Photoreleasable Protecting Groups Triggered by Sequential Two-Photon Absorption of Visible Light: Release of Carboxylic Acids from a Linked Anthraquinone- N-Alkylpicolinium Ester Molecule.

A photoreleasable protecting group activated by sequential absorption of two visible photons is designed, synthesized, and tested. Specifically, an anthraquinone-based chromophore is covalently attached to an N-alkylpicolinium ester. Photolysis of this linked system results in the clean release of a corresponding carboxylic acid. Through product analysis, laser flash photolysis, and steady-state UV-vis spectroscopy, it is demonstrated that carboxylate ion release is affected by sequential absorption of two photons. The initial photochemical step results in reduction of an anthraquinone chromophore to the corresponding hydroquinone. The latter either reacts with O2 to regenerate the starting material, or absorption of a second photon causes an electron transfer to the picolinium group triggering C-O bond scission and release of the carboxylate.