Deactivation behavior and excited-state properties of (coumarin-4-yl)methyl derivatives. 1. Photocleavage of (7-methoxycoumarin-4-yl)methyl-caged acids with fluorescence enhancement

The photochemistry of the (7-methoxycoumarin-4-yl)methyl (MCM) carboxylates 3a-d, the mesylate 4, and the phosphates 5a-e has been examined under near physiological conditions in acetonitrile or methanol/aqueous HEPES buffer solution, respectively Analysis of photoproducts as well as measurements of photochemical quantum yields, fluorescence quantum yields, and lifetimes for the excited singlet state verified the similar photochemical and photophysical behavior of all the esters studied here. 4-(Hydroxymethyl)-7-methoxycoumarin (2) and the corresponding free acids were obtained as major products upon irradiation. The rates of deactivation of the excited MCM derivatives 3a-5e were found to be dependent on the:leaving group ability of the anion concerned as well as on the solvent polarity. The polarity dependence and the exclusive formation of O-18-labeled 2 during irradiation of 5a in O-18-labeled water indicate that photocleavage of the excited singlet state of the MCM caged compounds 3a-5e proceeds via a photo S(N)1 mechanism (solvent-assisted photoheterolysis).