Photochemical Sciences Ph.D. Dissertations

Early Events in Photochemistry of Aryl Azides Used as Photoaffinity Labeling Agents

Date of Award

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Alexander Tarnovsky

Second Advisor

Marshall Wilson (Committee Member)

Third Advisor

Craig Zirbel (Committee Member)

Fourth Advisor

Mikhail Zamkov (Committee Member)

Abstract

Aryl azides have been in focus of many studies for many years as an effective and reliable photoaffinity labeling and photocross-linking systems. It appears that the typical mechanism of cross-linking proceeds via ring expansion branch of nitrene chemistry. However, the photochemistry of aryl azides with powerful electron-donating groups deviates from the ring expansion mechanism and results in generation of basic nitrenes that protonate to form nitrenium ions. In the current work detailed studies of the ultrafast processes in photochemistry of 4-diethylamino-3-nitrophenyl azide and 5-azido-2-(N,N-diethylamino)pyridine have been conducted using ultrafast time-resolved spectroscopy and quantum chemical computation methods in order to better understand the photoreaction mechanisms of these compounds. Photolysis of 4-diethylamino-3-nitrophenyl azide leads to instantaneous formation of the closed-shell singlet nitrene which occurs on the second S2 singlet excited state surface in violation of the Kasha’s rule. The closed-shell singlet nitrene as a powerful base undergoes extremely fast protonation (5 ps, in methanol) in protic solvents forming nitrenium ion. The positive charge on the nitrogen atom in the resulting nitrenium ion is stabilized by the strong electron-donating 4-amino group providing a diiminoquinone-like species that relatively slowly collapses with formation of ultimate cross-linked product. Photolysis of 5-azido-2-(N,N-diethylamino)pyridine exhibits similar spectroscopic behavior at all chosen excitation wavelengths resulting in formation of the closed-shell singlet nitrene on the first S1 singlet excited state surface. The cooling of the initially formed hot closed-shell singlet nitrene is followed by intersystem crossing to the triplet nitrene. In polar protic solvents the formed triplet undergoes reverse intersystem crossing to the closed-shell singlet nitrene which can be protonated by the solvent molecules on 10-100 ns time domain forming nitrenium ion. Alternatively, the deactivation of the closed-shell singlet nitrene in protic solvents may proceed via competition between ISC and protonation processes which occur on the same time scale (17 ps in 2-PrOH). As a result, the protonation reaction would lead to the formation of a contact ion pair (CIP). This CIP will rapidly equilibrate to the solvent-separated ion pair (SSIP). The resulting nitrenium ion is stabilized by the strong electron-donating 4-amino group providing a diiminoquinone-like species that slowly collapses with formation of the final cross-linked product.

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