Water oxidation chemistry of photosystem II
Light is indeed the energy required for the process to occur! The photosystem chlorophyll involved is P680. The light energy excites an electron to a higher level, and this electron is captured by pheophytin, forming P680+. The redox potential for P680+ is huge, 1.3V, and it becomes a very strong oxidizing agent, regenerating it's lost electron from water during the oxygen-evolving process. The electron that went to pheophytin ends up transferring to QA, then to QB (these are involved elsewhere).
So now, the P680+ oxidizes this redox-active tyrosine residue called Yz by taking an electron. This forces the Yz into a radical state Yz*. As the article goes on to provide, data showed the Yz hydroxyl group pKa to be greater than 9, meaning it should be protonated at physiological pH. As a result, in concert with the electron transfer outlined above, a proton is donated from Yz* to a nearby base: histidine 190. In this manner, Yz* is capable of quickly oxidizing the nearby tetranuclear manganese cluster. An oxygen-evolving complex is formed by Yz, Mn4, Ca, Cl, and a couple of additional amino acids. Yz oxidizes the manganese cluster resulting in transition states denoted S1-S4:
The heavily oxidized Mn cluster then oxidizes water to return to an S0 state, forming O2 as a part of the process.
The authors detail:
that O–O bond formation occurs in the S4 state via nucleophilic
attack on an electron-deficient MnV=O species by
a Ca2-bound water molecule... O–O bond formation begins by bringing the second
substrate water closer to the MnV=O in an SN2-like reaction
(S4 state in figure 2). We propose that this occurs
through contraction of the Mn–Cl bond upon formation
of the high-valent MnV=O moiety.
In conclusion, we see light powers the reaction, but it is mediated by protein. The net reaction is an oxidation of a metalloenzyme complex which essentially rips the hydrogens off of water molecules to return to it's standard state. The article used for this answer was the best explanation, with a lot of details, that I could personally find, but it is from 2002. If someone has a more accurate explanation let me know!