K there is evidence that triplet emission can occur when the molecule is excited at long wavelengths. Luminescence lifetime measurements of the emission from the lanthanide ions: erbium, neodymium and ytterbium all show effective lifetimes of the order of microseconds which is very fast compared to the lifetimes of the free ions. Using excitation directly into the lanthanide ion (e.g. ∼980 nm excitation for erbium) and via the organic ligands (∼400 nm excitation) we have seen that there are no changes in the emission lifetimes and hence the exciton transfer from the ligand to the lanthanide ion is not a rate limiting step