[smellycat6464]

First of all, hi there! I'm a newbie here

I was wondering if there was a specific "energy threshold" for conjugated systems. If so, what is it called?
I am doing a research project on the radical activity of some active ingredients in sunscreens. When exposed to UV rays, the chromophores degrade and the pi bonds split and form radicals/ros.

I have a theory it has something to do with the excitation energy and/or transition between the HOMO and LUMO? What do you think?

Am I even making sense haha? Sorry if I'm not being clear!
 Thanks in advance!!!

[Corribus]

Photochemistry is complicated.  You are on the right track but you've got your order wrong - "pi bonds splitting" is usually the result of ROS generation, not the cause of it.  You might want to check out photodynamic therapy as a related topic.  Especially, see if you can find the mechanism of how photofrin works (this is a pharmaceutical photosensitizer).

[smellycat6464]

Thank you, I think knowing that mechanism will bring light to the situation--I'll get researching. Would you know if there was a term for the type of excitation photofrin expresses when exposes to light. I am hoping to find a specific energy value required to make photosensitizes generate ROS. For example, I found that the carbonyl on benzophenones is the site where the radical arises, I would love to know how much energy it takes to place it there. Or is it not that simple?

I apologize for my ignorance, as you can tell, I am not experienced in this domain of chemistry haha!
Thanks a million

[Corribus]

Generally what you are looking for is generation of singlet (¹Δ_g) oxygen.  The ground electronic state of oxygen (dioxygen) is a singlet state (³Σ_g^-).  You cannot photogenerate singlet oxygen directly because the transition is forbidden by symmetry and by spin (and by parity – three strikes against it!).  But you can photogenerate it indirectly by an energy transfer reaction from a photogenerated triplet state in a photosensitizer.

Here's a quick lesson in photophysics you may find useful.

If S is our photosensitizer, in the absence of oxygen (* = excited state):

1) ¹S + hν₁  ¹S* (photoexcitation)

One of three things happens now:

2) ¹S* ¹S + heat (internal conversion)
3) ¹S* ¹S + hν₂ (fluorescence)
4) ¹S* ³S* + heat (intersystem crossing)

If (4), then one of two things generally happen:

5) ³S* ¹S + hν₃ (phosphorescence)
6) ³S*  ¹S + heat (intersystem crossing)

Each process has an associated rate constant that is determined by a lot of molecular structural parameters and quantum phenomena.  What's important to know is that anything that when it comes to direct transitions involving light, anything with a singlet to singlet conversion is USUALLY fast and anything with a singlet to triplet (or vice-versa) conversion is USUALLY slow.  Typically heat-producting relaxations have fast rates, so phosphorescence and fluorescence (particularly the former) are usually minor processes EXCEPT in more rigid molecules and molecules with larger HOMO-LUMO gaps, which have slower rates of internal conversion for various reasons that you probably don't care about.

Anyway, in molecules that have efficient intersystem crossing - including porphyrins with closed-shell metals - pathway 4 becomes competitive and a large population of long lived excited triplet states can be generated.  If this happens, and oxygen is present, the following photochemical conversion can take place:

7) ³S* + ³Σ_g^- O₂   ¹S + ¹Δ_g O₂*   

It may not be immediately apparent that spin is conserved here but it is.  The sensitizer goes from an excited triplet to a ground singlet and oxygen goes from a ground triplet to an excited triplet.  So the total change in spin is conserved, so the reaction is fast.  This can only happen if the sensitizer is a triplet.  It can also only happen obviously if the sensitizer triplet state is higher energy than the oxygen singlet state, which is approximately 95 kJ/mol.  That’s pretty low and most conjugated chromophores have lowest lying electronic states well above that, which means most chromophores COULD sensitize singlet oxygen.  The limiting factor really is efficient intersystem crossing in the excited chromophore, which (thankfully for us, as it turns out, because if we had singlet oxygen being generated everywhere, it’d tear us all to shreds), most aren’t.

Oh and if you’re wondering why singlet oxygen is such a powerful oxidizing agent, it’s because, unlike ground-state (triplet) oxygen, its reactions with organic substrates (mostly ground state singlets) are not (again) spin forbidden.  Most reactions between ground state triplet oxygen and organic substrates are thermodynamically favorable but kinetically slow because spin isn’t conserved when these reactions take place.  Singlet oxygen on the other hand has loads of energy to spare and it’s kinetically fast because there’s no spin-associated kinetic barrier.   

[smellycat6464]

Thank you so much, I really appreciate all of that effort you put in that response. I apologize if this is getting to "physical" for an organic board. That just shows how little I knew coming in to this. But you gave me a lot of research ideas, and I'll definitely look more into intersystem crossing, because I think the mechanics behind that will point me to my answers.
Thank you!

[Corribus]

No problem, happy to help.  If you have any follow up questions, let me know.

And by the way, I see I had a bad typo in the second sentence of my last post:

The ground electronic state of oxygen (dioxygen) is a singlet state (³Σg-).

The bold word should be "triplet", not "singlet".  This should be evident from the term symbol but I wanted to correct it anyway lest there be any confusion.

[smellycat6464]

Thank you for that clarification