The Synthesis, X-ray Structure Analysis, and Photoisomerization Assessment of the Acyclic Triazenes 4-[(E)-2-(biphenyl-4-yl)diazenyl]-morpholine and 1,3-Bis(biphenyl-4-yl)triazene

Abstract

The acyclic triazenes 4-[(E)-2-(biphenyl-4-yl)diazenyl]-morpholine (1) and 1,3-bis(biphenyl-4-yl)triazene (2) were prepared and their X-ray crystal structures determined. Compound 1 crystallized in the monoclinic space group P21/c with a = 29.153 (5) A, b = 6.0444 (14) A, c = 7.5997 (15) A, and β = 96.03 (1)°. Compound 2 crystallized as the 1/4 cyclohexane solvate in the triclinic space group P $$\bar{1}$$ with a = 10.3228 (10) A, b = 13.4541 (14) A, c = 15.4323 (15) A, α = 87.158 (4)°, β = 76.030 (4)° and γ = 71.492 (5)°. The N–N double bond in 1 and 2 adopted an (E)-configuration and had bond angles that ranged from 112.19 (9)° to 114.01 (13)°, which deviated from optimal trigonal planar geometry by 6°–8°. A degree of π-conjugation across the acyclic triazene moieties in compounds 1 and 2 was indicated by N–N double and single bond lengths that were respectively longer and shorter than the literature values for these types of bonds. The room temperature electronic absorption spectra of 1 and 2 in various solvents were both dominated by an intense band above 300 nm that was assigned to a combination of π → π* and n → π* transitions. Compounds 1 and 2 in acetonitrile and methanol solution were found to undergo trans → cis photoisomerization. The magnitudes of trans → cis rate constants were rationalized in terms of the degree of solvent stabilization of the dipolar resonance hybrid structure of the acyclic triazene subunits in 1 and 2. Irradiation of chloroform solutions of 1 and 2 caused trans → cis photoisomerization and radical decomposition to occur. The X-ray structures of compounds 1 and 2 were determined and their photoisomerization kinetics explored.