Oxidation of guanine with singlet oxygen
You're right that ground state $\ce{^3O2}$ doesn't undergo this reaction.

There is no direct excitation of oxygen either.

The key in these processes is the electronic excitation of tetraphenyl porphyrine (TPP). The excited singlet state of the dye undergoes intersystem crossing to a triplet state. The latter interacts with ground state oxygen and generates singlet oxygen.

$$\ce{TPP -&gt;[h\nu] ^1TPP^*}$$
$$\ce{^1TPP^* -&gt;[ISC] ^3TPP^*}$$
$$\ce{^3TPP^* + ^3O2-&gt;[sens] TPP + ^1O2}$$

Singlet oxygen then acts like the typical dienophile in a Diels-Alder reaction.