The photodecontamination of cellular blood components: mechanisms and use of photosensitization in transfusion medicine.

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p HOTOSENSITIZATION is a term that covers many phenomena, but a general definition is action of a component (photosensitizer) of a system that causes another component of the system to react to light. The photosensitizer in its ground state serves as a chromophore (light absorber). On absorption, the sensitizer becomes electronically excited to a higher energy level. The excited sensitizer molecule can react directly with a substrate or with some other molecule (frequently oxygen) in the reaction mixture, giving products that, in turn, can react with the substrate. Thus, photosensitized processes typically have an initial light step followed by one or more steps. The photosensitizer molecules in the dark are almost always in the singlet state, ~ in which the molecule has no unpaired electron spins. Absorption of a photon (hv) promotes an electron to a higher molecular orbital without a change in its spin. Thus, the first excited state is also a singlet, tS. Few photosensitized reactions are directly mediated by this state because of its short lifetime (1 to 100 ns). Excited singlet states can decay to the ground state, emitting heat or (fluorescence). In the case of an effective photosensitizer, there is a fast spin inversion of the high-energy electron, resulting in a metastable triplet state, 3S, which has two unpaired electrons and a much longer lifetime (1 to 1000 ps). The sensitizer in its excited triplet state can undergo many collisions with other molecules during its lifetime and, as a result, can mediate photosensitized reactions with high efficiency. These processes are shown schematically.