Metal complexes with sterically demanding phenanthroline ligands: a combined spectroscopic study

Abstract The interactions of a series of sterically hindered phenanthroline derivatives with first-row transition metal ions were thoroughly investigated through a combined spectroscopic study. UV-Vis titrations reveal that all the investigated systems form stable complexes with a stoichiometric metal to ligand (M:L) ratio of 1:1; furthermore, the 1:2 species can be easily obtained in solution by choosing the appropriate ratio between copper ions and the three ligands. The great sensitivity of the fluorescence emission quenching of these compounds to the presence of cations can be exploited for analytical purposes to determine the unknown concentration of cations. Femtosecond transient absorption measurements reveal peculiar excite state dynamics depending on the charge density of the metal ion considered: a higher charge density is responsible for extremely fast decays, in agreement with fluorescence quenching, while lower charge density leads to the population of a triplet excited state. Particular attention has been devoted to the copper complexes and the picture described so far was further confirmed by NMR and CW EPR measurements, although different experimental settings have to be employed. In addition, the EPR study provided: i) a deep insight into the stereochemistry of the copper(II) complexes ii) first evidence on the ligand reducing ability towards copper and iii) clear indication on the possible formation of the 1:3 complex species.