Low-temperature photochemistry of nitro-substituted aromatic azides; subsequent reactions of intermediates

Abstract

Triplet 4-nitrophenylnitrene ( 1B) and triplet 4-nitrene-4′-nitrostilbene ( 2B) are characterized by their low-temperature UV-Vis and ESR spectra as well as by their consecutive thermal and photochemical reactions. The main thermal reaction of 1B and 2B between 77 and 90 K is a stepwise, twofold hydrogen abstraction from the methyltetrahydrofuran (2-MTHF) matrix solvent, leading to the corresponding primary amines. The activation energy of the first hydrogen abstraction of 1B and 2B is not influenced by their structure and is about 30 kJ mol −1. A similar amount of energy is required for the second hydrogen abstraction. Since the activation energy required for diffusion in a solid 2-MTHF matrix is about 87 kJ mol −1, bimolecular processes that require diffusion are less efficient between 77 and 87 K. While 2B is photochemically stable, 1B reacts after optical excitation under hydrogen abstraction from the solvent to form an imino radical 1C. The photochemically induced hydrogen abstraction after the excitation of 1B and the absence of the analogous reaction after the excitation of 2B can be explained by means of two models; either as a hot ground state reaction or as a reaction starting from an electronically excited state of 1B. In the latter model, it is assumed that the activation energy of the hydrogen abstraction is lower in the first excited state than in the electronic ground state of 1B. Both models predict that the hydrogen abstraction is more efficient if the difference in energy between the electronic ground and electronically excited states is high enough. This is the case in 1B but not in 2B. The experimental results are discussed in connection with theoretical statements arising from quantum chemical calculations.