o‐Nitrobenzyl Photolabile Protecting Groups with Red‐Shifted Absorption: Syntheses and Uncaging Cross‐Sections for One‐ and Two‐Photon Excitation

Abstract

We evaluated the o‐nitrobenzyl platform for designing photolabile protecting groups with red‐shifted absorption that could be photolyzed upon one‐ and two‐photon excitation. Several synthetic pathways to build different conjugated o‐nitrobenzyl backbones, as well as to vary the benzylic position, are reported. Relative to the reference 4,5‐dimethoxy‐2‐nitrobenzyl group, several o‐nitrobenzyl derivatives exhibit a large and red‐shifted one‐photon absorption within the near‐UV range. Uncaging after one‐photon excitation was studied by measuring UV‐visible absorption and steady‐state fluorescence emission on model caged ethers and esters. In the whole series investigated, the caged substrates were released cleanly upon photolysis. Quantum yields of uncaging after one‐photon absorption lie within the 0.1–1 % range. We observed that these drop as the maximum wavelength absorption of the o‐nitrobenzyl protecting group is increased. A new method based on fluorescence correlation spectroscopy (FCS) after two‐photon excitation was used to measure the action uncaging cross section for two‐photon excitation. The series of o‐nitrobenzyl caged fluorescent coumarins investigated exhibit values within the 0.1–0.01 Goeppert–Mayer (GM) range. Such results are in line with the low quantum yields of uncaging associated with cross‐sections of 1–50 GM for two‐photon absorption. Although the cross‐sections for one‐ and two‐photon absorption of o‐nitrobenzyl photolabile protecting groups can be readily improved, we emphasize the difficulty in enlarging the corresponding action uncaging cross‐sections in view of the observed trend of their quantum yield of uncaging.