DTPA is an octa-dentate ligand used in lanthanide separation for its high separation efficiency for ion binding. During the separation, DTPA must bind to the lanthanide ion, which is associated with a large scale conformational change of the ligand. Understanding this change is crucial for efficient lanthanide separation. To this end, we applied ultrafast polarization dependent mid-IR pump probe spectroscopy to monitor the five carboxylate asymmetric stretching (COOasym-str.) modes of DTPA. These modes can serve as molecular level probes for the structural change of DTPA in free and complexed form. From the experiment, excited state dynamics of COOasym-str. is sensitive to metal binding where the metal binding shields the carboxylate from the solvent–prolonging the vibrational lifetime. Metal binding also significantly changes the intramolecular coupling among COOasym-str. modes leading to slower energy relaxation (Figure 1A, 1B). Further, we extract the anisotropy decay in metal free and metal bound conformations. Compared to free DTPA (1 ps), lanthanide binding leads to faster anisotropy decay with time constant of &gt; 500 fs (Figure 1A, 1B). The ultrafast anisotropy decay is attributed to energy transfer within COOstr. manifold. DFT calculations and stochastic quantum dynamic simulations further confirm this mechanism of anisotropy decay in metal bound DTPAs. The ultrafast energy transfer in metal bound DTPA is attributed to the large conformational change upon metal binding, which induces vibrational delocalization of DTPA carboxylates. Thus, the rate of vibrational anisotropy decay for DTPA maps the strength of the metal-ligand interaction and is sensitive to the charge density of the metal ion (Figure 1C).

Vibrational anisotropy can detect ion binding induced vibrational energy transfer in lanthanide binding: A ligand’s perspective to preorganization

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.