J-like Aggregation of Azobenzene-Linked Amphiphile, C₆AzoC₅N⁺Br^- in Single Crystal

The crystal structure of an azobenzene-linked, single-chain ammonium amphiphile (C₆AzoC₅N⁺Br^-), C₂₇H₄₂N₃O₃Br^- was determined by X-ray diffraction. The crystal is triclinic with the space group PI, Z=2, a=2.7446(9), b=0.8592(1), c=0.6112(1) nm, α = 106. 91(2), β=87. 05(2), and γ = 93.18(4)°. The structure was solved by the direct method and refined by the block-diagonal least-squares method to the R-value of 0.060 for 3535 reflections (sin θ/λ0.56). In contrast to other azobenzene derivatives, azobenzene part of the C₆-AzoC₅N⁺Br^- molecule has very good planarity; tortion angles between the azo moiety and the benzene rings are within 2°. The zigzag planes of the spacer alkyl and the tail alkyl parts are linked to the azobenzene plane with small tortion angles, 0.1 and 6.3°, respectively; this shapes the whole molecule a long thin plate. The crystal structure consists of regularly stacking bimolecular layers along the a-direction in which azobenzene chromophores aggregate in the J-like (head-to-tail) fashion between adjacent molecules. The detailed molecular and crystal structure of C₆-AzoC₅N⁺Br^- is very similar to that of the previously analyzed homologous compound, C₁₂AzoC₅N⁺Br^-.