to place the halogen bond donor. This tactic allows medicinal chemists to predict which donor to incorporate and where to place it on a substrate. Hobza has used another approach by employing the semiempirical family of PM6 functions to make halogen bond computations accessible without using computationally expensive quantum mechanical (QM) calculations [162,163]. Using this method, they demonstrated that reasonable modeling can be achieved using lower levels of theory on non‐halogen bonding components.
Other methodologies for studying the halogen bond in biology are effective and highly utilized. For example, Boeckler developed an evaluation tool called XBScore, which rates halogen bond interactions in proteins using QM/molecular mechanics (MM) calculations [164]. QM/MM uses computationally cheap MM to model most of the protein and expensive QM to model the binding site and halogen bonding substrate [165]. In comparison, other techniques like optimized potentials for liquid simulations‐all atoms (OPLS‐AA) [166] or assisted model building with energy refinement (AMBER) [167] have used a positive extra point approach by adding a pseudoatom at the halogen atom surface to inexpensively simulate a σ‐hole. Ho further developed these force field systems by deriving MM/MD equations specifically for the halogen bond [168]. The above computational techniques highlight how the ingenuity of the chemists has overcome limitations of computational power to provide reasonable predictions in a timely fashion.
1.4.9 Computational Conclusion
To date, researchers have generated a variety of computational and experimental tools to study the halogen bond, and they are constantly being improved. One can look at the aphorism by the statistician George Box, which states, “All models are wrong, but some are useful.” Computational models depend on the experimental systems they come from and make assumptions to limit the computational resources required. However, these limitations are being lifted to obtain useful information for drug design and fundamental interaction studies. Future halogen bonding computational models will be developed, which combine the better processing power of future hardware with a greater understanding of the principles that make up the halogen bond. Computational studies of the halogen bond and other noncovalent interactions will be necessary for rational molecular design across many synthetic fields. Furthermore, these studies provide a strong foundation to understand the solution‐based halogen bonding presented in later chapters of this book.
1.5 Materials
1.5.1 Introduction
Materials like liquid crystals (LCs), polymers, and gels frequently exhibit properties governed by noncovalent forces, most often the hydrogen bond. As expected, the distinct characteristics of the halogen bond, such as high directionality, strength, polarizability, and hydrophobicity, provide enticing prospects for the development of novel materials. In this section, select examples of halogen bonding materials are presented. For more information pertaining to halogen bond materials, recent reviews have been published [7,169].
1.5.2 Liquid Crystals
The LC state is a mesophase, having properties of both crystalline solids and isotropic liquids, with extensive real‐world applications. A variety of different noncovalent interactions are employed to achieve desired LC properties (e.g. low temperature formation, unique light modification, predictable phase transition, etc.), but hydrogen bonding is by far the most common [170]. The success of hydrogen bond‐mediated LCs is largely attributed to its directionality, thereby inspiring evaluations using the more stringent halogen bond. In fact, LCs incorporating halogen bonds have exhibited unique properties dissimilar to hydrogen bonding derivatives. This section provides select examples of how the halogen bond has been applied to produce different classes of LCs. For further reference, a recent review of the topic has been published [171].
The first example of LCs assembled by halogen bonding was reported by the Bruce lab in 2004 [172]. Here, alkoxystilbazole derivatives were used as halogen bond acceptors and pentafluoroiodobenzene as the donor (Figure 1.17). X‐ray crystallographic studies suggested the LC formation resulted from a CI⋯N halogen bond. The dimeric complex formed LCs only when cooling, known as monotropic formation. However, alkoxystilbazoles with longer alkyl chains (n > 6) resulted in enantiotropic LCs (occurring at both heating and cooling cycles). Exchanging the iodine donor for a weaker bromopentafluorobenzene precluded LC formation. The LC formation temperature was lower for the halogen bond derivatives than hydrogen bonding analogues – a property that is generally beneficial for LCs operating near room temperature (e.g. liquid crystal display [LCD] displays). Many other early halogen bond LCs incorporated iodoperfluorobenzene donors, as they form moderately strong halogen bonds and are readily available for purchase [173–176]. However, in 2013, Bruce used molecular iodine as a halogen bond donor to create LCs with stilbazole acceptors [177]. The high temperature stability (>200 °C) of the mesophase was attributed to the intermolecular iodine–iodine contacts. These initial examples demonstrate the ability of the halogen bond to facilitate LC formation with favorable properties.
Figure 1.17 The first example of a halogen bonding LC developed by Bruce. Alkyl chains R related to LC behavior.
The Bruce lab, along with Metrangolo and Resnati, also contributed to the first example of ionic LCs using halogen bonds [173]. Using tricomponent imidazolium cations and neutral perfluoroiodo halogen bond donors bound to iodide, they showed that the alkyl chains on the organocations did not drive liquid crystallinity and even smaller chain lengths (n = 2) formed a mesophase. Later studies made these systems light responsive [178].
Photoresponsive LCs, used in displays, nanotechnology, and photo‐driven devices, provide on–off switchable liquid crystallinity. Toward this end, the halogen bond has been integrated into photoresponsive LCs. For example, Priimagi et al. [175] paired the photoactive azo group on a halogen bond donor with an alkoxystilbazole acceptor to produce UV‐active halogen bonding LCs (Figure 1.18). Separately, neither of these molecules exhibited an LC phase, but together they induced anisotropy when irradiated with polarized UV light. Replacing a CH hydrogen bond donor in tetrafluorobenzene with an iodo halogen bond donor in iodotetrafluorobenzene resulted in a decrease of the phase‐transition temperature that was dependent on the concentration of iodoperfluorobenzene, along with improved chiral light absorption [179].
The Li group doped commercially available achiral LCs with chiral halogen bonding molecular switches to produce helical cholesteric LCs (CLCs) [180]. The CLCs operate reversibly under thermal or light response. Reflection colors for these CLCs were temperature dependent, producing red, green, and blue colors. Additionally, the helical twisting power (HLC), known as the amount of chiral LC formation, could be altered by UV light interacting with the halogen bond CLCs. This concept shows that halogen bonding can be used to optimize doped LC systems to create photonic devices.
1.5.3 Supramolecular Polymers
1.5.3.1 LC Polymers
Supramolecular polymers are arrays of small molecules or linear polymeric chains held together by noncovalent interactions. LC polymers have many of the characteristics of other polymers, including mechanical strength at high temperatures, chemical resistance, and flame resistance, while maintaining LC order. The strength and directionality of the halogen bond make it an interesting noncovalent interaction to be used in polymer science. Yet, there are very limited examples, as detailed by a review in 2012 [181]. Xu et al. created the first LC polymer mediated by halogen bonds using bis(iodotetrafluorophenoxy) alkane donors, which formed halogen
