Let us pause to consider the weirdness of diamond. Not because diamonds are rare – they’re not, at least compared to many other minerals and gemstones. But diamond itself has very unusual physical properties, and that comes down to its structure. As is well known to chemists, it’s a three-dimensional lattice of bonded sp3 carbons, which sort of stretches ones idea of what a molecule is. Hold that thought, though: can you name another crystalline organic substance that’s assembled from a framework of covalent bonds?
I’ll bet not. In fact, until very recently there have been
no examples at all of any single crystals of a substance like that (other than diamond itself). Update: I missed the azodioxy-linked ones reported by the Wuest group at Montréal!) You can get terrific crystals of metal-organic frameworks, where organic spacer groups are held together at the junction points by coordination to metal ions – I was able to spend some time a couple of years ago growing such things, and it’s a blast. But a crystal of a sheer covalent-bond-framework material, that’s something else again. People have made such substances, but they almost always come out as stuff that can only be characterized by powder X-ray diffraction at best, and amorphous gorp at worst.
The problem is assembling the framework. If you picture the process down on the molecular level, you have to have each unit come in, find its proper place, form bonds between the appropriate neighbors, and wait for something to come bond with it in turn. If you have a unit come in and dangle off the surface a bit, then the next building block that comes in might bond with it instead of with the growing surface, and now both of those have to come down together into the right place. And what’s surely going on, even more than that, is the start of untold millions of tiny frameworks all over the bulk solution, nucleation sites everywhere, with zero chance of them every fitting together into something larger in any kind of ordered fashion.
There are only a few ways out of this situation, as far as anyone knows. One is the diamond-forming process, where the final product is a thermodynamic sink. You can just take less orderly forms of carbon and heat and squeeze them under extreme conditions until the bonds rearrange into the lowest-energy structure. It helps if you let geology do the work for you. Diamonds are believed to be distributed near the surface by kimberlite eruptions, most of which seem to have taken place a very long time ago (and a good thing, too), and the diamonds themselves – along with many other other interesting igneous minerals – are apparently formed at great depths in the Earth’s mantle. This is not going to be a general process for covalent organic frameworks.
The other main way you can imagine making them would be if you could have some sort of reversible bond formation involved. That way, you could in theory adjust the rate of crystal growth by changing the conditions and thus let things come along in a more ordered way. That’s the secret to growing good crystals of anything, actually – slow and careful, with plenty of chances for mistakes to be corrected down at the surface. And now it’s been done with covalent organic materials.
Here’s a new paper from a large multicenter team (involving the Yaghi group at Berkeley who are very experienced in the related metal-organic framework field). They take the tetra-amine building block at right and the dialdehyde to form the large open framework shown, held together by imine bonds, but if you just mix those two together, you’re going to get a terrible mess. The key is to add aniline (about 15 equivalents) during the crystal-growing process. This forms a reversible imine with the free aldehyde groups, and slows the crystal formation down enough to allow things to settle into a reasonable form. They have very nice resolution of crystals of the parent and its hydrate, with water molecules filling the empty spaces. You’d look at that structure and say “That’s a lot of room”, but that’s a schematic: the real structure is interlaced and interpenetrated (sevenfold, actually), so there’s not a lot of empty space in the crystal itself. The same thing happens very often with metal-organic frameworks; it can be tricky to grow crystals with huge open spaces in them, although that depends completely on the geometries involved. Interestingly, if you look at this structure, it’s basically a huge diamondoid lattice: what you’re looking at is a whopper adamantane, the repeating diamond unit, since the branch points are also tetrahedral. (Actual diamond doesn’t have the room to have an interpenetrated lattice). Update: the nitroso/azodioxy framework reported earlier are also from a tetraphenylmethane scaffold, and form a similar diamondoid lattice, fourfold interpenetrated.
If you switch the functional groups, with a tetrabenzaldehyde-methane and the 1,4-diamine, you get crystals as well under the aniline conditions. In fact, they are almost identical in every way – the only way you can tell one from the other, apparently, is through single-crystal X-ray. The team went on to form other covalent organic framework crystals with different building blocks, so the slow-it-down-with-aniline conditions seem to be reasonably general for imine frameworks. No doubt you could mess around with more or less reactive anilines or other amines to refine the conditions for a wide range of these things. Crystal growing, as has been mentioned around here before, is a brutally empirical art.
And simultaneously, here’s a paper from Northwestern/Cornell/Argonne showing macrocrystalline two-dimensional covalent organic frameworks for the first time. You get things like this in the metal-organic framework field as well. They can look like three-dimensional crystals, but if they’re put into a slightly stronger solvent than the one used to form them or given any mechanical stress, they delaminate into vast numbers of sheets. Watching a clear crystal turn into a pile of what looks like Danish pastry or a spilled-over deck of cards under the microscope is an odd experience, I can report. The strong bonds are in two dimensions, and the sheets are able to just slide over each other: instead of diamond, the exact carbon equivalent is graphite, with sheets of graphene. And as with the three-dimensional cases, graphite has been the only organic example known that’s not an amorphous/microcrystalline material.
These new materials remain as individual two-dimensional sheets, though, at least in some cases (other tend to stack up on each other when imaged on a surface). The team prepares them by forming boronate esters, and adjusting things by very slow addition of the building blocks involved. The first reaction goes through a large number of nucleation sites, as mentioned earlier, and if you scale that up or keep throwing in monomers you just get more tiny particles. But if you take that initial solution and switch to slow addition, you can grow the existing nano-sized seed crystals (which are about 30 nm wide) rather than forming new ones. This took the average size up to about 400 nm, and further tweaking of the conditions on these allowed even larger particles to be produced. These are now large enough to be well-defined by electron microscopy images, and optical transient absorption experiments showed them to be far better behaved than any such materials ever tried before, with a thousandfold increase in signal/noise.
So both of these papers report completely new materials, which will have properties that remain to be explored. As with the metal-organic frameworks and the two-dimensional inorganic materials, the structural, optical, and electronic behavior of these things is an open book, and getting a handle on these things could lead to exotic materials that do things that no one has ever been able to accomplish.