Editorial

The importance of co-crystals continues to grow within the field of crystal engineering, and co-crystalline materials are now emerging across a wide range of applications. There is an increasing focus on the properties and practical applications of co-crystals, with a strong emphasis on the design and synthesis of functional co-crystals. A recurring theme is therefore whether co-crystals can offer advantages in function and performance over single components. The often heated debates about the nomenclature of co-crystals, salts, and solvates are now subsiding, with recognition that a particular classification may actually depend on the conditions of temperature (and even pressure) under which these materials are studied. The collection of papers presented in this themed issue encompasses many important aspects of functional co-crystals and reflects current research interests in this field.

The area that is currently attracting most interest is that of pharmaceutical co-crystals, where the control of solubility (and hence bioavailability) of the active pharmaceutical ingredient (API) is of critical importance. Several of the papers describe co-crystallisation strategies that increase the solubility of the API, and one of the papers demonstrates that on occasions it can be desirable to decrease the solubility of the API in order to facilitate slower release of the drug. The suppression of phase changes in APIs can also be achieved through the formation of solid solutions. Particularly interesting are “ionic co-crystals”, which feature components coordinated with simple metal salts. These materials are becoming more widespread and extend the scope for delivery of bio-active compounds. Another aspect is represented by API molecules that have only limited capacity to engage in hydrogen bonding – the strategy is therefore instead to exploit halogen-bonding interactions to form co-crystals. A recurring challenge is the requirement to establish predictable links between molecular structure and macroscopic physical properties – this is successfully exemplified through the positive correlation between molecular characteristics and bulk physical properties, e.g. melting point. A critical evaluation is also presented about the barriers to widespread adoption of pharmaceutical co-crystals as realistic options for drug delivery. One of the emerging challenges for co-crystals is scale-up from laboratory to large-scale manufacture. This is exemplified in one of the papers, which highlights the strong drive towards continuous manufacture, its associated challenges, and how these challenges may be overcome.

Perhaps one of the most striking areas in which co-crystals play a role is in the modification of the optical properties of materials. Examples in this issue include: thermochromism, as a result of either proton migration or order–disorder transitions; modulated fluorescence, achieved by suppression of π-stacked aromatic molecules; and enhanced non-linear optical (NLO) effects achieved by co-crystallisation of two components that are themselves NLO materials. Magnetic properties of radical species may also be altered through co-crystallisation, resulting in 1-D antiferromagnetic exchange interactions between radical sites.

Another emerging area is the control of reactivity through co-crystallisation. This is exemplified in the field of energetic materials (explosives and propellants) where the sensitivity to initiation can be modified by co-crystallisation. A further example of the control of reactivity is provided by the photo-dimerisation of a substituted thiophene, in which the use of a second component to modify the supramolecular packing generates a photo-reactive assembly in the co-crystal.

The rational design and prediction of co-crystals remains challenging, but several of the papers highlight how different predictive methodologies can be successfully applied. In particular, with over 700 000 small molecule crystal structures now available, it is now possible to use structural data mining very effectively in the context of co-crystal design and prediction.

I hope that readers will find this themed issue both interesting and stimulating. I should like to thank all the contributors, reviewers, and the editorial team for their high quality and professional work.

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