Do Columns of Azobenzene Stars Disassemble under Light Illumination?

Abstract

The clustering properties of star-shaped molecules comprising three photochromic azobenzene-containing arms are investigated with specific focus on the influence of light on these structures. Previous experimental works report self-assembly of azobenzene stars in aqueous solution into long columnar clusters that are detectable using optical microscopy. These clusters appear to vanish under UV irradiation, which is known to induce trans-to-cis photoisomerization of the azobenzene groups. We have performed MD simulations, density functional theory, and density functional tight binding calculations to determine conformational properties and binding energies of these clusters. Our simulation data suggest that the binding strength of the clusters is large enough to prevent a breaking along their main axis. We conclude that very likely other mechanisms lead to the apparent disappearance of the clusters.

Stacks of Azobenzene Stars: Self-Assembly Scenario and Stabilising Forces Quantified in Computer Modelling

“…In this work we investigate aqueous solutions of azobenzene star molecules with a planar benzene-1,3,5-tricarboxamide (BTA) core and three azo arms (azo-BTA star) ( Figure 1a). This star can be classified as rigid and quasi planar in its all-trans state, as we have found recently [14,29]. Its BTA core is a well-known supramolecular entity in colloid science, used for the fabrication of dynamic and responsive materials [40][41][42][43][44][45][46].…”

“…The quantification of the interactions is our next task. Partially, this task has been solved recently by us [29] for the same object of the study. In that article we have shown that the binding along the column is much stronger (up to a factor of six) as compared to the intercolumnar coupling.…”

“…These descriptors have been used by many researchers for the classification of the self-assembly mechanisms in BTA-containing systems [44,61,64,67]. The stacks of azo-BTA stars are analysed using data sets from molecular dynamics (MD) simulations [29] and from experiments [17] applying density functional theory and classical modelling techniques.…”

“…There are several classifications of star-shaped azos: they may differ in (i) the number of arms (three [9,10,[14][15][16][17][18][19][20], four [7,21,22], six [23][24][25], nine [26]), (ii) in their centres (nitrogen [9,19], phosphorus [27], silicon [28] or carbon atoms [21,22], benzene [14,16,20,[29][30][31][32], polycyclic aromatic hydrocarbons [25,33], heterocyclic hydrocarbons [34][35][36][37], some chiral groups [6] or bio-active residues [38]), (iii) in the way azobenzenes are attached to the centre (covalently or noncovalently [39]), (iv) in their conformational rigidity (flexible or rigid), (v) in their overall geometry (quasi-planar or 3D). Both the conformational rigidity and the shape of the star are closely related to the chemical nature of the star core, for instance, the planar conjugated fragment in the absence of any fatty linkers between the centre and the azobenzene arms supports the planarity of the star and its rigidity, and vice versa, long hydrocarbon linkers provokes its conformational flexibility [8,30,31].…”

“…Here, the optimised geometry is shown [14]. A snapshot of the columnar aggregate with N=20 from MD trajectory [29] is illustrated in panel (b). The azobenzene star studied experimentally by Lee et al [17] is depicted in panel (c).…”