Abstract
In this paper, the columnar supramolecular aggregates of photosensitive star-shaped azobenzenes with benzene-1,3,5-tricarboxamide core and azobenzene arms are analyzed theoretically by applying a combination of computer simulation techniques. Without a light stimulus, the azobenzene arms adopt the trans-state and build one-dimensional columns of stacked molecules during the first stage of the noncovalent association. These columnar aggregates represent the structural elements of more complex experimentally observed morphologies—fibers, spheres, gels, and others. Here, we determine the most favorable mutual orientations of the trans-stars in the stack in terms of (i) the – distance between the cores lengthwise the aggregate, (ii) the lateral displacements due to slippage and (iii) the rotation promoting the helical twist and chirality of the aggregate. To this end, we calculate the binding energy diagrams using density functional theory. The model predictions are further compared with available experimental data. The intermolecular forces responsible for the stability of the stacks in crystals are quantified using Hirshfeld surface analysis. Finally, to characterize the self-assembly mechanism of the stars in solution, we calculate the hydrogen bond lengths, the normalized dipole moments and the binding energies as functions of the columnar length. For this, molecular dynamics trajectories are analyzed. Finally, we conclude about the cooperative nature of the self-assembly of star-shaped azobenzenes with benzene-1,3,5-tricarboxamide core in aqueous solution.
Highlights
In recent years, azobenzene-containing molecules, which are reversibly responding to the light stimulus, have been widely applied in science and engineering to construct materials with various photosensitive characteristics [1,2,3,4,5]
Employing a number of computer simulation techniques, we have analyzed the intermolecular interactions between the azo–BTA stars in columns in experimentally resolved crystals [17] and as obtained in our recent molecular dynamics study [29]
Our results suggest that during the formation of ordered phases, these molecules prefer to build 1D columns with the equilibrium distance between the star cores of 3.6 Å, in which stars are held together via weak interactions. These noncovalent interactions include among others the hydrogen bonding, whose length shortens with increasing the column length
Summary
Azobenzene-containing molecules (azos), which are reversibly responding to the light stimulus, have been widely applied in science and engineering to construct materials with various photosensitive characteristics [1,2,3,4,5]. Star-shaped azos, i.e., multi-photochromic compounds with azobenzene moieties attached to the central unit as arms, have received considerable attention in the research of the past decade. On one hand, such molecules combine mesomorphism and photoswitching [6,7,8], they can more effectively reply to the light stimulus (for example, during inscribing a surface relief grating) as compared to monomeric azos. Star-shaped azos are different from linear or cyclic azo n-mers with the same number of azo-fragments (trimers, tetramers, etc.) First of all, this is manifested in different supramolecular structures the stars and the linear or cyclic [11] n-mers can build. The branching results in lower melting temperatures, and lower viscosity [6,12,13], meaning that these properties are topology-dependent
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