Tailor made synthesis of amphiphilic azoaromatics via regioselective C–N bond fusion. Comparative studies of surface properties of the two positional isomers and cobalt complexes

Abstract

Tailor made synthesis of the isomeric azoaromatics, HL¹–HL⁴ [HL = (arylamino)phenylazopyridine] containing a single hydrophobic tail (C_n = C₁₀ and C₁₂) is described. The coordination induced C–N bond fusion synthetic protocol has been successfully used for the synthesis of the compounds, which are subsequently characterized using various spectroscopic techniques. The single crystal X-ray structure of compound HL⁴ has revealed that the hydrophobic chain in it orients itself with all-trans conformation of alkyl groups. Studies of their surface properties clearly demonstrate that these behave as surfactants. Amphiphilic properties of the compounds are followed by the studies of compression isotherms and their surface morphologies are studied with the use of high resolution field emission scanning electron microscopy (FE-SEM) as well as atomic force microscopy (AFM). Distinct differences in surface properties in the two HL isomers are observed and disposition of the hydrophobic tail with respect to the head group is shown to play a significant role in the organization process of the molecules at the air–water interface. Transferred monolayers of the above two isomeric compounds show agglomerated nano-domain structures. This phenomenon has been explained considering hydrophobic tail–tail repulsive interaction within the adjacent molecules. Surface properties of the double tail complex, [Co(L¹)₂]ClO₄ (1) along with that of the single tail complex, [Co(L¹)(L⁵)]ClO₄ (3) are also reported. Amphiphilic behavior of the above azoaromatics are distinctly different than those in their metal free state. Notably, the double tail complex (1) favors bi-layer formation even at low surface pressure region (∼10 mN m⁻¹). The single tail cobalt complex (3), on the other hand, forms a monolayer at high surface pressure region leading finally to the collapse at a very high pressure ∼60 mN m⁻¹.