Substituent effects intrans-p,p′-disubstituted azobenzenes: X-ray structures at 100 K and DFT-calculated structures

Abstract

The crystal and molecular structures of two para-substituted azobenzenes with π-electron-donating -NEt2 and π-electron-withdrawing -COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the -NEt2 group in N,N,N',N'-tetraethyl-4,4'-(diazenediyl)dianiline, C20H28N4, (I), may be caused by steric hindrance and the π-electron-donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp(2)-hybridization and the other is slightly shifted from the plane of the surrounding atoms. The molecule of the second azobenzene, diethyl 4,4'-(diazenediyl)dibenzoate, C18H18N2O4, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecules. The most significant changes are observed in the values of the N=N-C-C torsion angles, which for the isolated molecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II).

Azo-Label Heterometal–Organic Rhomboids Exhibiting Photoswitchable NIR Luminescence in Crystalline State

“…As expected, the azobenzene block presents in the form of trans -conformation. The length of the NN bond in complex 1 is 1.257 Å, which is consistent with the corresponding distance observed for azobenzene and its derivatives …”

“…The length of the NN bond in complex 1 is 1.257 Å, which is consistent with the corresponding distance observed for azobenzene and its derivatives. 28 The UV−vis absorption spectra of free ligand azo-H 2 L and its complex azo-Zn 2 Eu 2 (1) in DMF at room temperature are depicted in Figure 2. The analysis of the spectra of azo-free and azo-label ligands indicates the additional peaks at 376 nm and at 532 and 467 nm in azo-H 2 L are tentatively ascribed to π−π* and n−π* transition bands of the azo group, respectively.…”

Synthesis of Bis-β-Diketonate Lanthanide Complexes with an Azobenzene Bridge and Studies of Their Reversible Photo/Thermal Isomerization Properties

“…In the three complexes, all the C–N=N–C dihedral angles were close to 180° and N=N–C angles were in the range 111.7°–115.9°, in good agreement with the values found in azobenzene derivatives. 48…”

Self-assembly mechanism based on charge density topological interaction energies

“…Meaningful changes in the aromaticity indices [HOMA, NICS(0), NICS(1), NICS(1)zz, and PDI] are observed when the carboxylate groups participate in intermolecular hydrogen bonding (model III). This effect has been previously observed in the structure of 4,4′-(diazenediyl)dibenzoate [52] wherein the carbonyl oxygen atom of an ester group participates in hydrogen bonding to surrounding molecules. In the present case, hydrogen bonding to the carboxylate oxygen atoms slightly reduces the electron withdrawing ability of the carboxylate group and therefore reduces conjugation with the aromatic ring.…”