3 research outputs found
5-azido-2-aminopyridine, A New Nitrene/nitrenium Ion Photoaffinity Labeling Agent That Exhibits Reversible Intersystem Crossing Between Singlet And Triplet Nitrenes
The photochemistry of a new photoaffinity labeling (PAL) agent, 5-azido-2-(N,N-diethylamino)pyridine, was studied in aprotic and protic solvents using femtosecond-to-microsecond transient absorption and product analysis, in conjunction with ab initio multiconfigurational and multireference quantum chemical calculations. The excited singlet SI state is spectroscopically dark, whereas photoexcitation to higher-lying singlet excited S-2 and S-3 states drives the photochemical reaction toward a barrierless ultrafast relaxation path via two conical intersections to S-1, where N-2 elimination leads to the formation of the closed-shell singlet nitrene. The singlet nitrene undergoes intersystem crossing (ISC) to the triplet nitrene in aprotic and protic solvents as well as protonation to form the nitrenium ion. The ISC rate constants in aprotic solvents increase with solvent polarity, displaying a direct gap effect, whereas an inverse gap effect is observed in protic solvents. Transient absorption actinometry experiments suggest that a solvent-dependent fraction from 20% to 50% of nitrenium ions is generated on a time scale of a few tens of picoseconds. The closed-shell singlet and triplet nitrene are separated by a small energy gap in protic solvents. As a result, the unreactive triplet state nitrene undergoes delayed, thermally activated reverse ISC to reform the reactive closed-shell singlet nitrene, which subsequently protonates, forming the remaining fraction of nitrenium ions. The product studies demonstrate that the resulting nitrenium ion stabilized by the electron-donating 4-amino group yields the final cross-linked product with high, almost quantitative efficiency. The enhanced PAL function of this new azide with respect to the widely applied 4-amino-3-nitrophenyl azide is discussed
5‑Azido-2-aminopyridine, a New Nitrene/Nitrenium Ion Photoaffinity Labeling Agent That Exhibits Reversible Intersystem Crossing between Singlet and Triplet Nitrenes
The
photochemistry of a new photoaffinity labeling (PAL) agent,
5-azido-2-(<i>N</i>,<i>N</i>-diethylamino)Âpyridine,
was studied in aprotic and protic solvents using femtosecond-to-microsecond
transient absorption and product analysis, in conjunction with <i>ab initio</i> multiconfigurational and multireference quantum
chemical calculations. The excited singlet S<sub>1</sub> state is
spectroscopically dark, whereas photoexcitation to higher-lying singlet
excited S<sub>2</sub> and S<sub>3</sub> states drives the photochemical
reaction toward a barrierless ultrafast relaxation path via two conical
intersections to S<sub>1</sub>, where N<sub>2</sub> elimination leads
to the formation of the closed-shell singlet nitrene. The singlet
nitrene undergoes intersystem crossing (ISC) to the triplet nitrene
in aprotic and protic solvents as well as protonation to form the
nitrenium ion. The ISC rate constants in aprotic solvents increase
with solvent polarity, displaying a “direct” gap effect,
whereas an “inverse” gap effect is observed in protic
solvents. Transient absorption actinometry experiments suggest that
a solvent-dependent fraction from 20% to 50% of nitrenium ions is
generated on a time scale of a few tens of picoseconds. The closed-shell
singlet and triplet nitrene are separated by a small energy gap in
protic solvents. As a result, the unreactive triplet state nitrene
undergoes delayed, thermally activated reverse ISC to reform the reactive
closed-shell singlet nitrene, which subsequently protonates, forming
the remaining fraction of nitrenium ions. The product studies demonstrate
that the resulting nitrenium ion stabilized by the electron-donating
4-amino group yields the final cross-linked product with high, almost
quantitative efficiency. The enhanced PAL function of this new azide
with respect to the widely applied 4-amino-3-nitrophenyl azide is
discussed