The 2008 Nobel Prize in Chemistry was awarded for the discovery and development of Green Flourescent Proteins. An important question is
Why does the chromophore cease to be flourescent when it is in the gas phase or is removed from the protein and dissolved in water?
The most natural (partial) explaination is that inside the protein non-radiative decay channels are quenched. In particular, quantum chemical calculations suggest that in the gas phase the chromophore undergoes rapid photo-isomerization. i.e., when it absorbs a photon and makes a transition into the lowest excited singlet state it twists about the central carbon atom (methine bridge).
Such molecules are strongly correlated electron systems.
Calculating the relevant potential energy surfaces (which determine the excited state quantum dynamics) at a level of approximation that gives qualitatively
reliable results for such a large molecule is at the boundary of what is currently possible in computational chemistry. Furthermore, even if one can do such a calculation is it possible to obtain some chemical and physical insight as to the key issues. I believe that Seth Olsen and I recently made significant progress in this regard. Our paper has been accepted for publication in the Journal of Chemical Physics. The figure below provides a simple picture of the relevant resonating valence bond states relevant to the three lowest singlet states.
Seth will soon be giving seminars on this work at two of the worlds leading groups on computational photochemistry, Mike Robb's group at Imperial College London, and Massimo Olivucci's group in Sienna, Italy. I look forward to hearing their feedback.
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