Light-induced release of photolabile protecting groups is a key step in the photolithographic in-situ synthesis of oligonucleotides in the production of high-density DNA-chips. [1,2] Protecting groups of the o-nitrophenyl type that are preferentially used for this purpose have rather low absorption coefficients at 366 nm, the wavelength of the most conveniently used mercury line. The weak absorptivity of these compounds leads to principal limitations of their light sensitivity. Since they have photoreactive triplet states, however, this problem can be overcome by introducing a triplet sensitizer that efficiently absorbs the light and transfers the electronic energy to the reactive protecting group.The sensitized photodeprotection was studied in detail for the o-nitrophenyl-2-propoxycarbonyl (NPPOC) group [3] where the photo cleavage occurs via b-elimination of the corresponding nitrostyrene derivative as shown in the scheme. The first step of this reaction is photochemically initiated intramolecular H-atom transfer from the a-position to the nitro group whereby an aci-nitro compound is formed [3] . Protolytic dissociation of the aci-nitro compound is followed by the b-elimination from the aci-nitro anion. As an example of the time-resolved
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