Halogen bonding with carbon: directional assembly of non-derivatised aromatic carbon systems into robust supramolecular ladder architectures

Carbon, although the central element in organic chemistry, has been traditionally neglected as a target for directional supramolecular interactions. The design of supramolecular structures involving carbon-rich molecules, such as arene hydrocarbons, has been limited almost exclusively to non-directional π-stacking, or derivatisation with heteroatoms to introduce molecular assembly recognition sites. As a result, the predictable assembly of non-derivatised, carbon-only π-systems using directional non-covalent interactions remains an unsolved fundamental challenge of solid-state supramolecular chemistry. Here, we propose and validate a different paradigm for the reliable assembly of carbon-only aromatic systems into predictable supramolecular architectures: not through non-directional π-stacking, but via specific and directional halogen bonding. We present a systematic experimental, theoretical and database study of halogen bonds to carbon-only π-systems (C–I⋯πC bonds), focusing on the synthesis and structural analysis of cocrystals with diversely-sized and -shaped non-derivatised arenes, from one-ring (benzene) to 15-ring (dicoronylene) polycyclic atomatic hydrocarbons (PAHs), and fullerene C60, along with theoretical calculations and a systematic analysis of the Cambridge Structural Database. This study establishes C–I⋯πC bonds as directional interactions to arrange planar and curved carbon-only aromatic systems into predictable supramolecular motifs. In >90% of herein presented structures, the C–I⋯πC bonds to PAHs lead to a general ladder motif, in which the arenes act as the rungs and halogen bond donors as the rails, establishing a unique example of a supramolecular synthon based on carbon-only molecules. Besides fundamental importance in the solid-state and supramolecular chemistry of arenes, this synthon enables access to materials with exciting properties based on simple, non-derivatised aromatic systems, as seen from large red and blue shifts in solid-state luminescence and room-temperature phosphorescence upon cocrystallisation.


Introduction
2][3] PAH-based molecules are ubiquitous in the design of organic electronics 4,5 and organic light emitting diodes (OLEDs). 6,7While the arrangement of PAH units in solids is of critical importance for their optical and electronic properties, 8 controlling the assembly of PAHs in crystalline materials is a persistent challenge of organic solidstate chemistry. 9In most cases, guiding the arrangement of PAH units in the solid state requires derivatisation, either to introduce sterically demanding groups that modify molecular packing, 10 or to introduce recognition sites for the formation of multi-component crystals (cocrystals) by directional interactions such as hydrogen (HB) [11][12][13] or halogen bonding (XB). 14uch derivatisation strategies are, however, not applicable to the assembly of pristine, non-derivatised PAHs.Indeed, while carbon is central to organic chemistry, it is rarely considered a target of supramolecular recognition.Directional interactions such as HB 15 and XB [16][17][18] are generally regarded as insufficiently robust to enable predictable, directional assembly of carbononly p-systems.As a result, approaches to molecular recognition, and in particular to cocrystallisation, of PAHs have traditionally been limited to non-directional face-to-face p-p stacking. 19][22] Halogen bonding has emerged as a versatile directional interaction in the crystalline solid state, offering access to a wider range of acceptor atoms than seen for hydrogen bonding. 236][37][38][39][40][41][42] Recently, 43 we observed that cocrystals of azulene with XB donors 1,4-diiodotetrauorobenzene (tb) or trans-octauoro-4,4 ′ -diiodoazobenzene (oab) (Fig. 1b and c) exhibit a ladder-like halogen-bonded motif involving aromatic carbon, in which the PAHs act as the rungs and the XB donors as rails, identical to the motif seen in a cocrystal of naphthalene 30 with 14tb.This unexpected observation of the same C-I/p C ladder-like motif in three different cocrystals of naphthalene and azulene suggested an unexpected role for XB as a unique, possibly general tool for directional assembly of nonderivatised PAHscontrasting the traditionally relied upon non-directional p-stacking.
We now provide an extensive and systematic study that demonstrates halogen bonding as a long-overlooked, reliable tool for the directional assembly of non-derivatised carbon-only aromatic systems.Our study, addressing diversely-shaped and -sized PAHs, as well as fullerene C 60 (Fig. 1a), reveals the persistent formation of cocrystals based on a highly reproducible [44][45][46] supramolecular ladder-like motif (Fig. 1b, c, and 2) of C-I/p C halogen bonds.The robustness of this motif presents it as a unique example of a supramolecular synthon 47 for the assembly of exclusively carbon-based aromatic systems through directional interactions, independent of molecular size, shape, or presence of p/p or C-H/p contacts.The presented set of cocrystals involving different combinations of 9 XB acceptors and 2 donors, along with theoretical calculations and a systematic analysis of the Cambridge Structural Database (CSD), 48 establishes a different paradigm for the crystal engineering based on aromatic carbon: not via surface-based pstacking, but by directional halogen bonding.The potential of this approach is evident by luminescence studies of select cocrystals based on pyrene, coronene and perylene, demonstrating extensive modication of emission properties due to halogen bond-directed assembly.

Linear arenes (acenes)
Our rst target was cocrystallisation of 14tb with benzene (benz), the smallest possible arene that could be addressed in this study.Dissolving 14tb in benz followed by slow evaporation produced large, colorless plate-like crystals.Removal of the crystals from the crystallisation vessel at room temperature, however, resulted in immediate degradation, evident by a loss of crystal transparency.Cooling the sample in dry ice enabled successful handling of the crystal and collection of X-ray single crystal diffraction data at 180 K (see ESI, Table S1 †).Structure determination revealed that the crystals exhibit the composition (benz)(14tb), based on one-dimensional (1D) supramolecular chains of alternating benz and 14tb molecules, held by C-I/p C halogen bonds between 14tb iodine atoms and each face of benz molecules (Fig. 2a).Specically, each 14tb  1, also ESI Table S2.†).The d I/C separations are shorter than the expected sum of van der Waals radii (3.68 Å) for carbon and iodine. 49he simplicity of components makes it surprising that (benz)(14tb) was not reported previously.The formation of halogen-bonded cocrystals with benz, however, is not without precedent, as 1D motifs similar to that in (benz)(14tb) have been found with Br 2 (ref.50) and 1,4-diiodotetrachlorobenzene as XB donors. 51While the d I/C distances in (benz)(14tb) are ∼5% shorter than the sum of corresponding van der Waals radii, the shortest distance between the iodine atom of a 14tb donor and a benz acceptor molecule is to the centroid of a carbon-carbon bond (3.42(1)Å).This is consistent with previously noted optimal geometry for halogen bonding to psystems, 36,37 and with the calculated electrostatic surface potential (ESP) surface of benz (Fig. 1a), showing that the areas of most negative ESP, expected to be benecial for halogen bonding, are located on the arene p-bonds.
Bulk analysis of (benz)(14tb) was hindered by rapid decomposition in air.Eventually, the formation of bulk cocrystal in a slurry of 14tb in excess benz was conrmed by powder X-ray diffraction (PXRD) analysis of a sample under a plastic-wrap cover (see ESI, Fig. S1 †). 52The (benz)(14tb) structure conrms that the simplest aromatic hydrocarbon can be used as an acceptor in forming C-I/p C chains (Fig. 2a) reminiscent of those in cocrystals of naphthalene and azulene with 14tb or oab. 30,43ext, we targeted anthracene (anthra) and tetracene (tet) as XB acceptors.For anthra, a cocrystal of composition (anthra)(14tb) 2 was obtained by mechanochemical liquidassisted grinding (LAG), 53,54 using nitromethane as a liquid additive (ESI, Fig. S2 †). 55Diffraction-quality single crystals of (anthra)(14tb) 2 were obtained by dissolving the mechanochemically made material in hot CH 2 Cl 2 , rapidly cooling in an ice bath for ca. 10 seconds, followed by slow evaporation.Structure analysis revealed that (anthra)(14tb) 2 is based on the ladder-like motif (Fig. 2b) with each face of the arene forming two C-I/p C bonds with 14tb molecules.The d I/C distances ranged from 3.427(9) Å to 3.583(7) Å, with : C-I/C angles in the range 157°to 173°.The distance between the iodine atom and the centroid of the relevant carbon-carbon bond was even shorter, at 3.366(5) Å, and no other short contacts were observed that would indicate C-H/p hydrogen bonding or p/p stacking between anthra molecules.The structure of (anthra)(14tb) 2 was also recently reported by Azzali et al. and our experimental data fully agrees with it. 55n the case of tet, the cocrystal (tet)(14tb) was obtained in the form of diffraction-quality single crystals by slow evaporation of a solution of tet and 14tb in 1,2,4-trichlorobenzene.Unlike the cocrystal containing anthra, the (tet)(14tb) cocrystal does not exhibit the ladder-like motif, but instead presents a zig-zag architecture of alternating tet and 14tb molecules interconnected via short, linear C-I/p C contacts (Fig. 2c, Table 1 and ESI Table S2 †).The individual I/C distances are again shorter than the sum of van der Waals radii for C and I atoms, at 3.410(6) Å and 3.509(7) Å, with respective : C-I/C angles of 163°a nd 174°.The corresponding distance of the 14tb iodine atom to the centroid of the relevant C-C bond on tet is again even shorter, at 3.387(5) Å, supporting the targeting of the p-system on the PAH.While tet is the only PAH in our study that did not produce the expected supramolecular ladder architecture, the  (tet)(14tb) structure conrms the general reliability of C-I/p C halogen bonds for PAH cocrystallisation.

Non-linear arenes
In order to explore the possibility of using XB for the assembly of arenes beyond the acene family, we targeted cocrystallisation with pyrene (pyr), benzanthracene (bant), perylene (pery) and coronene (cor), constituted of four, ve and six fused ring systems.Mechanochemical screening in all cases produced new cocrystal phases, with compositions (pyr)(14tb) 2 , (bant) 2 (14-tb) 5 , (pery)(14tb) 2 and (cor)(14tb) 2 (Fig. 2d-g).All four cocrystals were obtained in the form of diffraction-quality single crystals by crystallisation from CH 2 Cl 2 , and structural analysis revealed in each case the ladder motif analogous to the one seen with naphthalene, 30 azulene, 43 and anthra, with each face of the PAH participating in at least two linear C-I/p C halogen bonds.The halogen-bonded distances were in each cocrystal comparable, in the range from 3.4 Å to 3.6 Å, with : C-I/C angles in the range from 156°to 174°(see Table 1 and ESI Table S2 †).Each cocrystal was also obtained mechanochemically as a bulk powder, exhibiting PXRD patterns consistent with herein determined crystal structures (see ESI †).The stability and composition of each cocrystal was validated by differential scanning calorimetry and thermogravimetric analysis (DSC and TGA, respectively, see ESI †).The DSC analysis revealed that the cocrystals were generally stable up to ca. 110 °C, and upon decomposition exhibited a weight loss step consistent with a loss of 14tb.Structures of (pyr)(14tb) 2 , (bant) 2 (14tb) 5 , (pery)(14tb) 2 , (cor)(14tb) 2 and (dicor)(14tb) 3 (Fig. 2d-h) further highlight the robustness of the supramolecular ladder motif based on C-I/p C XB interactions, even with expanded aromatic systems as XB acceptors.
In the case of pyr, two other cocrystals with 14tb have previously been reported, with different compositions: (pyr)(14tb) 29 and (pyr) 4 (14tb). 56Both of these previously reported stoichiomorphs contain 14tb molecules involved in either extended (for (pyr)(14tb) cocrystal) or discrete (for (pyr) 4 (14tb) cocrystal) arene-peruoroarene p-stacking motifs with pyr molecules.The 14tb molecules in these two prior cocrystals also form C-I/C halogen bonds of ca.3.5 Å to neighboring pyr units.It is, therefore, notable, that the (pyr)(14tb) 2 phase reported here presents C-I/p C halogen bonding as the dominant intermolecular interaction, without any notable herringbone C-H/p or arene-peruoroarene pstacking interactions seen in other stoichiomorphs (Fig. 2d, also see ESI Fig. S4 †). 29,56ocrystallisation of bant, an isomer of pyr and tet, with 14tb resulted in a halogen-bonded cocrystal of composition (bant) 2 (14tb) 5 .The cocrystal was found to again contain the anticipated ladder motif (Fig. 2e).The ladder motif in (bant) 2 (14tb) 5 is unique, however, with different faces of each bant unit alternatively taking part in either two or three C-I/p C halogen bonds.
Crystal structures of cocrystals (pery)(14tb) 2 and (cor)(14tb) 2 (Fig. 2f and g) are, to the best of our knowledge, the rst examples of halogen bonding being used to form cocrystals with large, non-substituted PAHs comprising >4 aromatic rings.To further examine the ability of C-I/p C bonds to organise large arenes, we focused on dicoronylene (dicor), comprised of 15 aromatic rings (Fig. 1a).A sample of dicor was synthesised according to the reported procedure, 57 by reaction of cor at 170 °C in a 3 : 1 by weight melt of AlCl 3 and NaCl, followed by sublimation (see ESI †).Attempts to obtain cocrystals of 14tb and dicor from common organic solvents were not successful due to poor arene solubility, with crystallisation eventually achieved from molten 14tb at 110 °C.Single crystal X-ray analysis of the ruby-red crystals revealed a cocrystal structure with composition (dicor)(14tb) 3 , exhibiting the C-I/ p C ladder motif, but with each face of the arene now engaged in XBs with 3 separate 14tb donors (Fig. 2h).The XB distances lie in the range from 3.424(7) Å to 3.560(9) Å, with corresponding distances to the centroids of carbon-carbon bonds being in the range 3.327(6) Å to 3.579(8) Å.The halogen bonds exhibit a linear geometry, with : C-I/C angles adopting values from 159-178°.Notably, the C-I/p C bonds form on the edges of each dicor molecule, consistent with the calculated arene ESP surface, which shows that the highest negative potential is localised on the molecular rim.This means that a large fraction of the dicor molecule surface is not likely to engage in halogen bonding, and could be accessible for other types of interactions.Indeed, the dicor molecules are also engaged in p-p stacking between adjacent halogen-bonded ladders.
Of the three symmetrically distinct 14tb molecules in the (dicor)(14tb) 3 structure, two also participate in F/F and C-H/F contacts 58 with neighboring 14tb and dicor molecules, while one 14tb molecule also participates in F/I contacts.
The formation of this (dicor)(14tb) 3 shows that C-I/p C halogen bonds are sufficiently robust to enable the cocrystallisation of large PAHsto the best of our knowledge, no cocrystal of a non-derivatised PAH of similar size to dicor has ever previously been reported.Attempts to produce a bulk sample of (dicor)(14tb) 3 have so far been unsuccessful.

Non-planar aromatic systems: C 60
Next, we targeted a non-planar aromatic acceptor, buckminsterfullerene (C 60 ).While previous work reported that C 60 and 14tb do not form a cocrystal, 39 we found that milling of the two components produces a cocrystal of composition (C 60 )(14tb) 2 .Dark-red block-shaped crystals of (C 60 )(14tb) 2 were subsequently obtained by slow evaporation of a toluene solution of the mechanochemically made material.Crystal structure analysis revealed that (C 60 )(14tb) 2 is based on interpenetrated square-grid topology (sql) nets of C-I/p C bonds (Fig. 3a) with d I/C separation of 3.54(1) Å and halogen bond angle of 163°.The sql-nets are formed by C 60 molecules acting as four-fold nodes with 14tb molecules as framework linkers (Fig. 3b and  c).The structure also exhibits short C/F contacts between 14tb and C 60 molecules in adjacent nets (d F/C = 3.17(1) Å; : C-F/C = 102.8(4)°).0][61][62] Analysis of (C 60 )(14tb) 2 by DSC and TGA reveals decomposition around 110 °C, evidenced by a single endothermic event in the DSC thermogram, accompanied by the loss of 50% of sample weight, consistent with the theoretical content of 14tb (52.7%).

Replacement of the halogen bond donor
In order to further explore the generality of the supramolecular ladder motif formed with PAHs, we next attempted cocrystallisation of anthra and pyr with oab, a longer XB donor.In each case, milling of the XB donor and acceptor followed by PXRD analysis revealed a new cocrystal phase, with compositions (anthra)(oab) 2 and (pyr)(oab) 2 .Crystallographic analysis of single crystals grown by recrystallisation of the mechanochemically-made material from CH 2 Cl 2 revealed the expected C-I/p C ladder-like architectures in both cases (Fig. 3d, e, Table 1 and ESI Table S2 †).Importantly, the measured I/C and I$$$centroid distances are not appreciably different from those in (anthra)(14tb) 2 and (pyr)(14tb) 2 .Both (anthra)(oab) 2 and (pyr)(oab) 2 were found to melt around 160-170 °C, as established by DSC, with TGA showing that samples completely evaporate, in a single step, at temperatures around 200 °C.

Periodic DFT calculations
The of herein reported cocrystals was also investigated by plane-wave periodic-density functional theory (DFT) calculations in CASTEP, 63 using PBE 64 functionals combined with Grimme D3 semiempirical dispersion correction. 65ble 1 Calculated energies of cocrystal formation (E calc , in kJ mol −1 ) from starting materials, halogen bond interaction energies (E XB , in kJ mol −1 ), and experimental C-I/C (in Å) distances for herein reported cocrystals.a For structures where an arene molecules forms C-I/p C halogen bonds with multiple XB donors, the average E XB is provided, with individual E XB values in brackets  Periodic DFT was previously shown 66 to be effective for understanding the thermodynamic stability of halogen-bonded cocrystals.The calculated cocrystal formation energies (E calc ) with respect to individual crystalline components (Table 1) were found to be negative in all but one case, indicating that cocrystallisation was enthalpically favorable in general.The exception is (dicor)(14tb) 3 , with E calc of +6.48 kJ mol −1 suggesting that its formation may be entropy driven.Vibrational contributions to the Free energies related to cocrystallisation were, however, not taken into account due to the high computational cost of periodic DFT phonon calculations.The energies associated with individual halogen bonds (E XB ) were obtained by subtracting the total energies of individual component molecules from the total energy of each distinct XB donor-acceptor dimer unit found in the cocrystal structure.The E XB values were highly consistent across all explored XB acceptors, with all dimer interaction energies being exothermic and falling within the range of −13 kJ mol −1 to −21 kJ mol −1 .

Analysis of the Cambridge structural database (CSD)
Our systematic experimental study reveals the robust supramolecular synthon based on C-I/p C halogen bonds for the directional assembly of carbon-only aromatic systems of various shapes and sizes.It is highly surprising that this expansive potential for directional arene assembly by C-I/p C bonds has not previously been noted, leading us to conduct an in-depth search of the Cambridge Structural Database (CSD) to probe for the appearance and geometry of such interactions involving the iodine atom of a C-I fragment as a donor and a carbon atom of a 5-, 6-or a 7-membered ring, a C]C, or a C^C moiety, as the acceptor.As XBs are expected to be shorter than the sum of the van der Waals radii of interacting atoms, the search was limited to I/C distances up to 4.68 Å, which is ca. 1 Å longer than the sum of van der Waals radii for carbon (1.7 Å) and iodine (1.98 Å). 49 The CSD searches revealed that short contacts between iodine and 6-membered rings are the most prevalent, representing ca.71% of the total short contacts examined, followed by contacts to C^C bonds (13%), C]C bonds (13%), while contacts involving 5-and 7-membered rings represented ca.2% and 1% of all identied short contacts (Fig. 4a).The CSD searches indicated two geometry-distinct regimes of C-I/p C interactions, dependent on the I/C p distance.Specically, there was no particular geometrical preference for longer I/C separations.However, shorter I/C distances were generally associated with a preference for linear C-I/C contacts, with angles in the range of ca.140-180°.Such behavior was particularly pronounced for C^C bonds and 6-membered rings as acceptors, where the majority of I/C distances below ∼3.7 Å were associated with a C-I/C angle of 140°or higher (Fig. 4b  and c).Such preference for linear geometries at lower I/C distances is consistent with XBs, 14d reinforcing the herein proposed role of halogen bonds as an overlooked, yet consistent, strategy for directional assembly of carbon-based aromatic system, and suggest the possible existence of other C-I/p C driven supramolecular synthons.In addition to structures identied in the CSD, during preparation of this manuscript two crystal structures were reported 67 that strongly reinforce the view of XBs as reliable interactions to form supramolecular architectures based on PAHs (Fig. 4d and e).These structures are based on phenanthrene and on chrysene, an isomer of tet and bant, and exhibit the C-I/p C ladder motif with PAH as the XB acceptor and 14tb as the donor.

Luminescence properties
Organic materials with emissive triplet states and long emission lifetimes (phosphorescence) are being sought for applications in OLEDs, 68 biological imaging, 69 anticounterfeiting 70 and more. 715][76] In this context, cocrystallisation of PAHs offers a promising platform for achieving organic phosphorescent materials, as the solid-state arrangement of aromatic units directly inuences diverse optoelectronic properties, such as excitation/emission wavelengths, and lifetime kinetics. 77,78alogen bonded arene cocrystals have been reported to exhibit phosphorescence decays on the scale of several milliseconds, [27][28][29][30][31][32]42,67,77 with an isolated case of decays >200 ms. 77b Recetly, Abe et al. reported luminescence properties of a (phenanthrene)(14tb) 2 cocrystal, and found that partial replacement (up to 25%) of phenanthrene with pyr leads to an increase of the phosphorescence quantum yield from ca. 6 to >20%.67 Further replacement of phenanthrene with pyr (up to 50%), however, led to a drop of phosphorescence efficiency.Notably, a cocrystal of composition (pyr)(14tb) 2 , in which all phenanthrene would be replaced by pyr, was not reportedbut (pyr)(14tb) showed a low luminescence quantum yield <1% and short lifetime of 50 ms 67 (500 ms 29 ).
Having herein synthesised the previously missing cocrystal of composition (pyr)(14tb) 2 , we explored its photoluminescence properties, along with those of the heavier congeners (pery)(14tb) 2 and (cor)(14tb) 2 (Fig. 5).The cocrystals of cor, pyr, or pery with 14tb all exhibited strong luminescence shis when compared to the pristine solid PAH.The previously not accessible (pyr)(14tb) 2 exhibited a remarkable bathochromic shi of almost 200 nm, with the maximum emission wavelength (l max ) at 678 nm (red), compared to 498 nm (blue) for solid pyr (Fig. 5a and b).The red-shied l max and the emission life-time (10.4 ms) are similar to the previously reported behavior of (pyr)(14tb) 29,67 and suggest the phosphorescent nature of the emission.A similar behavior was displayed by (cor)(14tb) 2 cocrystals: a signicant ∼150 nm red shi of l max (from 513 nm in pristine solid cor to 660 nm in the cocrystal, Fig. 5c and d) and very long emission life-time (4.2 ms).Notably, the emission of (cor)(14tb) 2 extends in the NIR region, beyond 750 nm, which is typically difficult to achieve because of the energy-gap law. 78n a sharp contrast to the red-shied emission (pyr)(14tb) 2 and (cor)(14tb) 2 cocrystals, the (pery)(14tb) 2 cocrystals exhibited a strong ∼100 nm hypsochromic shi of emission, from l max = 607 nm (orange) in pristine pery solid (a-pery polymorph) to 521 nm (green) of the cocrystal (Fig. 5e and f).Such blue-shiing behavior is unusual, as cocrystals exhibiting C-I/p C interactions generally exhibit red-shied emission compared to the pure emissive component solid. 79This behavior of (pery)(14tb) 2 is reminiscent of the J-aggregate emission of the b-pery crystals 80 in which the pery molecules are arranged in off-diagonal interaggregate interactions, similar to (pery)(14tb) 2 (see ESI, Fig. S36 †).Such J-aggregated states are known to enhance the uorescence emission radiative rates, outcompeting intersystem crossing rates from populating triplet excitons. 81Indeed, the emission lifetime observed in (pery)(14tb) 2 was <0.62 ns (see ESI, Fig. S35 †).The observed blue shi of (pery)(14tb) 2 cocrystals (as well as b-pery crystals) compared to a-pery is explained by disruption of the excimer emission resulting from the p-dimer packing motif of the latter structure.
Consequently, the incorporation of the C-I/p C interactions in co-crystals of the PAHs can enable their room-temperature phosphorescence (RTP) through the heavy atom effect, and is commonly manifested in the red-shied emission of cocrystals compared to pure PAH.However, the RTP properties also depend on a balance of inter-system crossing vs. uorescence transition which varies with the molecular and crystal structure.Thus, the fast uorescence transition in (pery)(14tb) 2 brought about by the specic packing of PAH molecules (J-aggregation) prevents the observation of RTP.

Conclusions
In summary, we have presented an extensive and systematic experimental, theoretical, and database study that establishes halogen bonding to aromatic carbon systems as a powerful, but historically overlooked approach for the design of supramolecular architectures based on carbon as the molecular recognition site.This work coalesces and advances scattered reports of halogen bonds to p-systems, and demonstrates not only that C-I/p C halogen bonding can be reliably used as a directional interaction for the synthesis of cocrystals of differently-sized and -shaped carbon-only aromatic systems, but also establishes the supramolecular ladder structure based on C-I/p C halogen bonds as a unique supramolecular synthon generally applicable to non-substituted carbon systems.
In particular, C-I/p C halogen bonding has been effective in cocrystallisation of all 9 explored aromatic carbon molecules, from benzene (one aromatic ring) to dicoronylene (15 fused aromatic rings), resulting in altogether 11 cocrystals.Focusing on PAHs only (i.e., excluding benzene and C 60 ), C-I/p C halogen bonding led to the formation of the expected supramolecular ladder motif in 8 out of 9 cases, with PAH units acting as the rungs and halogen bond donor molecules as the rails.5][46] From that perspective, the synthesis of C-I/p C ladder motifs based on non-substituted arene hydrocarbons is highly reliable, with an 89% supramolecular yield.If one includes in this overview the recently reported cocrystals of naphthalene, 30 azulene (two cocrystals), 43 phenanthrene and chrysene, 67 which were all found to exhibit the C-I/p C ladder motif, the supramolecular yield rises to 13 out of 14 cases, i.e. 93%.
The high reliability of this supramolecular cocrystal-forming reaction is surprising when considering that traditionally the only reliable way to achieve cocrystallisation of arenes was through p-stacking.The C-I/p C halogen-bonded ladder now suggests a route for crystal engineers to rationally use nonsubstituted PAHs as building blocks in designing new carbonbased supramolecular architectures, different from p-stacked arrays.‡ The C-I/p C halogen bonds underlying the ladder motif oen coincide with the sites of greatest negative ESP on the arene, and a single side of a six-membered carbon ring is never observed to participate in more than one halogen bond.These observations establish a different element of design for the assembly of supramolecular architectures from nonsubstituted arenes, based on the directional interaction of halogen bonding, rather than the traditionally used and nondirectional p-stacking.The ability to generate new structures based on simple PAH components is of signicance in the context of materials with new properties.That is illustrated by very large red or blue shis (in the range of ca. 100 nm to 200 nm), as well as room-temperature phosphorescence achieved for PAHs engaged in the supramolecular C-I/p C ladder structures.In particular, the phosphorescence lifetime of >4 ms observed for the coronene-based cocrystal provides further incentive to explore the design of fully organic phosphorescent systems based on halogen bonding to aromatic system.

Fig. 1
Fig. 1 (a) Halogen bond acceptor and donor molecules explored in this work, along with corresponding electrostatic surface potential (ESP) maps, with isosurfaces plotted at 0.01 a.u.Fragments of the halogen-bonded C-I/p C supramolecular ladder seen in the cocrystals of: (b) (azulene)(14tfib) 2 and (c) (azulene)(ofiab) 2 along with corresponding molecular diagrams.43

Fig. 4
Fig. 4 Outcomes of CSD searches for short C-I/C contacts to alkene, alkyne, 5-, 6-and 7-membered ring acceptors based on aromatic carbon and selected representative structures.(a) Distribution of the I/C contact lengths shorter than 4.68 Å across different acceptor types.For each XB acceptor type, the fraction of contacts with a linear geometry (C-I/C angle in the range from 140°to 180°) corresponding to halogen bonding, is indicated on the outer circle by a dotted pattern.Plots of the distribution of lengths and angles for C-I/C contacts to: (b) 6membered ring and (c) alkyne (C^C) acceptors.To guide the eye, contacts shorter than the van der Waals limit of 3.68 Å are shown in purple.Plots of the distribution of C-I/C contact lengths and angles for 5-and 7-membered ring, as well as alkene (C]C) acceptors, are given in the ESI.† Fragment of the crystal structures 67 of: (d) (phenanthrene)(14tfib) 2 and (e) (chrysene)(14tfib) 3 , illustrating the appearance of the C-I/p C ladder motif.