Mechanism of photocleavage of (coumarin-4-yl)methyl esters

(Coumarin-4-yl)methyl esters (CM-A) are caged compounds that, upon excitation, release the masked biologically active acid HA and the highly fluorescent (coumarin-4-yl)methyl alcohol CM-OH very rapidly and in part with high efficiency. The results of photostationary and time-resolved investigations of 25 CM-A esters and corresponding CM-OH alcohols with varying substitution on the (coumarin-4-yl)methyl moiety and a wide variation in the structure of the acidic part have been analyzed. The initial step of the photoreaction is heterolytic ester cleavage leading to the singlet ion pair (1)[CM+ A(-)] with rate constant k(1). (1)[CM+ A(-)] hydrolyzes to CM-OH and HA with rate constant k(2) or recombines to ground-state CM-A with rate constant k(rec). (1)[CM+ A(-)] is the key intermediate of the reaction. Stabilization of both CM+ by using electron-donating substituents and A(-) by increasing the acid strength leads to a strong enhancement of k(1) and simultaneously to a diminution of k(rec). Therefore, stabilization of the ion pair has a two-fold positive effect on the photocleavage of (coumarin-4-yl)methyl esters: increasing the rate of the initial reaction step, which might require less than 30 ps, and increasing the efficiency of product formation.