The drug actually pops off the liposome and onto low-density lipoproteins in the blood. The drug-laden LDL is then absorbed by the LDL receptors on the cells of the newly formed blood vessels. Intersystem crossing is a relatively long-life excited state, microseconds rather than the nanoseconds of fluorescence, but shorter than the milliseconds of phosphorescence. This is key because oxygen diffusion occurs next. An oxygen molecule collides with the sensitizer molecule, which excites the O 2 molecule from the ground state to the singlet oxygen state.
Singlet oxygen is highly toxic to cells.
Prostaglandins are produced that close blood vessels down. Mitochondria, which control cell metabolism, shut down. Spin-prohibited transitions are singlet to triplet or vice versa via phosphorescence, p, or intersystem crossing, isc. Only fluorescence and phosphorescence are radiation processes. The photosensitizing agents are porphyrins or porphyrin-like derivatives.