Saturday, May 22, 2010

Watching excited state structural changes

Organometallic complexes are a key functional component of many organic LED's and photovoltaic cells. Understanding their excited state dynamics is a major challenge. Two key questions:

What is the mechanism of the ultrafast (10-100 fsec) intersystem crossing from the singlet to triplet metal-ligand charge transfer state (MLCT) state?

What is the non-radiative decay path of the (phosphorescent) triplet state to the ground state?

What structural changes are associated with these transitions?

A paper in Science last year helps answer the second and third questions for an Fe(II) complex. The results are summarised in the Figure below. This experiment is based on new advances which allow monitoring X-ray Absorption Near Edge Structure (XANES) as a function of the time delay between laser pump and x-ray probe.

A key reaction co-ordinate is the Fe-N distance. Increasing it reduces the crystal field splitting which allows excitation of high spin states associated with excitation of the eg states.

We now need an effective Hamiltonian (backed up by quantum chemistry calculations) to describe this schematic of potential energy surfaces.

No comments:

Post a Comment