Excited state dynamics of transition metal complexes show many complicated and fast relaxation processes, such as intersystem crossing, structure change, etc. Ultrafast spectroscopy is essential to understanding such complicated excited state dynamics. We monitored the ultrafast emission dynamics of a Cu complex that is very important for fundamental chemistry. Bis(2,9-dimethyl-1,10-phenanthroline)copper(I) has a strong absorption band in the visible region assigned to metal-to-ligand charge transfer, meaning the metal center is formally oxidized to Cu(II).

Fig.1 Bis(2,9-dimethyl-1,10-phenanthroline)copper(I) and its photo-induced structural change

A Jahn-Teller distortion induced in the excited state resulted in a change in the time-resolved emission spectrum, which was observed using the up-conversion fluorescence technique.

Fig.2 Real time change in the emission spectrum due to the photo-induced structural change of Bis(2,9-dimethyl-1,10-phenanthroline)copper(I) in dichloromethane

These results show that a transient state populated just prior to the structural change has a finite lifetime of 660 fs, which shows that the potential energy surface is more complicated than Jahn-Teller theory predicts.

Fig.3 Proposed potential energy surfaces of Bis(2,9-dimethyl-1,10-phenanthroline)copper(I)

[1] M. Iwamura, S. Takeuchi, and T. Tahara, J. Am. Chem. Soc., 129(16), 5248-5256 (2007).