Chemical Communications Blog

Imagine if a pile of laundry could spontaneously organise itself into a neat, folded pile of clothes (and how useful this would be!). This principle can actually happen for some molecules in a process known as self-assembly, whereby a molecule or molecules will organise themselves spontaneously to form a supramolecular structure. Molecules can be specifically designed to self-assemble, by relying on the inclusion of functional groups and motifs that create repulsions and interactions to form unique nanostructures and functional materials.

Researchers in Japan have been studying scissor-shaped azobenzene dyads that can self-assemble by folding, using π-π stacking and hydrogen-bonding (Figure 1a and 1b). They previously observed two key self-assembly processes for two example dyads with formation of either toroidal (doughnut-shaped) structures through intramolecular folding, which can subsequently stack onto each other to create tubular motifs; or fibre/ribbon-type structures through one-dimensional helical stacking. The researchers hypothesised that these different self-assembly pathways could be attributed to differences in the degree of intermolecular aggregation of the dyads and set about verifying this using alkylated or perfluoroalkylated azobenzene dyads (2 and 3, Figure 1a) that have different aggregation properties.

The researchers prepared the supramolecular assemblies of the new dyads by cooling hot solutions (at 90 °C) of 2 or 3 in methylcyclohexane. Both absorption spectroscopy and dynamic light scattering (DLS) measurements indicated self-assembly of the dyads after cooling to 20 °C, and the resulting assemblies precipitated from the cooled solutions with minutes. The structures of these assemblies were then revealed by atomic force microscopy (AFM), showing different nanostructures for 2 and 3. The alkylated dyad 2 assembled into both toroidal and cylindrically extended nanostructures (Figure 2a and b), where the cylinders were composed of stacked toroidal components (as in Figure 1c). On the other hand, the perfluoroalkylated dyad 3 assembled into entangled fibres after cooling (Figure 2c and d), with no well-defined nanostructures observed even with moderate heating.

The researchers investigated the two different self-assembly pathways of 2 and 3 using multiple characterisation techniques. They found that the perfluoroalkylated dyad 3 self-assembles directly into extended fibres as a result of the enhanced intermolecular interactions provided by fluorophilic interactions, whereas the unique toroidal assembly for the alkylated dyad 2 is a result of weak molecular interactions. The extended fibres of self-assembled dyad 3 were found to form organogels at higher concentrations, in which the researchers also studied the photoresponsive properties. Overall, the results confirm the original hypothesis of different self-assembly pathways for dyads with different aggregation properties, and this study will guide future work into creating targets that can switch their self-assembly pathways under external stimuli.

 

To find out more, please read:

Self-assembly of alkylated and perfluoroalkylated scissor-shaped azobenzene dyads into distinct structures

Hironari Arima, Takuho Saito, Takashi Kajitani and Shiki Yagai *

Chem. Commun., 2020, 56, 15619-15622

 

About the blogger:

Dr. Samantha Apps recently finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

 

Nanomaterials are small and mighty. Scaling down to the molecular level imparts desirable properties to materials, for example high conductivity that is useful for electronic applications. Conjugated polymers (CPs) are one-dimensional nanomaterials with promising potential in optoelectronics and spintronics, due to their enhanced conductivity as a result of the delocalised π-electrons along the backbone of the polymer. Doubly-linked CPs, referred to as π-conjugated ladder polymers, have more unique features compared to their singly-linked counterparts, but such nanomaterials are often difficult to synthesise.

Polycyclic aromatic molecules such as acenes have shown potential as organic semiconductors and are therefore ideal materials for new CPs, but polymer formation by coupling the acenes is limited by repulsions between hydrogen atoms of adjacent acene motifs. These constraints can be overcome by installing wider ethynylene or cumulene π-conjugated bridges between the acenes, as demonstrated by researchers from Spain and the Czech Republic in their design and synthesis of a new one-dimensional π-conjugated ladder polymer (Figure 1). This ladder polymer is based on doubly-linked pentacenes (5 linearly-fused benzene rings).

The new conjugated ladder polymer was synthesised by vacuum deposition of the precursor molecule (5,7,12,14-tetrakis(dibromomethylene)-5,7,12,14-tetrahydropentacene, 8BrPN) onto a gold surface, Au(111). Subsequent thermal treatment up to 360 °C resulted in the formation of ethynylene linkages between the pentacenes, forming a ladder polymer on the gold surface (Figure 1a). The researchers determined the ladder polymer structure using scanning tunnelling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) techniques, as shown in Figure 1b-e. The STM images showed mostly one-dimensional chains with some disordered segments (Figure 1b), and the close-up image in Figure 1c confirms the double-linkages between each pentacene. The nc-AFM images show even more detail, confirming that the backbone of the polymer contains the pentacenes, doubly-linked to adjacent moieties in the 5,7,12 and 14 positions (Figure 1d/e). The nc-AFM characterisation also confirmed ethynylene (–C≡C–) rather than cumulene (=C=C=) linkages, shown by the brighter regions in the middle of the connections that correspond to the ethynylene triple bonds.

The researchers also probed the electronic structure of the new polymer on the gold surface using scanning tunnelling spectroscopy (STS). They determined an electronic bandgap that is 0.22 eV larger than that of a singly cumulene-linked pentacene polymer, thus demonstrating the advantages of the ladder-type structure in this instance. The technique used to create the pentacene ladder polymer in this report could be adopted for the creation of other π-conjugated ladder polymers with varying acenes, which could ultimately be useful for new nanomaterials in optoelectronics and spintronics.

 

To find out more, please read:

On-surface synthesis of doubly-linked one-dimensional pentacene ladder polymers

Kalyan Biswas, José I. Urgel, Ana Sánchez-Grande, Shayan Edalatmanesh, José Santos, Borja Cirera, Pingo Mutombo, Koen Lauwaet, Rodolfo Miranda, Pavel Jelínek,* Nazario Martín* and David Écija*

Chem. Commun., 2020, 56, 15309–15312

 

About the blogger:

Dr. Samantha Apps recently finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.