Masafumi Minoshima

The precise control of DNA recombination enables the cell‐ or time‐dependent regulation of gene expression in studies of gene function. Caged estrogen receptor ligands combined with a Cre‐ERT2/loxP system are useful tools for light‐triggered DNA recombination. However, the photolysis of most caged compounds requires ultraviolet or blue light, which is toxic and displays low tissue penetration. Although a cyanine‐based photo‐responsive protecting group (PPG) can release estrogen receptor ligands with longer‐wavelength light, its low photolytic efficiency requires long illumination times. We developed a caged estrogen receptor ligand with improved green light‐responsive PPGs. The rational modification of Hydroxylated Thiazole Orange (HTO) photocages using electron‐donating groups (EDGs), such as dimethoxy (DiMeO)‐substituted HTO, resulted in high photolytic efficiency (up to ϵ Φ ≈320 M −1 cm −1 ). Theoretical calculations demonstrated that the enhanced photolytic efficiencies were derived from the increased intramolecular charge transfer by EDGs upon excitation. The efficient uncaging of estrogen receptor ligands enabled the control of gene recombination in a ligand‐dependent Cre‐ERT2/loxP system in live cells.