Understanding an efficient light harvesting material

When we fill porous materials with dye molecules of the right size, we obtain useful compounds for solar energy technology. These compounds can transfer solar energy efficiently because pores and channels fit to the dyes “like a glove”. In this way, molecules are forced to stay in line, and energy can easily pass from a molecule to the next one in the line. If we knew in detail the structure of the dye arrays, we’d have better chances to improve these compounds.

Unfortunately, the precise positioning of the molecules inside the pores is very hard to determine.  Recently, we solved this problem for a class of particularly efficient dyes filling the channels of zeolite L.  Key to success was diversity within the team, which favored the combination of multiple techniques involving both experiments and calculations.

The useful properties of these materials arise from the arrangement of dye molecules inside the porous host, which depends on the interactions among molecules and with the porous host. After this work, now it seems we understand a little better these complex materials. Indeed, our dyes are linear, symmetric and fit to the zeolite channels. Yet they adopt a slightly asymmetric positioning to maximize the interactions with the zeolite cations, which stabilize the compound.

Perylene-bisimide dye (cyan) in zeolite L (gray). The purple spheres represent the zeolite potassium cations

This work also suggests some possible ideas to improve these compounds by modifying either the porous container (the “host”) or the dye molecule (the “guest”). In my view, this is also a good example of how computational modeling may help to rationalize experimental results in apparent contrast with each other, yielding a consistent picture of a useful and intriguing material.


Gigli et al. (2018)  “Structure and Host–Guest Interactions of Perylene–Diimide Dyes in Zeolite L Nanochannels”  J. Phys. Chem. C 122, 6, 3401-3418

How large molecules cross narrow pore entrances

How can a snake swallow a mouse bigger than its mouth?

Weird as it seems, questions like this emerge very often at the molecular scale. For example, we can fill porous materials with molecules larger than the diameter of the pores: in this way, we may obtain devices for energy and health applications. What makes this useful process possible? Flexibility is the key: both the porous host (the “snake”) and the molecule (the “mouse”) must deform for the process to occur. But here, contrary to the mouse-snake case, cooperation between the two partners is needed.

We captured the passage of a bulky molecule through the very narrow opening of one of these pores. We did this by computer simulations, because it is very hard to get such information experimentally. To get an idea of what we found, you don’t even need to read the paper – and i’m not kidding. Just look at the movie below!

What we’ve seen first, is that the pore is slightly larger at its entrance. This surely helps the molecule to go in.

Second: contrary to the mouse, which would escape the snake as fast as it could, the molecule is indeed “magically” drawn to the pore entrance – by electrostatic forces.

“So what?” – you may say.

Keep in mind that the molecule is still larger than the pore opening. No kind of “fatal attraction” could do the trick, in a world of rigid bodies.

We’ve found that the molecule can pass through the opening and slip inside the pore only because it’s flexible, and its motion is “in tune” with the vibrations of the porous matrix. All this factors cope to make the entrance process more favorable than the exit process – that’s why the molecule gets finally swallowed by the pore, and remains trapped inside the material.

For me, it was very nice to see how bulky molecules manage to pass through narrow openings and travel inside a porous material. But finding out the reason why they stay inside was, probably, even more exciting:  because it explains how materials of this kind can form and remain stable. Which is exactly one of the things you may need, in the quest of  easier and smarter ways to produce better materials.

 As we have to give credit where credit is due, i must confess that i borrowed the mouse-and-snake idea used in this post. But you’ll never know from whom. Me neither: (s)he was an anonymous referee of the paper. I am very grateful to this person: i can hardly imagine a nicest way to sketch our work.
Many thanks, of course, also to ChemComm for the cover!