Compounds made up of organic molecules bound to metal ions—often referred to as ‘metal-ligand complexes’—are an important class of chemical substances. Certain copper complexes are used by some plants for photosynthesis, and are also promising candidates for solar cell applications.
The number of electrons associated with a given metal ion (oxidation state) often determines the structure of metal-ligandcomplexes because it strongly influences how the organic ligands are arranged. This phenomenon is observed in copper complexes, where the structure depends on whether the metal ion is in the +1 or +2 oxidation state; referred to as Cu(I) and Cu(II), respectively.
The molecule 2,9-dimethyl-1,10-phenanthroline forms a complex with Cu(I) ions (Fig. 1 - click on link below). Interestingly, by shining visible light on this complex, the copper ion can be changed into the +2 oxidation state through a process known as metal-ligand charge transfer, in which an electron jumps from the metal to the ligands. This reorganizes the initially perpendicular organic ligands to give a flattened—but not completely planar—structure.
Although this kind of photo-induced structural change is widely known, there is not a well-developed understanding of how it occurs. As Tahei Tahara from RIKEN’s Discovery Research Institute in Wako points out, “our knowledge of the photochemical dynamics of metal-ligand complexes is limited in comparison with what we know about organic compounds.”
Now, by studying the fluorescence of the copper complex, Tahara and co-workers have developed a detailed description of how these structural changes happen. This was done using ultrafast time-resolved spectroscopy experiments in which the Cu(I) complex was illuminated with visible light. The resulting fluorescence emitted from the complex was monitored over very short (femtosecond) time-scales.
The results, reported in the Journal of the American Chemical Society (1), show that the Cu(I) complex is converted into a higher energy excited state in which the copper ion is oxidized to the +2 state, but retains the original structure in which the ligands are perpendicular to one another. After what Tahara describes as, “an unexpected waiting time,” this complex finally distorts into the ‘flattened’ structure (Fig. 1).
The finite lifetime of this excited-state species suggests a potential energy profile for these structural changes that contradicts the present theoretical understanding of the dynamics of metal-ligand complexes. Therefore, this study not only sheds new light on the fundamental question of how metal-ligand complexes change shape in real time, but could have implications for practical applications that use these materials.
Reference
1. Iwamura, M., Takeuchi, S. & Tahara, T. Real-time observation of the photoinduced structural change of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) by femtosecond fluorescence spectroscopy: A realistic potential curve of the Jahn–Teller distortion. Journal of the American Chemical Society 129, 5248–5256 (2007).
