Read the article entitled Appreciating Oxygen, the article On the state energies of molecular oxygen, and learn about the role of oxygen in photodynamic therapy.
Here is a short reading on the color of liquid oxygen reading on the color of liquid oxygen and the chemiluminescence observed from the Mallet reaction.
Examine the photoelectron spectrum of O2; here are lecture notes. Here are the lecture slides used in class.
Further resources: interactive molecular orbitals of O2, molecular orbitals of ozone, reading on the topic of porphyria, reading on the Structure of PSII OEC, lecture notes by Dan Nocera, and supplemental exercises.