[Enthalpy]

Hello nice people!

I imagined to have grasped how photosensitizers help light-induced reactions, but it doesn't fit with an existing case. Maybe you can help debug my mental representation?

Where I am: a photosensitizer absorbs easily the photon produced by the light source, gets excited, transfers energy to the reactant, which is excited hence can undergo the desired reaction. Whether the sensitizer touches the reactant when it absorbs the photon is unclear to me, and I suppose the reactant uses to separate from the sensitizer before the reaction, or the sensitizer would be called a catalyst instead.

In this description, people use Michler's ketone to absorb light from a medium-pressure mercury lamp emitting mainly around 336nm, excite a butadiene molecule, which then reacts with an other butadiene in ground state to produce 1,2-trans-divinyl-cyclobutane:
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv5p0528
and now I don't understand any more, because butadiene demands a 217nm photon to be excited, so 336nm shouldn't have sufficed - I miss 2eV!

I suppose 336nm is just fine for this reaction, because of the choice of medium-pressure mercury. A low-pressure lamp instead would be globally less strong but produce nevertheless more power at 254nm.

So what happens? Is a second photon absorbed at an other step, for instance when energy transfers from this sensitizer to this reactant? Or does thermal energy (only 26meV mean @300K) help a bit at several steps? Or does this sensitizer stick to the reactant until it cyclizes, acting as a catalyst as well?

Thank you!