Single-crystal data for (I 4F 16cb)♲(bpe) were collected with a Bruker APEXII Kappa diffractometer equipped with an Oxford Cryostream low-temperature device using Mo K α radiation ( λ = 0.71073 Å). The movements span the outer geometries of N⋯I bonds that have been reported to date. We demonstrate that the components engaged in halogen bonding exhibit structural flexibility, their geometries undergoing a linear-to-bent type of deformation in the SCSC transformation. We show the components of (I 4F 16cb)♲(bpe), which assemble via N⋯I halogen bonds to form a cocrystal that reacts quantitatively to form tpcb (tpcb = rctt-tetrakis(4-pyridyl)cyclobutane). In this contribution, we report a halogen-bonded cocrystal with components that support an SCSC photodimerization (Scheme 1). We are unaware of an SCSC photodimerization supported by halogen bonds. Examples of halogen-bonded solids that support single-crystal processes, however, remain rare and to date have involved molecules that provide internal degrees of molecular flexibility and freedom ( e.g. hydrogen bonds, halogen bonds) can be considered fundamentally important in the development of machine-like materials (Khuong et al., 2006 ). The bending of components engaged in noncovalent forces ( e.g. It is not clear whether the components of N⋯I halogen bonds, however, can tolerate types of bending motions similar to hydrogen-bonded components in the solid state, although rotary movements of halogen-bonded molecular components have been recently recognized as important in the development of artificial molecular machines (Catalano et al., 2015 ). Computational studies have recently demonstrated that the energies of the N⋯I bond exhibit a relatively high dependence on the C-I⋯N angle θ ( ca 150° < θ < 180°) (Tsuzuki et al., 2016 ). The pyridine–iodopolyfluorophenyl (N⋯I) halogen bond has been exploited extensively in the field of crystal engineering to sustain discrete and extended supramolecular frameworks (Cavallo et al., 2016 ).

Solids that undergo SCSC transitions bear relevance to applications of organic solid-state materials in areas such as data storage, sensors and shape memory (Kole et al., 2014 ). Flexibility in the solid state can give rise to atomic and molecular movements that support single-crystal-to-single-crystal (SCSC) transformations (Friščić & MacGillivray, 2005 ). X = O, N) to accommodate the orientation and assembly of components into supramolecular structures. Hydrogen bonds generally exploit the flexibility of X-H donor groups ( e.g. liquid crystals, polymers).Ī hallmark feature of noncovalent bonding when controlling self-assembly is the structural flexibility of components engaged in intermolecular forces (Desiraju, 2011 ). The strength and directionality of halogen bonds parallel that of hydrogen bonds such that halogen bonding provides a reliable force to organize molecules in the construction of functional crystalline solids (Cavallo et al., 2016 ) and other condensed materials ( e.g. Halogen bonds are employed in supramolecular chemistry to direct the assembly of molecules in solution and the solid state (Gilday et al., 2015, Resnati et al., 2015 ). Specifically, a linear-to-bent type of deformation of the halogen-bonded components was observed, which is expected to support the development of functional halogen-bonded materials containing molecules that can undergo movements in close-packed crystal environments.

A comparison is provided of the geometric changes experienced by the halogen-bonded components in the single-crystal reaction to the current crystal landscape of N⋯I halogen bonds, as derived from the Cambridge Structural Database. Following an investigation of solid-state reactions involving organic syntheses and the development of photoresponsive materials, this work demonstrates the ability of the components of intermolecular N⋯I halogen bonding – a `workhorse' interaction for the crystal engineer – to support a single-crystal-to-single-crystal photodimerization. Although the structure-directing roles of halogen bonds are often compared to hydrogen bonds, general knowledge concerning the fundamental structural behavior of halogen bonds has had limited opportunity to develop. Halogen bonds have emerged as noncovalent forces that govern the assembly of molecules in organic solids with a degree of reliability akin to hydrogen bonds.

Singlecrystal 2.3