Red light-activated phosphorothioate oligodeoxyribonucleotides. 
Hairpin-structured phosphorothioate oligodeoxyribonucleotides containing a singlet oxygen-sensitive linker in the loop were prepared.  These compounds do not bind complementary nucleic acids in the dark.  Upon irradiation with red light in the presence of chlorine e6 the linker within these compounds is cleaved and a single-stranded oligodeoxyribonucleotide is produced.  The latter compound is an efficient binder of complementary nucleic acids.  This is the first example of 'caged' phosphorothioate oligodeoxyribonucleotides, whose nucleic acid binding ability is triggered by red light.