Photoisomerization of azobenzenes isolated in cryogenic matrices

Abstract

2,2'-Dihydroxyazobenzene (DAB), 2,2'-azotoluene (AT) and azobenzene (AB) were isolated in argon and xenon matrices and their molecular structures and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. All these compounds can adopt the E and Z isomeric forms around the central CNNC moiety, which can be enriched by several conformational and tautomeric modifications for DAB and AT. A number of DAB and AT isomeric forms were identified for the first time. For DAB, the E azo-enol isomer with two intramolecular six-membered quasi-rings formed via OHN hydrogen bonds was found after deposition. Irradiation with UV light generated a different E azo-enol form with two intramolecular H-bonded five-membered quasi-rings. Phototransformation was shown to be reversible and the forms could be interconverted by irradiation at different wavelengths. The isomerization between these two forms constitutes a direct experimental observation of an E → E isomerization in azobenzene-type molecules. Further irradiation generated a form(s) bearing both OH and NH groups. For AT, two E isomers with the CH3 groups forming five-membered and five/six-membered quasi-rings with the azo group were observed in the as-deposited matrices. Irradiation of AT with UV light generated a Z form that can be converted back to the E form at different irradiation wavelengths. E-AB was deposited in a xenon matrix and both E → Z and Z → E phototransformations were observed (contrary to what was previously reported in an argon matrix where only the Z → E conversion occurred). AB photoisomerization becomes more pronounced at elevated temperatures, thus indicating that the matrix effects responsible for hindering the AB photoisomerization are essentially due to steric restrictions. The different photoisomerization channels observed for these compounds are discussed in terms of a small-amplitude pedal motion.

An Intramolecular Hydrogen‐Shift in a Peroxy Radical at Cryogenic Temperatures: The Reaction of 2‐Hydroxyphenyl Radical with O2

“…In agreement with previous gas‐phase studies, thermolysis of 2‐iodophenol does not yield 1 but leads to the isomeric phenoxy radical which further decomposes to CO and cyclopentadienyl . We also subjected 2,2′‐dihydroxyazobenzene to photolysis and pyrolysis conditions but only found hitherto described isomerization products . Therefore, we had to use the inefficient photolytic preparation of 1 to probe its reactivity towards oxygen.…”

Photoisomerization of a Chiral Imine Molecular Switch Followed by Matrix‐Isolation VCD Spectroscopy

“…The matrix‐isolation (MI) technique is a well‐established method for studying photochemical rearrangement reactions and identifying reactive intermediates . It involves the isolation of the target molecules or their precursors in a solid inert‐gas matrix at very low temperatures (e.g.…”

“…[5] In contrast, most ultrafast spectroscopies are currently not sensitive enough to address chirality (with afew exceptions) [6] and can only be used to prove the existence of atransient species. [7] Them atrix-isolation (MI) technique [8] is aw ell-established method for studying photochemical rearrangement reactions [9] and identifying reactive intermediates. [10] It involves the isolation of the target molecules or their precursors in asolid inert-gas matrix at very low temperatures (e.g.…”

The curious case of the photochemistry of 2-hydroxyphenylazo-3,5-dimethylisoxazole: unravelling the process among tautomerization, photoisomerization, and conformational changes

“…In yet another study, the same group has also explored the photochemistry of 2,2′-dihydroxyazobenzene (DAB) and azotoluene (AT). 16 Upon irradiation, DAB exhibited reversible E–E isomerization (conformational changes) and tautomerization. Surprisingly, no Z form was observed due to the presence of strong intramolecular hydrogen bonding (OH⋯N) interactions.…”