Figure 3.

ROS production in terms of simplified energy-level diagram. Excitation (denoted by absorption of a photon, hυ_a) occurs to one of several higher vibronic states followed by rapid internal conversion (compact squiggly arrow) resulting in the molecule dropping to the lowest excited singlet state (S₁) in which it exists for several nanoseconds. From this state, several fates are possible: the excited chromophore can emit (denoted by emission of a photon, hυ_f), transfer its energy to the solvent without emission (extended squiggly arrow), stimulate electron transfer from ³O₂ (depicted as the broken line from S₁ to ³O₂) to produce superoxide anion (O₂^•−) or undergo intersystem crossing (isc) to the triplet manifold. (Note, to simplify the diagram, generation of ¹O₂ from the chromophore single state is not depicted.) The triplet state is long lived and, for this reason, much photochemistry proceeds from it, including ROS production; emission from the triplet state is called phosphorescence (denoted by emission of a photon, hυ_p).