The first thing to consider is that gaps between electronic states (in the Born-Oppenheimer approximation) are not the same at every geometry. At the ground state minimum there is typically a gap of several eV to the first excited state and correspondingly excitation is in the UV. If you distort the geometry, the ground state energy increases (since we were at the minimum). Since the minimum of the excited state does in general not coincide with the minimum of the ground state, it can relax to a lower excited state energy. Ground state and excited state move together and the gap decreases. Fluorescence will be at a lower energy than absorption. This is just the semi-classical explanation of the Stokes shift.
You can go on trying to decrease the gap. And it turns out that for many molecules of interest there are actually geometries where the ground and first excited states are degenerate. These are called conical intersections. And there is not just one such geometry but in the general case the crossing seam forms a hyperline in the space of all nuclear geometries (i.e. an N-2 dimensional subspace, where N is the number of nuclear degrees of freedom). Conical intersections are usually structures strongly distorted from the ground state geometry (hence the ground state energy increases). And they are adapted to the excited state (to keep its energy low).
Nucleic acid bases are of interest in this context. As I read it in a nice popular science article: When a photon from the sun is absorbed by a molecule, this essentially means that this molecule is heated to the temperature of the surface of the sun. Conjugated polar molecules like nucleic acid bases definitely absorb UV light. So why are our cell nuclei not scorching away when we are out tanning a little bit? Well the answer is that they are very efficient in giving away the excess energy and thereby returning to the ground state.
Actually people are arguing that photostability was one main points of selection pressure in the early biosphere. A support of this is that the nucleic acid bases have very short decay times compared to analogues with different substitution patterns. Analogues were destroyed by photochemistry while the bases that are now actually in use could protect themselves with photophysics (i.e. non-radiative decay).
Cytosine is an interesting example [1]. In this case there are three excited states of interest and all of them cross with the ground state. The question is which one is actually the decay channel and there is not really consensus. Well I just want to show the geometries.
The ground state minimum is almost planar with same pyramidalization on the amino group.




[1] The method was MCSCF in Columbus but I cannot really give computational details here. This is more thought as a "community outreach" - just showing some nice pictures.
And you can for example check out this ref if you want to know more about cytosine.
[2] an ε > 0, if you like calculus
2 comments:
Hello,
there is a very good (but also very long) overview article: Pacher, T., Cederbaum, L. S., and Koppel, H. Adv. Chem. Phys. 84, 293
(1993).
I wrote my bachelor-thesis about NH$_3^+$. Due to vibronic coupling, this molecule also shows no fluorescence from the excited states. For now I do not have any cool pictures for you, I not sure what we will publish from the results. Sorry. But perhaps you can find some here:
Viel, A., Eisfeld, W., Neumann, S., Domcke, W., and Manthe, U.
\Photoionization-induced dynamics of the ammonia cation: ab initio
potential-energy surfaces and time-dependent wave-packet calculations", J.
Chem. Phys. 124, 214306 (2006).
Have a nice day,
Till
Hi, yeah that Domcke article looks pretty cool. it will give me an enjoyable way home in the subway ... we are doing more of the on-the-fly classical type stuff so it's nice to see what else you can do
i cannot access the other one but that's ok because long reviews tend to be long sometimes ...
have a nice day
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