In Search of Visible Light Activatable Photocages: Structure‐Activity Relationship Study on C‐8 Substituted Indene‐fused‐coumarinyl Photoremovable Protecting Groups

A series of C-8 substituted indeno[1,2-g]coumarin-based photoremovable protecting groups (PPGs) were synthesized. para-Substituted benzoic acids were employed as leaving groups to evaluate their photolytic efficiency. Substitution of phenyl groups was proved to have negative impacts on photochemical properties of the PPGs, including but not limited to: retarded photolysis course, decreased uncaging quantum yield, and unsatisfactory cargo release yield. Electron-donating diethylamino substituted PPG 3d, a structural analogue of the widely used 7-diethylaminocoumarin PPG (DEACM), exhibited red-shifted absorption maximum and improved optical properties. Photochemical characterization revealed that PPG 3d not only showed comparable photolytic efficiency to DEACM at 365 nm and 405 nm, but also demonstrated superior sensitivity towards 465 nm wavelength, to which DEACM is unable to absorb and therefore, non-responsive. The >450 nm photosensitivity makes 3d a complement to DEACM for long wavelength excitation and a promising PPG for biological applications.