Monday, January 3, 2011

Molecular movies: watching excited state dynamics

Tomorrow I am visiting Ahmed Zewail's group at Caltech. This motivated re-reading a beautiful Science paper from 2005, Dark Structures in Molecular Radiationless Transitions Determined by Ultrafast Diffraction It contains the Figure below (which I have used on this blog and in talks before) of schematic potential energy surfaces for a ground and excited state.
A key issue in photochemistry and photophysics is how for a particular molecule one actually deduce such potential energy surfaces. Calculating them accurately from quantum chemistry is extremely difficult, even for small molecules. Trying to infer the topography of the surfaces from experiment is often ambiguous. Over the past decade Zewail's group has pioneered a powerful method for doing this. They optically excite the molecule and then monitor the molecular structure using ultra-fast electron diffraction.

Two key questions are addressed about the different photophysics of pyridine, 
2-methylpyridine, and 2,6-dimethylpyridine.

What is the origin of the abrupt increase in the radiationless decay above a certain internal energy threshold [channel three threshold]? 

How are the radiationless processes influenced by subtle changes in the molecular structure associated with the methyl groups?

The Figure below shows the structure of the three molecules after 50? psec. Note that the top two molecules [pyridine and methyl-pyridine] undergo ring opening whereas the third one [bi-methyl-pyridine]  does not.

This observation is in stark contrast to the prevailing view that an ultrafast internal conversion pathway, mediated by the proximity of the first and second excited-state surfaces, opens up at the channel-three threshold and leads to vibrationally hot ground-state molecules.

It would be interesting to apply this technique to organometallic complexes that are used in LEDs. In particular, it should be possible to see the M-C bond rupture that has been proposed to be an important non-radiative decay channel.

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