Twisting charges apart

This follows up with the earlier post about a paper, Conical Intersections, Charge Localization and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye by Seth Olsen and I, which been accepted for publication in Journal of Chemical Physics.

An important issue is after a organic molecule absorbs a photon what conformational change will occur. Below are several options involving bond twists.

A second issue is how the charge distribution in the molecule changes upon twisting.

This kind of physics is at the heart of how your eye works. When retinal absorbs a photon it undergoes a conformational change which produces a charge separation which eventually leads to an electrical signal in your brain. It is also at the heart of designing better organic solar cells.

We considered a model Hamiltonian for a large class of dyes. The figure below shows contour plots for the first excited state potential energy surface for several parameter values. The lower part of the figure shows how the charge distribution in the molecule changes for different conformations. It is amazing how such a simple model Hamiltonian can capture such rich physics.