Tuesday, February 7, 2012

Seeing the outcome of an encounter with a conical intersection


There is a nice JACS paper Population Branching in the Conical Intersection of the Retinal Chromophore Revealed by Multipulse Ultrafast Optical Spectroscopy
by Goran Zgrablić, Anna Maria Novello, and Fulvio Parmigiani.

It looks at how the polarity of a solvent changes the outcome of the photo-isomerisation (trans-cis) reaction of a retinal molecule. A similar reaction is responsible for vision: it seems that the protein environment is key to the speed and selectivity of the reaction.

This experimental study finds that as one increases the polarity of the solvent (as measured by the static dielectric constant) the branching ratio for the blue to green/purple curves in the figure above increases.

This is argued to be broadly consistent with a theoretical study published earlier this year, Dynamical Friction Effects on the Photoisomerization of a Model Protonated Schiff Base in Solution, by João Pedro Malhado, Riccardo Spezia, and James T. Hynes.

A key insight seems to be the schematic Figure below which contrasts the case of water (top) and acetonitrile (below). It shows the ground and excited state PESs as a function of the twist angle (torsion) associated with cis-trans isomerisation.
Water is claim to have "fast" dynamics which increases the probability of direct transition to the lower surface. In acetonitrile the system can stay longer on the excited state surface and away from the conical intersection seam allowing it to follow the dashed trajectory above, leading to a larger formation of cis.

However, the authors point out how their study involves several key debatable assumptions including
  • nuclear and solved dynamics can be modelled by a Generalised Langevin Equation (GLE); this means it is classical (I am not sure to what extent it is overdamped)
  • non-adiabatic transitions between the excited and ground state potential energy surfaces (PESs) can be modelled by the surface hopping algorithm of Tully
  • a simple parametrisation of the PESs
  • rough estimates for the frictional parameters for the GLE
To me all this underscores just how a quantitative description of the quantum dynamics near a conical intersection in the presence of a dissipative environment is such a basic and important outstanding problem.

2 comments:

  1. Hi!
    thanks for being interested in this work (I am co-author of the theoretical paper) !
    Riccardo

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  2. Thank you for mentioning our experimental work on retinal in various solvents. Still, there are many thing we don't understand well, especially why acetonitrile is so different from all other solvents.
    Goran Z.

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