Schmidt Reinhard, Geissler Daniel, Hagen Volker, Bendig Jürgen
Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue-Strasse 7, D60438 Frankfurt am Main, Germany.
J Phys Chem A. 2007 Jul 5;111(26):5768-74. doi: 10.1021/jp071521c. Epub 2007 Jun 12.
(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.
（香豆素 - 4 - 基）甲酯（CM - A）是一种笼形化合物，在激发时会非常迅速地释放出被掩盖的生物活性酸HA和高荧光性的（香豆素 - 4 - 基）甲醇CM - OH，部分释放效率很高。对25种（香豆素 - 4 - 基）甲酯及其相应的CM - OH醇进行了光稳态和时间分辨研究，这些化合物在（香豆素 - 4 - 基）甲基部分具有不同的取代基，酸性部分的结构也有很大差异，对研究结果进行了分析。光反应的初始步骤是异裂酯裂解，生成单线态离子对1[CM⁺A⁻]，速率常数为k1。1[CM⁺A⁻]以速率常数k2水解生成CM - OH和HA，或以速率常数krec重新组合成基态的CM - A。1[CM⁺A⁻]是该反应的关键中间体。通过使用供电子取代基稳定CM⁺以及通过提高酸强度稳定A⁻，会导致k1显著增强，同时krec减小。因此，离子对的稳定对（香豆素 - 4 - 基）甲酯的光裂解具有双重积极作用：提高初始反应步骤的速率（可能需要不到30皮秒），并提高产物形成的效率。
