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.

Controlled two-photon photodegradation of PEG hydrogels to study and manipulate subcellular interactions on soft materials

“…30 This moiety was chosen due to its capability of undergoing two-photon photolysis 37 (at 740nm the two-photon cross section δ u Φ u ≈ 0.01 to 0.03 GM 38 ) and its previous use in biological systems. 39 Upon two-photon excitation, PEGdiPDA cleaves between the NBE and acrylate functionalities, releasing modified PEG and polyacrylate chains with pendant PEG.…”

“…Based on a statistical treatment of chain growth networks, the critical extent of cleavage (p c ) is dictated by the weight average number of crosslinks per polyacrylate 1 kinetic chain ( N = 20 for this system 33 ), such that ${\text{p}}_{\text{c}}=\frac{1}{\sqrt{N-1}}$. 41 The degradation reaction as a function of irradiation time ( t ) is modeled as a first-order process as previously described 38,42 : $$\frac{d[\text{NBE}]}{dt}=-k_e[\text{NBE}]$$ with…”

“…38,42 For these studies, a conservative estimate of 0.01 GM for the two-photon cross section was used based on previous literature. 38 Equation (3) was solved to give a form for the extent of degradation (p): $$\text{p}=\frac{[\text{NBE}]}{{false[\text{NBE}false]}_0}=e^{-k_{e}t}$$ such that one can combine Eqns. (4) and (5) for the relationship between the critical exposure time ( t = t c ) required to erode the material completely (p = p c ) and the average laser power ( P avg ).…”

Photodegradable Macromers and Hydrogels for Live Cell Encapsulation and Release

“…Several different o-NB groups have been reported [14][15][16] , and small modifications to the structure of the o-NB linker have a significant impact on the photolysis rate. 14 The benzylic position (photodegradation site) of o-NB groups can be primary or secondary (Chart 1a, R 1 = −H or R 1 = −CH 3 ). Additionally, the o-NB group may contain no other aromatic substituents (R 2 =R 3 =−H), one or two aryl ethers, or other substituents.…”

Polyurea microcapsules with a photocleavable shell: UV-triggered release

“…29 Moreover, recent studies reported that NVOC-based moieties are also sensitive to NIR light with slower degradation kinetics in comparison to UV-light irradiation. 30,31 In this study, a straightforward method will be reported to design photocleavable microcapsules by the introduction of NVOC units in the shell structure. For this purpose, an NVOC-based diisocyanate oligomer has been synthesized from a photolabile diol and toluene 2,4-diisocyanate (TDI) in a condensation polymerization and used for the synthesis of photocleavable polyurea microcapsules.…”