Theoretical investigation of the excited states of 2-nitrobenzyl and 4,5-methylendioxy-2-nitrobenzyl caging groups

Abstract

The photochemistry of caged compounds of the o-nitrobenzyl type has been investigated thoroughly in the past. However, even recently new side reactions have been discovered. Earlier, we reported [Bley, F., K. Schaper, and H. Görner (2008), Photochem. Photobiol.84 162-171] that we found long-lived triplet states which do not lead to product formation for the bathochromic absorbing compounds with 4,5-methylendioxy-2-nitrobenzyl caging group. Here, we report on theoretical studies which explain the special behavior of these compounds. These studies reveal that the bathochromic shift of absorption for these compounds compared with o-nitrobenzyl compounds themselves is not due to a shift in energy of the involved states, but due to a substantial change of oscillator strength of the respective transitions. The lack of reactivity of the triplet state of 4,5-methylendioxy-2-nitrobenzyl compounds can be attributed to state switching. In the triplet manifold the lowest state is a nonreactive charge transfer state, while the lowest state in the singlet manifold is a reactive local excitation in the nitro-group. From these results we conclude that it will be most likely not possible to create derivatives of caged compounds based on the o-nitrobenzyl caging group which have absorption which is shifted even more strongly to longer wavelengths.