Halogen bonding in the co-crystallization of potentially ditopic diiodotetrafluorobenzene: a powerful tool for constructing multicomponent supramolecular assemblies

Abstract Halogen bonding is emerging as a significant driving force for supramolecular self-assembly and has aroused great interest during the last two decades. Among the various halogen-bonding donors, we take notice of the ability of 1,4-diiodotetrafluorobenzene (1,4-DITFB) to co-crystallize with diverse halogen-bonding acceptors in the range from neutral Lewis bases (nitrogen-containing compounds, N-oxides, chalcogenides, aromatic hydrocarbons and organometallic complexes) to anions (halide ions, thio/selenocyanate ions and tetrahedral oxyanions), leading to a great variety of supramolecular architectures such as discrete assemblies, 1D infinite chains and 2D/3D networks. Some of them act as promising functional materials (e.g. fluorescence, phosphorescence, optical waveguide, laser, non-linear optics, dielectric and magnetism) and soft materials (e.g. liquid crystal and supramolecular gel). Here we focus on the supramolecular structures of multicomponent complexes and their related physicochemical properties, highlight representative examples and show clearly the main directions that remain to be developed and improved in this area. From the point of view of crystal engineering and supramolecular chemistry, the complexes summarized here should give helpful information for further design and investigation of the elusive category of halogen-bonding supramolecular functional materials.

The co-crystallization process is greatly related to molecular recognition and supramolecular self-assembly between components, which are driven by non-covalent interactions, for example, halogen bonds, hydrogen bonds, π -π stacking, van der Waals forces and so forth. Therefore, understanding of non-covalent interactions is a matter of considerable importance [20][21][22]. In recent years, the research focus has been extended towards halogen bonds from the well-known hydrogen bonds as they have been proven to be another powerful tool in crystal engineering and supramolecular chemistry [23][24][25], encompassing a wide range from fundamental studies (e.g. the nature of the halogen bond [26,27]) to materials science (photoelectric materials [28][29][30], liquid crystals [31,32], supramolecular gels [33], anion recognition [34,35], etc.) to biological systems [36]. Ding et al. 1907 Nitrogen-containing compounds One common synthetic approach to achieve halogen-bonding co-crystallization systems is to utilize halogen-bonding donors and acceptors with complementary functional groups [8]. As far as halogen-bonding donors are concerned, the halogen-bonding strength depends on the electronegativity of the halogen atoms, increasing in the order of Cl < Br < I [37], and can be further enhanced by introducing electron-withdrawing groups, for example, fluorine atoms [38,39]. Consequently, perfluorinated iodobenzenes were regarded as ideal halogen-bonding donors and we have summarized the use of tritopic 1,3,5trifluoro-2,4,6-triiodobenzene (1,3,5-TFTIB) in the design of multicomponent supramolecular complexes [40]. In this review, we will concentrate on the linear ditopic 1,4-diiodotetrafluorobenzene (1,, which has been more widely exploited to construct a diversity of supramolecular architectures through co-crystallizing with various halogen-bonding acceptors in the range from neutral Lewis bases (nitrogen-containing compounds, N-oxides, chalcogenides, aromatic hydrocarbons and organometallic complexes) to anions (halide ions, thio/selenocyanate ions and tetrahedral oxyanions) (Scheme 1; chemical structures of halogen-bonding acceptors seen in Supporting Information). We hope that the information given here will be useful in understanding the 'structureassembly-property' correlation in this kind of halogen-bonding co-crystals as well as stimulating further research into the field of multicomponent crystalline materials.
halogen bonds with C-H· · ·F and N-H· · ·F hydrogen bonds, F· · ·F and π · · ·π interactions, drives the formation of a 3D supramolecular structure. In spite of sharing a similar discrete structure with 3APy, 5A2MPy shows different intermolecular interactions with 1,4-DITFB. The amino group unexpectedly turned out to be a decent halogen-bonding acceptor for the iodine atom (d I· · ·N = 2.97Å, ∠C-I· · ·N = 179 • ) rather than the pyridine nitrogen atom that forms the N-H· · ·N hydrogen bond with the NH 2 group (Fig. 1b). The methoxy group is free and not involved in any supramolecular interactions. In (DMAPy) 2 ·(1,4-DITFB), the shortest C-I· · ·N halogen bond was observed between N py and I atoms (d I· · ·N = 2.67Å, ∠C-I· · ·N = 179 • ), known in the co-crystals of fluorinated iodoarenes, to which solid-state packing effects may be a contributing factor. Several amide-substituted pyridines (INA, Py3A and Py4MA) were also utilized to react with 1,4-DITFB, resulting in three co-crystals with a 2 : 1 molar ratio [45,46]. The amide moieties are engaged in the self-complementary N-H· · ·O = C hydrogen-bonding interactions while pyridine N atoms are halogen bonded to I atoms, generating 3D supramolecular networks (Fig. 2a). Meanwhile, based on Py2A together with PyDCA, the first ternary co-crystal comprising 1,4-DITFB was synthesized [46], in which the carboxylic groups from PyDCA have a propensity to form a heteromeric trimer with the acetamido pyridine moiety of Py2A. It is interesting that a bifurcated halogen bond C-I· · ·N appears between one pyridine N atom and two 1,4-DITFB molecules, and the iodine atoms on the other side interact with the O = C moiety of the amide group (Fig. 2b).
Furthermore, the behavior of MIN with an ester carbonyl was investigated in the 1 : 1 co-crystal   (MIN)·(1,4-DITFB) [47]. Both pyridine nitrogen and carbonyl oxygen atoms of MIN act as the halogen-bonding acceptors for iodine atoms, affording an infinite zigzag chain. The C-I· · ·N halogen bond is stronger than C-I· · ·O (d I· · ·N = 2.919Å, ∠C-I· · ·N = 173.8 • ; d I· · ·O = 3.045Å, ∠C-I· · ·O = 165.6 • ), suggesting that N py is the preferable binding site (Fig. 3a). This has been confirmed in the 2 : 1 co-crystal (MIN) 2 ·(1,4-DITFB), obtained by using a large excess of MIN [47]. In this case, two halogen-bonding binding sites of 1,4-DITFB are saturated by the better N atoms, leading to a discrete trimer (Fig. 3b). The similar trimeric structure was found in the co-crystallization of another carbonyl-containing PyPDONE with 1,4-DITFB [48]. Impressively, the β-diketone group is present as its enol tautomer and the carbonyl group is not involved in the halogen-bonding REVIEW Ding et al. 1909 interaction, forming a six-membered ring through the very short intramolecular hydrogen bond O-H· · ·O with the hydroxyl group (d H· · ·O = 1.642Å, ∠O-H· · ·O = 149.08 • ). The incorporation of MPyIMP into the 1,4-DITFB molecule gave a 2D co-crystal microplate [49], where the pyridine N atom is halogen bonded to the iodine atom and other functional groups (−OH, −OMe, imino group) take part in the formation of hydrogen-bonding interactions. The C-I· · ·N halogen bonds alleviate the intermolecular charge transfer (CT), bringing about a conducive four-level energy structure for the population inversion. Optically pumped lasing measurements of the microplate reveal a self-waveguided edge emission and strong 1D field confinement as well as highquality Fabry-Pérot (FP)-type microcavity effects so that laser oscillation is realized in the co-crystal (Fig. 4).
Subsequently, p-pyridyl-ended oligo pphenylenevinylene (OPV) derivatives (R = n-C 4 H 9 , OPV-1; R = (C 2 H 4 O) 3 CH 3 , OPV-2) were employed as the halogen-bonding acceptors [66]. When the hot solutions of OPV derivatives and 1,4-DITFB in equimolar amounts cooled slowly (EtOH for OPV-1; the addition of 20% H 2 O to EtOH for OPV-2), two co-crystals were obtained, forming infinite chains via C-I· · ·N halogen bonds. Interestingly, fast cooling of the hot solution for OPV-2 causes fibrous aggregates and eventually supramolecular halogels ( Fig. 7a and b). The methoxy substitution (R = CH 3 , OPV-3) generates a 2D organic parallelogram co-crystal that shows an asymmetric optical waveguide (optical-loss coefficients R Backward = 0.0346 dB μm −1 and R Forward = 0.0894 dB μm −1 ), which is ascribed to the unidirectional total internal reflection induced by the anisotropic molecular packing mode ( Fig. 7c-f) [67]. The asymmetric photon transport has been exploited as a microscale optical logic gate with multiple in/output channels.
The introduction of heteroatoms between two pyridine groups does not change the binding site of halogen-bonding acceptors, such as 4-(pyridin-4-ylsulfanyl)pyridine (PySPy) [71] and 4,4'-azopyridine (APy) [64]. The sulfur atom or azo moiety is not engaged in any inter/intramolecular interaction and linear-extended halogen-bonding chains still occur between the pyridine N atom and iodine atom of 1,4-DITFB. In addition, the structural equivalence of the azo (−N = N−) in APy and the ethylene (−C = C−) from B4PyEe mentioned above has been demonstrated by Varughese and co-workers in the co-crystallization process with 1,4-DITFB.
Replacing BPyMU with bis(pyridyl urea) derivatives (BPyU-1 and BPyU-2), two new co-crystals, (BPyU-1)·(1,4-DITFB) and (BPyU-2)·(1,4-DITFB)·2H 2 O, were isolated when hot solutions of BPyU and 1,4-DITFB in methanolwater mixtures cooled slowly [33]. In a manner similar to (o/m/p-BPyMU)·(1,4-DITFB) above, the former co-crystal with a needle morphology exhibits infinite halogen-bonding chains between 1,4-DITFB and BPyU-1 molecules, with a N· · ·I distance of 2.819Å. Such chains are connected with each other through the N-H· · ·O hydrogen bonds among ureas, inducing a 2D sheet structure with urea tapes (Fig. 9a). If the hot solution is allowed to cool rapidly, a supramolecular gel would come into being (Fig. 9b). In the latter case, 1,4-DITFB molecules possess two different environments: one is independent and not involved in any halogen bonds; the other acts as the halogen-bonding bridge between two BPyU-2 molecules, leading to a trimeric assembly that connects adjacent ones into a 1D undulated chain by solvent water via the O-H· · ·N hydrogen bonds (Fig. 9c). The absence of urea tapes perhaps explains the relatively weak nature of the gels formed by BPyU-2 and 1,4-DITFB.
In the two co-crystals, only one of three pyridyl groups is halogen bonded to 1,4-DITFB, affording the trimeric adducts.
With a view to predicting halogen-bonding selectivity effectively, Aakeröy et al. performed systematic co-crystallizations on pyridine-containing imidazoles with two different acceptor sites, [91]. Electrostatic potential calculations indicate that, if the potential-energy difference between two binding sites is not large enough (<75 kJ/mol), halogen-bonding selectivity will disappear and both potential acceptor sites will simultaneously participate in halogen-bonding interactions. As with every situation here except PIMPy, both pyridine  and imidazole N atoms are halogen bonded to 1,4-DITFB via C-I· · ·N interactions (Fig. 13).
Thereafter, 2,5-diphenyloxazole (DPO), a well-known UV fluorescent material, was utilized to co-crystallize with 1,4-DITFB through C-I· · ·N halogen bonds and explore novel photoelectric properties [98]. The halogen-bonding trimeric complex, (DPO)·(1,4-DITFB) 2 , shows UV/blue polarized emission, the second harmonic generation effect and reversible mechanochromic fluorescence (MCF) properties. Likewise, DPObased co-crystals can be constructed based on other co-formers (1,4-dibromotetrafluorobenzene, tetrafluoroterephthalic acid and so on), presenting tunable fluorescence properties in the UV/blue region [99]. Upon the formation of co-crystals, their dielectric constants have a significant increase due to the introduction of the hydrogen-and/or halogen-bonding interactions.  (HMPBTA) [101]) for DPO gives rise to two new co-crystals with similar halogen-bonding trimeric structures, where only one nitrogen atom is halogen bonded to 1,4-DITFB. In the latter case, the formation of co-crystal (HMPBTA)·(1,4-DITFB) 2 leads to enhanced excited-state intramolecular proton transfer (ESIPT) emission compared with the pristine HMPBTA (Fig. 16). The emission band of ∼413 nm is assigned to the fluorescence emission of its S 1 enol while the one at ∼605 nm stems from the emission of the S 1 keto species. The ESIPT process is computed to be barrierless, so the S 1 keto species are populated much more than the enol, which explains why the emission band of ∼413 nm nearly disappears. In Jin's group, a new co-crystal was successfully assembled by the butterfly-shaped non-planar phenothiazine and 1,4-DITFB [102]. Single-crystal Xray diffraction analysis reveals that a 1D chain exists in the co-crystal with the A· · ·2D· · ·A· · ·D· · · (A = acceptor, D = donor) arrangement owing to C-I· · ·N/S/π halogen bonds, wherein the iodine atom from 1,4-DITFB can participate in bifurcated or trifurcated interactions (Fig. 17). Study of the luminescent property indicates that the complex emits relatively strong delayed fluorescence with a small stokes shift and weak phosphorescence, obviously different from those of co-crystals constructed by rigid planar molecules such as monoazaphenanthrenes [42] and carbazole [96]. The phenothiazine molecule has a large torsion angle of 150.94 • between two wings and the non-planarity makes the phosphorescent radiative transition process decrease, thus producing weak phosphorescence.
In order to explore the mechanism of mechanochemical co-crystallization, the cocrystal of Tmor with 1,4-DITFB was selected as the model system by Jones et al. [105]. Grinding 1 : 1 reactants for 30 min provided infinite zigzag chains of (Tmor)·(1,4-DITFB), whereas a shorter time of grinding (e.g. 4 min) offered not merely halogen-bonding chains, but also trimeric assemblies of (Tmor) 2 ·(1,4-DITFB) that is isostructural with (Piperidine) 2 ·(1,4-DITFB). The appearance of discrete assemblies as the intermediate reaction can be explained by the hierarchy of halogen bonds, namely the nitrogen atom as the better acceptor is the preferred binding site, and stronger C-I· · ·N halogen bonds initially drive the formation of trimeric (Tmor) 2 ·(1,4-DITFB); further grinding leads to polymerization of the assemblies via weaker C-I· · ·S bonds, forming (Tmor)·(1,4-DITFB). Consequently, a stepwise mechanism for the mechanochemical synthesis of halogen-bonding co-crystals has been proposed and demonstrates the competition between supramolecular interactions (Fig. 18).
The cyano group can be engaged in the formation of halogen-bonding interactions as well when no other functional groups exist in the compound, for instance, 2,3,5,6-tetramethylterephthalonitrile (TMTPN) [109], 1,4-bis(4-cyanostyryl)benzene (BCSB) [110,111] and 1-(2-cyanostyryl)-4-(4-cyanostyryl)benzene (CSCSB) [112]. In the co-crystal (BCSB)·(1,4-DITFB) with 1D halogen-bonding chain that was synthesized by the liquid-assisted grinding method [110], the introduction of 1,4-DITFB changed the stacking mode of chromophore BCSB and enlarged the separation between the BCSB molecules so as to reduce the degree of molecular aggregation, inducing a strong blue shift by 64 nm in contrast with pure BCSB. Upon excitation by an 800-nm laser, the co-crystal exhibits strong two-photon luminescence with two main narrow peaks at 470 and 497 nm (Fig. 20). Compared with the above macroscopic co-crystal, the nanosized counterpart obtained by an ultrasound-assisted crystallization method presents different photoemission properties such as one-/ two-phonon emission and fluorescence lifetime, which gives new insights into the size-dependent luminescence effects of multicomponent organic nanocrystals [113]. Recently, the microcrystalline form of (BCSB)·(1,4-DITFB), sky-blue-emissive microwire, has been reported [111]. In contrast with the pure BCSB organic microcrystal, its radiative decay (k r ) rate is enhanced from 0.04 to 0.12 ns −1 and the fluorescence lifetime goes from 14.0 to 0.9 ns. With regard to the system of its isomer CSCSB, a tetramolecular ring-like structure was observed between CSCSB and 1,4-DITFB molecules via C-I· · ·C≡N halogen bonds [112]. The two-component assemblies displayed a slight
Recently, pure organic host-guest co-crystals were assembled by 4-phenylpyridine N-oxide (PPyNO) and 1,4-DITFB under the mediation of linear guest molecules (phenazine, acridine and 2,2 -BPy) [119]. Robust bifurcated C-I· · · − O-N + halogen-bonding interactions between PPyNO and 1,4-DITFB facilitate the formation of 1D zigzag chains that are crossed and interlinked together by C-H· · ·F hydrogen-bonding interactions and π −hole· · ·π bonds to produce the hexagonal host channels (Fig. 22a). The guest molecules reside in the host channel through π · · ·π stacking and interact with the host via C-H· · ·π interactions to stabilize the host-guest structure. The host-guest systems show different photoluminescence colors (bright cyan, sapphire blue and pink, respectively) at room temperature, mainly originating from guest molecules (Fig. 22b).

Chalcogenides as the halogen-bonding acceptors
In addition to nitrogen-containing compounds, chalcogenides are another large category of halogen-bonding acceptors. In recent years, the potential of chalcogenides containing Y=O (Y=C, P, S) as halogen-bonding acceptors has attracted much attention. In terms of phosphine oxides, co-crystallization experiments by mechanochemical methods readily yield polymorphs. For instance, two co-crystals, (MDPPO)·(1,4-DITFB) and (MDPPO) 2 ·(1,4-DITFB) (methyldiphenylphosphine oxide (MDPPO)), were obtained by changing the relative quantities of the starting reactants in a liquid-assisted grinding process [120,121]. In the former case, intermolecular C-I· · ·O = P halogen bonds and C-I· · ·π contacts direct the formation of tetramolecular fragments, whereas, in the latter, the co-crystal oxygen atom is involved in the C-H· · ·O hydrogen-bonding interaction, leaving the π -system of the phenyl ring as the halogen-bonding acceptor (Fig. 23). Here, the competition between hydrogen and halogen bonds gives rise to stoichiometric variations.
For the purpose of testing the potential of the hydroxyl and methoxy group to form halogen bonds, Cinčić et al.  [132,133]. In all cases, the imine nitrogen participates in the formation of strong O-H· · ·N intramolecular hydrogen-bonding interaction with the hydroxyl group. The hydroxyl or/and methoxy oxygen atom can be halogen bonded to the iodine atom from 1,4-DITFB for the first three co-crystals, but if another functional group exists (e.g. carbonyl or cyano group), that would become the preferred binding site. Actually, as early as 2008, their research team also chose cyclic chalcogenides (1,4-dioxane, 1,4-dithiane and 1,4-thioxane) to react with 1,4-DITFB, producing 1D single chains or ladder-like chains through C-I· · ·O/S halogen-bonding interactions [103,122].
Impressively, a heavier congener of the oxygen atom, selenium, can behave as an excellent halogenbonding acceptor of organic iodides. In the cocrystal of triphenylphosphine selenide (TPPS) with 1,4-DITFB [134], both monotopic and ditopic selenium atoms were observed, affording termolecular adducts and zigzag chains through C-I· · ·Se halogen bonds, respectively, which are interwoven with each other (Fig. 24).
Phosphorescent co-crystals can also be assembled by fluorene or its heterocyclic analogues (dibenzofuran and dibenzothiophene) with 1,4-DITFB based on C-I· · ·π interactions, providing 1D zigzag chains or grid-like chains [139]. The O or S atom in dibenzofuran or dibenzothiophene does not take part in any halogen-bonding interactions and the C-I· · ·π contacts are easier to form in the corresponding system, which has been confirmed by calculations of bonding energies. The phosphorescence spectra of the three co-crystals are greatly red-shifted by about 50-90 nm with regard to the free monomer in the β-cyclodextrin aqueous solution and all the decays obey a mono-exponential law with lifetimes of 0.34, 0.51 and 2.50 ms, respectively. In (Dibenzothiophene)·(1,4-DITFB) 2 , the heavy-atom effect from the sulfur atom may be a contributing factor to the longer phosphorescence lifetime.
Later, the stoichiometric ratio of cocrystallization was utilized to tune the photophysical properties (fluorescence or/and phosphorescence) of the conjugated hydrocarbons diphenylacetylene  as well as trans-stilbene by d'Agostino et al. [140]. The 1 : 1 molar ratio between the conjugated hydrocarbon and 1,4-DITFB facilitates the formation of a dual luminescent material with both fluorescence and phosphorescence emission, whereas the 1 : 2 stoichiometry makes their fluorescence suppressed so that only phosphorescence is observed. Comparatively speaking, the 1 : 2 co-crystals have lower emission quantum yields and shorter phosphorescence lifetimes, on account of the increased T 1 → S 0 radiative deactivation rate that is induced by the doubled heavy-atom/hydrocarbon stoichiometry.

Organometallic complexes as the halogen-bonding acceptors
Organometallic complexes are promising building blocks for the construction of multicomponent supramolecular assemblies. A novel co-crystal yielded upon isothermal evaporation of the CCl 3 solution containing trans-[Pt(PCy 3 ) 2 (C≡C-4-Py) 2 ] (Cy = cyclohexyl) and 1,4-DITFB at room temperature, in which the Pt(II) ion has a rigid square-planar coordination environment [141]. The extremely strong C-I· · ·N interactions are primarily Reprinted with permission from reference [141]. Copyright 2012 American Chemical Society.
The broadband dielectric spectroscopic measurements, carried out at a temperature from 25 to 155 • C and frequency from 10 −2 to 10 7 Hz, show that the co-crystal exhibits a real component of dielectric permittivity (ε ) that is remarkably lower than that of SiO 2 . Thereafter, a Pt(IV) complex with octahedral coordination geometry, PtMe 3 I(Cl-TPy) (Cl-TPy = 4 -chloro-2,2 :6 ,2 -terpyridine), was exploited as the halogen-bonding acceptor, affording a discrete trimeric structure through the halogen-bonding interactions between uncoordinated pyridine nitrogen and iodine atoms from 1,4-DITFB [142]. The Pt-bound iodine is not involved in the halogen-bonding interaction, but results in a weak hydrogen bond with the solvent CCl 3 instead. The octahedral complex Fe(PyPDONE) 3 takes carbonyl oxygen as the coordination atom, leaving peripheral pyridine nitrogen as the potential halogen-bonding binding site [48]. Two of three pyridine nitrogen atoms are halogen bonded to I atoms of 1,4-DITFB to offer a 1D chain, with N· · ·I distances of a little less than 2.8Å that indicate that metal coordination leads to shorter and stronger halogen-bonding interactions. However, isostructural Al(PyPDONE) 3 ·3H 2 O acts differently [48]: only one pyridine nitrogen atom is engaged in the halogen-bonding interaction with 1,4-DITFB, while a carbonyl oxygen atom along with solvent water also conducts as a halogen-bonding acceptor, codetermining the formation of 2D grid-like networks in combination with an O-H· · ·N hydrogen bond between the water molecule and another pyridine nitrogen (Fig. 28a). Lately, the first neutral metalcontaining 3D halogen-bonding network with an α-Po pcu (primitive cubic unit) topology (4 12 ·6 3 ) has been reported by Clegg and co-workers based on a six-coordinated Fe(DPyPDONE) 3 (DPyP-DONE = di(4-pyridyl)propane-1,3-dione) [143]. Each of the six pyridyl N atoms is linked to I atoms from six different 1,4-DITFB molecules, producing the [Fe(DPyPDONE) 3 ] 8 (1,4-DITFB) 12 subunit, which encapsulates a volume of ∼16 600Å 3 that is occupied by another six identical networks (Fig. 28b). In this way, a 7-fold interpenetrated network structure appears and the high degree of interpenetration probably results from the large aspect ratio of the 1,4-DITFB linkers (Fig. 28c).
In Cinčić's research team, halogen-bonding metal-organic co-crystals were designed based on Schiff base complexes with pendant acetyl groups, M(NMAPE) 2 (M = Cu, Ni; HNMAPE = (E)-1-(4-((2-hydroxynaphthalen-1-yl) methyleneamino)phenyl)ethanone), where unsaturated Cu(II) or Ni(II) ion adopts a square-planar geometry [146,147]. Each M(NMAPE) 2 interacts with two 1,4-DITFB molecules via two acetyl oxygen atoms with O· · ·I distances of 3.084 and 3.117Å, respectively, generating 1D zigzag halogen-bonding chains. Unlike the imine in the Schiff base complexes M(NMAPE) 2 above, the nitrogen atoms in tetraimino ferrocenophane can be reliably utilized as the halogen-bonding acceptors for the iodine atoms of 1,4-DITFB [148]. Specifically, only two of the four imine nitrogen atoms take part in the C-I· · ·N interactions that drive the co-crystallization process and promote the formation of polymeric chains. Mössbauer spectroscopy reveals the sole presence of low-spin Fe(II) and the temperature dependence of the magnetic susceptibility corresponds to a quasi-diamagnetic compound.

Anions as the halogen-bonding acceptors
In general, anions do not merely balance the charge of cations, but also serve as halogen-bonding acceptors, whereas the cation takes virtually no active role in the formation of halogen bonds. As compared with neutral species, anions are better halogen-bonding acceptors since their Lewis basicity can be enhanced by the increased electron density on the electron-donor site.
In the bromide, tetradentate Br − ions bridge four different 1,4-DITFB molecules via C-I· · ·Br − halogen bonds with distances of 3.280 and 3.350Å, and sit at the nodes of a somewhat distorted square, giving slightly undulated 2D (4,4) grid-like sheets in which the I· · ·Br − · · ·I angles are 85.53 and 90.97 • , respectively (Fig. 32a). The cations locate perfectly at the center of the square and consequently interpenetration is prevented. As for (K.2.2.2.⊂KI) 2 ·(1,4-DITFB) 3 , tridentate iodide anions behave as the μ 3 -bridges and connect three different 1,4-DITFB molecules into regular 2D (6,3) honeycomb-like sheets via C-I· · ·I − halogen bonds (Fig. 32b), wherein the I· · ·I − · · ·I angles are close to the tetrahedral geometry (119.45 • , 107.62 and 101.65) and the I· · ·I − contacts span the range of 3.352-3.379Å. Two such sheets inter-  . Two hexagonal networks are linked together by CCl 4 solvent molecules through I − · · ·Cl-CCl 2 -Cl· · ·I − halogen-bonding interactions, resulting in a 3D diamond network (6 6 -dia) with 2-fold interpenetration. In the overall structure, the iodide anion is in a pentadentate mode and its electron density is distributed over five halogen bonds, which are responsible for the longer I − · · ·X (X = Cl or I) contacts ranging from 3.435 to 3.587Å.
Thiocyanate and selenocyanate aniontemplated (SCN − , SeCN − ) assembly of 1,4-DITFB anion binds in a similar fashion to SCN − described above [161]. Multinuclear solid-state magnetic resonance spectroscopy of these complexes shows that 13 C chemical shifts of the thiocyanates slightly increase and yet 15 N chemical shifts decrease in contrast with reference compounds with simple counterions, indicating the existence of halogen bonds. The opposite trends are captured for the selenocyanates and more substantial changes are found in the pseudo unique principal component of the 77 Se chemical shift tensor as well as the 77 Se isotropic chemical shift. − , ReO 4 − ) with 1,4-DITFB affords the undulated (6,3) networks via C-I· · ·O halogen bonds wherein tridentate anions sit at the network nodes bridged by bidentate 1,4-DITFB molecules and the ammonium cations occupy the space that is encircled by the (6,3) frames [163]. The I· · ·O distances span from 2.864 to 3.264Å and the C-I· · ·O angles are in the range of 160.53-172.35 • . The remarkably similar structures among the three oxyanions prompted them to synthesize several mixed crystals (e.g. (n-Bu 4 N + )·(ClO 4 − ) 0.72 ·(ReO 4 − ) 0.28 ·(1,4-DITFB) 1.5 ), which are isostructural with the halogen-bonding supramolecular salts discussed above. Thereinto, two different oxyanions statistically occupy the nodes of (6,3) networks with ratios, even though they have quite different sizes. Tetrahedral oxyanions hence proved to be effective and general tectons in the anion-templated assembly, which is driven by halogen-bonding interactions.

CONCLUSION
In this review, an attempt has been made to sketch out the efforts to obtain multicomponent supramolecular complexes through the co-crystallization of 1,4-DITFB with a variety of halogen-bonding acceptors in the range from neutral Lewis bases (nitrogen-containing compounds, N-oxides, chalcogenides, aromatic hydrocarbons and organometallic complexes) to anions (halide ions, thio/selenocyanate ions and tetrahedral oxyanions). The examples reviewed here illustrate that halogen bonds have a vital role in co-crystallizing processes, exhibiting a wide diversity of impressive supramolecular architectures (for instance, dimers, trimers, tetramers, pentamers, heptamers, 13-molecule finite chains, 1D infinite chains, highly undulated infinite ribbons, 2D and 3D networks).
It is not easy to chase down common features among this large family of halogen-bonding supramolecular complexes based on 1,4-DITFB and yet their topological structures can be modulated by the different halogen-bonding acceptors. In a general way, anionic acceptors form furcated halogen bonds with iodine atoms from 1,4-DITFB, readily giving rise to high-dimensional (2D or 3D) supramolecular networks. Furthermore, neutral species with multiple potential binding sites, for example tetrapyridyl compounds, favor the formation of high-dimensional supramolecular structures as well.
Many interesting physicochemical properties are found in these co-crystals, such as fluorescence, phosphorescence, magnetism, dielectric and nonlinear optical properties, as well as liquid crystals and supramolecular gels. Among them, π -conjugated aromatic hydrocarbons can be exploited to construct phosphorescent materials, while the use of N-oxides has made the achievement of multicomponent magnetic complexes possible. Moreover, co-crystals of pyridine derivatives with alkoxy chains exhibit liquid crystalline behavior.
In addition, some co-crystals based on pyridine derivatives are able to be applied in different photoelectric devices, e.g. optical waveguide, laser, optical logic gate and memory. The microplate (MPyIMP)·(1,4-DITFB) displays a self-waveguided edge emission, strong 1D field confinement and FP-type resonance, realizing organic co-crystal lasers. Besides, the asymmetric optical waveguide makes the co-crystal of OPV-3 suitable for the construction of a microscale optical logic gate, contributing to the development of organic integrated photonics. Two other supramolecular assemblies from isomeric PVA enrich the library of piezochromic materials for haptic memory.
Although the utilization of 1,4-DITFB in the co-crystallization area has been ongoing for almost two decades, the relationship among molecular structures, assembly processes and functional properties has not been yet well understood. Exploration of photo-electro-magnetic functional materials has only just begun and even less is known about photoelectric devices as well as soft matter. From the point of view of crystal engineering and supramolecular chemistry, these co-crystals will offer helpful information for further design and investigation of the elusive family of halogen-bonding complexes. We believe that multicomponent supramolecular complexes assisted by halogen bonds should have a great potential and bright future in functional materials and device applications.