Combined Theoretical and Experimental Investigationof the Photodecarboxylation of Nitrophenylacetates and Its Implicationsfor the Design of Improved ortho-Nitrobenzylic Caging Groups.

The photodecarboxylation mechanismof different structural isomersof nitrophenylacetate (NPA) has been investigated using quantum chemicalcalculations as well as time-resolved UV-pump VIS-probe spectroscopy.It is shown that for a proper theoretical description of the excitedstates of anionic NPA in aqueous solution a careful considerationof the influence of the solvent is indispensable. In this sense, NPAis an example that demonstrates how character and lifetime of theinvolved excited states affect the results of equilibrium and nonequilibriumsolvation approaches. An ultrafast decay channel via a repulsive singletstate has been found to be responsible for observed ultrafast CO2release, while another very efficient but slower CO2release channel is found to proceed via intersystem crossing andsubsequent decay via a repulsive triplet state. After all, differencesand similarities in the observed excited state dynamics of the isomersare conclusively explained. Most notably, the much smaller quantumyield of CO2release from the ortho-isomer is due to analternative excited-state hydrogen-transfer channel, which occursalong a triplet and singlet pathway. On the basis of theoretical andexperimental evidence suggesting that the multiplicity of the routetaken determines the photoproduct yield, we provide guidelines forthe design of ortho-nitrobenzylic caging groups with improved uncagingyield. [ABSTRACT FROM AUTHOR]