title: 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.
authors.affiliation: Département de Chimie (C.N.R.S. U.M.R. 8640), Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Aujard
authors.forename: Chouaha
authors.initials: C
authors.lastname: Benbrahim
authors.forename: Marine
authors.initials: M
authors.lastname: Gouget
authors.forename: Odile
authors.initials: O
authors.lastname: Ruel
authors.forename: Jean-Bernard
authors.initials: JB
authors.lastname: Baudin
authors.forename: Pierre
authors.initials: P
authors.lastname: Neveu
authors.forename: Ludovic
authors.initials: L
authors.lastname: Jullien