[1.1](3,3‘)-Azobenzenophane:  Novel Crystal Structure and Cis−Trans Isomerization of Distorted Azobenzene

Abstract

[formula: see text] [1.1](3,3')-Azobenzenophane, in which two azobenzenes are cyclically connected by -CH2- chains at the meta positions, has been synthesized. The crystal structures of all isomers have been revealed. This is the first report on the crystal structure of the cis isomer of macrocyclic azobenzenes. The trans,trans isomer was slightly distorted, the trans,cis isomer highly deformed, and the cis,cis isomer unstrained. The thermal stability of cis isomers in solutions are deducible from the crystal structures.

Photochemical and Thermal cis/trans Isomerization of Cyclic and Noncyclic Azobenzene Dimers: Effect of a Cyclic Structure on Isomerization

“…The small spectral difference between (Z,Z)-1 and (Z,Z)-2 is attributable to the unstrained structure of (Z,Z)-1, deducible from the crystal structure of (Z,Z)-1, in which the structure of an azobenzene unit is similar to the crystal structure of (Z)-azobenzene. [11] In general, upon irradiation at the π,π* band the photostationary state of azobenzenes lies close to the (Z) isomer, while irradiation at the n,π* band shifts it to the (E) isomer. The compounds in this study can each exist as three isomers, and the existence of the intermediate (E/Z) isomers makes the systems more complex.…”

“…The synthesis of 1 by reduction of the corresponding dinitro compound has been reported previously. [11] The noncyclic azobenzene dimer 2 consists of two azobenzene units connected through a single methylene bridge at their meta positions. The synthesis of 2 was achieved by employing a condensation between bis(3-aminophenyl)methane and 3-nitrosotoluene in acetic acid.…”

“…These observation is consistent with the crystal structures of 1, which indicated that (E,E)-1 was slightly deformed, (E,Z)-1 was significantly distorted, and (Z,Z)-1 was not deformed. [11] In (E,Z)-1, for example, the dihedral angle between the benzene ring and the N=N double bond is nearly 90°in the (Z)-azobenzene unit. [11] This indicates that the transition probability from n to π* orbitals is reduced by the deformed structure of (E,Z)-1, resulting in a relatively small molar extinction coefficient of (E,Z)-1.…”

“…[11] In (E,Z)-1, for example, the dihedral angle between the benzene ring and the N=N double bond is nearly 90°in the (Z)-azobenzene unit. [11] This indicates that the transition probability from n to π* orbitals is reduced by the deformed structure of (E,Z)-1, resulting in a relatively small molar extinction coefficient of (E,Z)-1. The small spectral difference between (Z,Z)-1 and (Z,Z)-2 is attributable to the unstrained structure of (Z,Z)-1, deducible from the crystal structure of (Z,Z)-1, in which the structure of an azobenzene unit is similar to the crystal structure of (Z)-azobenzene.…”

“…[11] Both the thermal and photochemical isomerization processes in solution were different from those of strain-free azobenzenes. [11] The effect seems to be due to the strain imposed by the cyclic structure.…”