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 k1. 1[CM+ A-] hydrolyzes to CM-OH and HA with rate constant k2 or recombines to ground-state CM-A with rate constant krec. 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 k1 and simultaneously to a diminution of krec. 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.
Schmidt
Reinhard
R
Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue-Strasse 7, D60438 Frankfurt am Main, Germany. r.schmidt@chemie.uni-frankfurt.de
Geissler
Daniel
D
Hagen
Volker
V
Bendig
Jürgen
J
D000143:Acids / Q000737:chemistry; D000438:Alcohols / Q000737:chemistry; D003374:Coumarins / Q000737:chemistry*; D000242:Cyclic AMP / Q000737:chemistry; D004952:Esters / Q000737:chemistry*; D008745:Methylation; D015394:Molecular Structure; D010782:Photolysis
D000143:Acids; D000438:Alcohols; D003374:Coumarins; D004952:Esters; D000242:Cyclic AMP