Retinal - Ultrafast is just about fast enough

Ultrafast, barrierless processes have the remarkable feature that they occur in less than a picosecond and can therefore be modelled with molecular dynamics. The cis-trans isomerisation of photo-excited retinal is ultrafast. This is not primarily God's grace toward theoretical chemists (who like to model the process) but it ensures that the process is efficient and vision works. [1]

The trick with an ultrafast process is that after excitation, the reaction occurs (almost) without a barrier. The time is determined by the skeletal deformations, cf. a 333/cm vibration has a period of 100 fs.

To understand what is going on, it is sufficient to consider a smaller system than retinal, for example the pentadiene-iminium ion (the N instead of O comes from the protein). The system is strongly polarised toward the N. The first excited state (shwon in red) is polarized the opposite way.

After excitation (γ means "photon" [2]) the cis-doublebond becomes a single bond. Because of that it stretches out. Now there is an ecliptical single bond with weak π-conjugation. It spontaniously rotates.

The crucial step comes now: The excited state decreases in energy as the rotation progresses. The ground state's energy increases. At about 90° torsion angle their energies are the same. This is called a conical intersection. Now a radiationless transition occurs and we are in the ground state again. Ground state means double bond. And that means that a linear alignment is preferred. The molecule can either move back to the cis form or it keeps rotating the same direction and becomes trans. The ratio depends on the exact shape of the potential energy surface.

During that whole process we move down on potential energy surfaces. The geometry is relaxed to a minimum in the excited state. Then the molecule switches to the ground state where it has a local maximum. And the reaction keeps going until we reach one of the stable ground state structures. The energy needed to drive the process comes from the photon. But as soon as the photon is absorbed everything happens spontaneously.

Because of the short reaction time no competing reactions are to be expected. Fluorescence is with 10^-8 seconds 5 orders of magnitude slower. But also collisions that could take up energy aren't likely in that kind of a time frame. That means we can expect high quantum yield. It is 65% for retinal in rhodopsin and 25% for retinal in solution.

The information was mostly from this article by Garavelli (10.1007/s00214-005-0030-z).

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[1] Sorry for making a religious reference in a scientific text. But where I come from the gap between religion and science isn't quite as big. Our religious people don't try to sabotage scientific theories for incomprehensible reasons and our scientists don't announce that they are atheists all the time. I don't know whose fault the situation in the US is. Maybe religious people should be nice and tolerant. Maybe scientists should be smart and on top of things. Maybe it's difficult for anyone to stop a fight once it has that scale and all you can do is defend your own interests.

[2] I don't like the hν for two reasons. First the energy is neither the only feature of a photon (it has momentum, spin, ...) nor is hν a unique identification for it. Second I don't like ν because it looks like a v and it does not abbreviate the word "frequency" like f does.