Photoreactions in lignins and wood

Wood contains 15-30% lignin, an aromatic UV- and visible-light-absorbing polymer with a very complex structure (see Chart 8.1), and photochemical alterations of wood are essentially determined by reactions initiated by bond breakages in the 

Because of the capability of lignins to absorb near-UV and visible light, even indoor yellowing and darkening of wood surfaces due to slow photooxidation processes is unavoidable. More detailed information concerning the photochemistry of lignins and wood is available in relevant review articles [16, 47]. 

Sensitizers can be employed for agricultural purposes as herbicides and insecticides, or for medical purposes as antibacterial and antiviral agents. Moreover, sensitizer-based methods serve as tools for the analysis of the interaction faces of polymer complejKs and the sequence-selective photocleavage of double-stranded DNA. The ways in which photosensitized reactions are utilized are illustrated by the following typical examples. The first case relates to the photochemotherapy of cancer cells in superficial solid tumors [48]. The so alled photodynamic therapy, PDT, is based on the selective incorporation of a photosensitizer into tumor cells, followed by exposure to light (commonly at A=600 nm). Cytotoxic products, namely singlet oxygen, O, and superoxide radical anions, 

Chart 8.7 Sensitizers employed in the photochemotherapy of cancer cells. TPP meso-tetraphenylporphine, TMPyP meso-tetra 4-/V-methylpyridyl)porphine, MB methylene blue, 

TB toluidine blue, ZnPc zinc(ll) phthalocyanine, TPPo tetraphenylporphyrene. 

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