Growing organic crystals

In a conference held on November 2014 at the Solvay room of the Université Libre de Bruxelles, most of the speakers graced their talks with the phrase “Crystallization is a miracle”. They were talking from the physical and chemistry point of view, of course; however, us humans have always been fairly fascinated with crystals.

Perhaps the most common example of crystals are diamonds. These precious stones are made exclusively out of carbon, the same material that we use for pencils and to burn in order to generate heat during winter time – carbon is special enough to crystallize in different “shapes”, thus leading to the million dollar material found in high class jewlery or to simple tools for writing.

Carbon goes beyound diamond and graphite, and it is a so interesting and important element that chemist have developed a branch of their research dedicated exclusively to matter containing carbon atoms: “organic chemistry” – and thus, by analogy, all matter containing carbon atoms is called “organic”.

Recently I’ve had the chance to work on “small organic molecules”. These were molecules, that contained carbon atoms, and which in comparison to polymers are small, that is, the number of atoms in these molecules is relatively low.This fact is neither good nor bad; however, it can provide some advantages. For example, these molecules readily crystallize. Going into detail, the name of my molecule was “2,7-dioctyloxy[1]benzothieno[3,2-b]-benzothiophene”, and they can form crystals in such a way a conductive channel can be created. This means that after crystallization, the molecules allow electrons to flow from one side to the other, or in other words, an electrical current can circulate through the molecules.

The first investigations regarding the crystallization of this molecule were performed by Andrew Jones. In his paper, Jones reveals that when the molecule is prepared as a very thin layer (1000 times smaller than a human hair), still preserves its inherent crystallization, but also new crystals can be formed. These new features are called “substrate-induced phases” and are believed to berelated to the interaction of the molecule with its supporting material, generally a silicon wafer. Our idea was somehow to change this classic system. Following previous works we decided to mix the organic molecule with polystyrene and, after deposition of a thin layer, we found the molecule immersed in the soft matrix of the polymer, contrary to the usual case of hard silicon. The image below is an optical microscopy photograph of the prepared thin layer. In a general view the roughness, resembling a snake-skin kind of pattern, shows the distribution of the small organic crystals on the polystyrene matrix.


The crystals distributed on the polymer were allowed to grow, using a technique called “solvent vapor annealing” (SVA). Here, the system is subjected to a solvent atmosphere allowing the molecules to move around and encounter one another. This leads (in theory) to the grow of bigger crystals. In the case of the molecule on the hard silicon substrate, Jones found that under SVA no strong changes took place; nonetheless, we have found the development of perfect tetragonal-shaped crystals, as the ones shown in the following picture.


The step-by-step evolution of this system is detailed on the paper we published in the journal ChemPhysChem. We have been able to obtain details on the crystallization process evolution, and we have related the possibility of growing these big, beautiful crystals to the action of the polystyrene on the movement of molecules. Since under SVA both the polymer and the molecule are allowed to move, the soft polymer matrix is serving as a “motor” to allow more and more molecule to get together and finally permit the development of the tetragonal-shaped crystals. This result is of importance towards proposing new methods for “a la carte” crystal grow, as well as for understanding the role of polymer thin layers in possible electronic applications.


Technical aspects

This work was carried out in 2015 at the Université libre de Bruxelles. The results are published in ChemPhysChem, part of Wiley editorial. Please follow this link to access the full paper. All images shown in this blog post belong to D. E. Martinez-Tong and cannot be used before asking for permission. Contact and inquiries:

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