Fragmentation of Protonated Dimethylaminoazobenzene during Laser Pulsed Photolysis in a Cryogenic Atmosphere of Rarefied
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OCHEMISTRY AND MAGNETOCHEMISTRY
Fragmentation of Protonated Dimethylaminoazobenzene during Laser Pulsed Photolysis in a Cryogenic Atmosphere of Rarefied Helium Yu. A. Mikheeva,* and Yu. A. Ershovb† a Emanuel
Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia b Bauman Moscow State Technical University, Moscow, 105005 Russia *е-mail: [email protected] Received January 9, 2020; revised January 9, 2020; accepted January 21, 2020
Abstract—An analysis is performed for data on the composition of cationic fragments formed during the pulsed photolysis of protonated form of dimethylaminoazobenzene dye in a rarefied helium atmosphere at T ≈ 40 K. It is shown that the interpretation of the mechanism of photolysis, which is based on the traditional representation of this dye as a monomeric azonium cation, is incorrect. An adequate mechanism of the process is obtained using a new concept, according to which the ground state of protonated dimethylaminoazobenzene and its substituted derivatives are rydimers with two protonated amino groups. It was established that the photoexcitation of these rydimers in an atmosphere of rarefied helium results their deprotonation, accompanied by the formation of electrically neutral monomers and monomers with positive charges on sp2 orbitals. Both types of monomers react in a helium atmosphere with released protons, splitting into different cationic fragments. Formulas for the structure of fragments and schemes of monomer fragmentation are presented. Keywords: protonated dimethylaminoazobenzene, rydimers, gas phase photofragmentation, cationic fragments DOI: 10.1134/S0036024420110254
INTRODUCTION Recent works have analyzed the results accumulated over the last century from experimental studies of the UV–Vis spectroscopic and structural-chemical properties of aminoazobenzene dyes [1–5]. They have shown the inadequacy of the concept formulated in the early stages of organic chemistry and still in use, according to which simple dyes based on aminoazobenzene in their ground state are monomers. In reality, their main state is Rydberg dimers (rydimers). In addition, the color characteristic of azo dyes is associated not with their quinoid monomers, as was believed for more than 100 years, but with phenylaminyl type cations (PhAT) belonging to the rydimers. The results in [1–5] created conditions for reviewing the literature interpretations of experimental data on the photochemistry of aminoazobenzene dyes in the context of their rydimer structure. Enhancement of the acidity of protonated aminoazobenzene rydimers upon their photoexcitation was established in [6]. Details have been revealed of the photonics of 4aminoazobenzene rydimers in an ethanol medium [7, 8], methyl orange in an aqueous medium [9], 4-N,N† Deceased.
diethylaminoazobenzene in isopropanol [10], and monomers of protonated azobenzene in a cryogenic atmosphere of helium [11]. In this work, we analyze data from [12] on the photofragmentation of protonated 4-N,N-dimethylaminoazobenzene ((ABN+R2H)2, R =
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