Sunday, July 5, 2009

A further twist on optically active molecules

Understanding the quantum dynamics of the excited states of complex molecular materials is a scientific challenge that is of great technological importance.
From a physics point of view we would like to know which details really do matter in determining functionalities such as the light induced charge separation required in solar cells.

Methine dyes are an important class of materials for organic photonics.
Previously I discussed how Seth Olsen and I published a paper

A diabatic three-state representation of photoisomerization in the green fluorescent protein chromophore

J. Chem. Phys. 130, 184302 (2009)

We have also done a very detailed study of the topology and geometry of the potential energy surfaces for the low-lying singlet states for an effective Hamiltonian describing the three valence bond states relevant to methine dyes.

Important degrees of freedom are the different twists shown below.
Even this simple model exhibits much of the rich geometry and several key properties found previously in very sophisticated quantum chemistry calculations: the existence of conical intersections between different states and charge localisation connected with excited state twisting. Just one case is shown below.

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