Impact of Hydrogen and Halogen Bonding Interactions on the Packing

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The Impact of Hydrogen and Halogen Bonding Interactions on the Packing and Ionicity of Charge-Transfer Cocrystals Rajesh Goud Nagula, and Adam J. Matzger Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01548 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Crystal Growth & Design

The Impact of Hydrogen and Halogen Bonding Interactions on the Packing and Ionicity of Charge-Transfer Cocrystals N. Rajesh Goud, and Adam J. Matzger * Department of Chemistry and the Macromolecular Science and Engineering Program, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, United States

ABSTRACT: With an aim of understanding the influence of robust charge-transfer (CT) interactions and auxiliary hydrogen and halogen bonds in tailoring the crystal packing, we have synthesized four CT cocrystals involving 1,5-diaminonapthalene (DAN) as donor and fluoranil (FA), chloranil (CA), bromanil (BA) and iodanil (IA) as acceptors. While the CT interactions take the primary role in guiding the 3dimensional assembly in all the cocrystals, N-H···O and C-H···O hydrogen bonds play a significant role as auxiliary interactions in stabilizing the mixed stack arrangement in DAN-FA, DAN-CA, and DAN-BA cocrystals, whereas I···N halogen bond assist the segregated stack supramolecular packing in DAN-IA. The experimentally determined ionicity values of mixed stack DAN-CA and DAN-BA cocrystals were found to be 0.27 e and 0.23 e indicating significant charge transfer that increases with electronegativity of the halogen substituents.

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INTRODUCTION Organic charge-transfer (CT) cocrystals are highly promising materials for a range of applications due to their technologically relevant properties related to ferroelectricity,1 magneto-conductance,2 memory and photo-switching,3 light emission,4 and charge transport.5-7 Of late there has been great interest in tailoring these donor-acceptor (DA) systems for ambipolar single crystal field-effect transistors.6-7 There is a strong relationship between the electrical properties and the packing motif of DA crystals. Semiconducting properties are usually displayed by mixed-stack systems in which the donor and acceptor molecules alternate along the stacking directions.5,7 Therefore, a deeper understanding of design principles in guiding the molecular assembly of these cocrystals is desirable. Hydrogen and halogen bonding interactions play a major role in tailoring the supramolecular architectures of CT complexes and their properties.8 From the earliest discovery of a neutral-ionic transition in the TTF-CA (tetrathiafulvalene-chloranil) CT cocrystal, where the phase transition is accompanied by the structural transition of a two-dimensional C-H···O hydrogen bonded layer,9,10 to the recent report of σhole mediated I···N halogen bond guided segregated packing, and as a consequence the semiconducting properties, of BPE-IFB (where BPE = 1,2-bis (4-pyridyl)ethylene and IFB = 1,3,5-trifluoro-2,4,6triiodobenzene) cocrystal,11 non-covalent interactions play a key role in advancing the design and discovery of novel charge transfer complexes. This approach took a major boost through the seminal findings of Whangbo et al., who demonstrated that the superconducting transition temperature (Tc) of β(BEDT-TTF)2AuI2 (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) was controlled by a set of weak Csp3-H···I hydrogen bonds.12 These findings led to the synthesis of TTF derivatives functionalized by hydrogen bond donors such as –CH2OH, -CONHR, etc., which are shown to form complexes with TCNQ (7,7,8,8-tetracyanoqunodimethane)

and

TCNQF4

(2,3,5,6-tetrafluoro-7,7,8,8-

tetracyanoquinodimethane).13,14 A number of literature reports have disclosed the importance of halogen bonding interactions as a supramolecular tool for engineering CT cocrystals, exemplified by complexes


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such as (EDT-TTFCl2)2(TCNQF4) (where EDT-TTFCl2 = dichlorinated ethylenedithiotetrathiafulvalene) and DDQ with iodinated TTFs (where DDQ = 2,3-dichloro-5,6-dicyano-p-benzoquinone).8,15 This crystal engineering approach towards designing CT cocrystals using hydrogen bonds and other noncovalent interactions has generally relied on using TTF derivatives or aromatic hydrocarbons as donors and TCNQ or DDQ derivatives as acceptors. However, expanding the structural landscape with respect to donors and acceptors would be influential in understanding the role of diverse non-covalent interactions in tailoring supramolecular CT architectures, and contribute to establishing improved ‘structure-property’ relationships in organic optoelectronic systems.16 With these considerations in mind, we have synthesized CT cocrystals using 1,5-diaminonapthalene (DAN) as donor and halogen substituted benzoquinones (XA) as acceptors (Figure 1) ; indeed DAN was found to be a potent donor for halogenated benzoquinone acceptors leading to the synthesis of cocrystals under all conditions explored. Scattered reports describe the importance of halogen substitution and its impact on the molecular orientation and crystal packing. However, the influence of a series of halogen substitutions (F to I) on donor or acceptor and understanding its influence on the crystal packing and ionicity of CT cocrystals is a gap in current knowledge filled in the present study. In addition to the strong charge-transfer interactions along the πstacking axis, these DA combinations were hypothesized to have dual advantages. First, from a crystal engineering point of view, the halogen substituents are known to participate in type I/type II17,18 or σ-hole based halogen bonding interactions19,20 (type I [θ1 ~ θ2] and type II [θ1 ~ 180°, θ2 ~ 90°] where θ1 and θ2 are the C‒X···X' and C‒X'···X angles [X, X' are Cl, Br and I]; the σ-hole bond is the non-covalent interaction between a covalently bonded atom of groups IV-VII and a negative site, e.g., a lone pair of a Lewis base or an anion). Similarly, the –C=O and –NH2 functional groups of the participating DA systems are known to form robust N-H···O hydrogen bonds.21 Hence, we have envisaged that the resulting complexes could serve as a prototype to understand the structural outcome of supramolecular competition and cooperation between hydrogen and halogen bonded non-covalent interactions.22 Second, the ionization potential (ID) of the donor (D) and the electron affinity (EA) of the acceptor (A) molecules



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generally dictate the degree of charge transfer (ρ) from donor to acceptor.5 Therefore, the halogen substitution would have the potential for energy gap engineering23 (Figure 2) which, in conjunction with their supramolecular assembly in CT complexes, could alter their electronic and transport properties. The information available in the literature on DAN-XA is fairly limited. In 1977, Tamura and Ogawa reported the structure (without the position of hydrogens) of the CT DAN-CA system.24 Kim et al. examined the structure and absorption spectra of 2,3-diaminonaphthalene with CA as the acceptor.25 However, the donor used by Kim is not the regioisomer of interest in this work.

Figure 1: Molecular structures of donor and acceptors selected for CT cocrystal synthesis. Solution crystallization experiments were performed on the D-A combinations in Figure 1 resulting in CT cocrystals as qualitatively evidenced by their characteristic dark color (Figure S1, Supporting Information). Detailed experimental procedures used to obtain these cocrystals are shown in the section 2, supporting information. As hypothesized, these complexes were sustained by CT π-π interactions, hydrogen bonds, and halogen bonds. While D-A π-π interactions and N-H···O hydrogen bonds guide the mixed stack crystal packing5 (…A-D-A-D…) in DAN-FA, DAN-CA and DAN-BA cocrystals, the 4 ACS Paragon Plus Environment

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supramolecular segregated stack assembly5 (…A-A-A…,…D-D-D…) of DAN-IA propagate through CT interactions and I···N halogen bonds. Structural analysis of these cocrystals from the perspective of noncovalent interactions, and understanding their influence on the 3-dimensional supramolecular assembly and ionicity form the content of this article.

Figure 2: Comparison of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of DAN (donor) and XA (acceptor) molecules calculated at the B3LYP/6-311G**26. Values were computed from individual molecules in the gas phase.

RESULTS AND DISCUSSION All the coformers except iodanil used in this study were obtained from commercial suppliers and used without further purification. Iodanil was synthesized from bromanil.27 Single crystal data of all the cocrystals were collected at 85 K. The structural parameters of all the cocrystals are presented in section 3,



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supporting information and the representative hydrogen and halogen bond distances in section 4, supporting information. They were further characterized using Raman Spectroscopy (section 5, supporting information), powder X-ray diffraction (section 6, supporting information), and differential scanning calorimetry (section 7, supporting information) techniques.

Crystal structure analysis: 1,5-diaminonapthalene:tetrafluoro-1,4-benzoquinone (DAN-FA, 1): This centrosymmetric cocrystal in the P21/c space group crystallizes in a 1:1 ratio with DAN and FA molecules lying on inversion centers. The donor and acceptor molecules along the π-stacking direction are offset with respect to one another (Figure 3a). The distance between the adjacent donor (DAN, positional centroid) and acceptor (FA, positional centroid) molecules is 3.12Å and the angle between the molecular planes is 2.92°. The inclination angles of the donor-acceptor molecules along the crystallographic axes have significant effect on the degree of charge transfer28 and these values along a-, b-, and c-axes are 49.7°, 26.0° and 29.4° for DAN and 53.8°, 23.8° and 26.7° for FA, respectively. Along the direction perpendicular to the π-stacking axis, DAN and FA molecules connect through N-H···O (N1-H1B···O1, 2.10 Å, 156.9°) and C-H···O (C4-H4···O1, 2.34 Å, 169.2°) hydrogen bonds (Figure 3b). These one-dimensional hydrogen bonded chains perpendicular to the π-stacking axis are further connected through bifurcated and unsymmetrical N-H···F (N1-H1A···F1, 2.38 Å, 141.6°; N1-H1A···F2, 2.58 Å, 152.1°) hydrogen bonds and quasi-type I/type II F···F halogen interactions,29 extending them into a two-dimensional architecture (Figure 3c). The F···F halogen interactions with θ1 and θ2 of 114° and 138.1° respectively (Figure 3d) and the θ1 - θ2 of 24.1° are within the expected range (15° ≤ | θ1 - θ2 | ≤ 30°) of quasi type I/type II halogen interactions.29 Guided by the π-π interactions along the charge-transfer axis and auxiliary hydrogen and halogen bonds extending them into the 3D space, the DAN-FA cocrystal adopts a mixed stack arrangement.


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(a)

(b)

(c)

(d)

Figure 3. a) Overlay of DAN and FA molecules along the π-stacking direction. b) DAN and FA molecules connected through N-H···O and C-H···O hydrogen bonds along the direction perpendicular to the π-stacking axis. c) Inter-stack 1D chains interact through N-H···F hydrogen bonds and F···F halogen interactions. d) Quasi type I/type II F···F contact between FA molecules.

1,5-diaminonapthalene:tetrachloro-1,4-benzoquinone (DAN-CA, 2): This 1:1 cocrystal crystallizes in the non-centrosymmetric polar Pn space group. Along the direction perpendicular to the stacking axis, although DAN and CA molecules form 1D chains like cocrystal 1 (Figure 4a), through N-H···O (N1H1B···O1, 2.02Å, 149.23°) and C-H···O hydrogen bonds, the C-H···O interaction with the O1 hydrogen 7 ACS Paragon Plus Environment


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bond acceptor (C15-H15A···O1, 2.48 Å, 164.8°) is larger than the O2 hydrogen acceptor (C10H10A···O2, 2.43 Å, 166.6°). Further, unlike the cocrystal 1, the halogen atoms on either side do not participate in bifurcated N-H···X (halogen) hydrogen bonds. Instead, the vinyl –Cl atoms form type I ( C5-Cl4···Cl2-C2, θ1 = 131.3°, θ2 = 126.4°, θ1 - θ2 = 4.9° are within the expected range of type I contacts, 0° ≤ | θ1 - θ2 | ≤ 15°) and type II (C4-Cl3···Cl1-C1, θ1 = 168.2°, θ2 = 106.4°, θ1 - θ2 = 61.8° are within the expected range of type II contacts,29 30° ≤ | θ1 - θ2 | ) halogen interactions on both sides with one set of vinyl –Cl atoms further involved in C-H···Cl (C9-H9A···Cl1, 2.86 Å, 126.2°) and N-H···Cl (N1H1A···Cl2, 2.77 Å, 157.6°) hydrogen bonds with DAN molecules (Figure 4b, 4c). A recent report indicates that it is extremely rare to simultaneously observe type I and type II interactions coexisting in the presence of robust hydrogen bonds.30 Cocrystal 2 exhibits this phenomenon. Despite significant differences with respect to the halogen interactions in comparison to cocrystal 1, DAN-CA also exhibits a mixed stack arrangement.

(a)

(b)


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(c) Figure 4. a) DAN and CA molecules connected through N-H···O and C-H···O hydrogen bonds along the direction perpendicular to the π-stacking axis. b) Hydrogen and halogen interactions extend the packing into a two-dimensional architecture. c) Type I/type II halogen interactions observed in the DAN-CA cocrystal.

1,5-diaminonapthalene:tetrabromo-1,4-benzoquinone (DAN-BA, 3): Like the previous cocrystals, DAN-BA exhibits a mixed stack arrangement stabilized by CT interactions along the π-stacking direction and the R21 (7) graph set ring motif31 mediated by N-H···O (N1-H2···O1, 2.01 Å, 157.4°) and C-H···O (C7-H7···O1, 2.32 Å, 169.6°) hydrogen bonds along the direction perpendicular to the π-stacking axis (Figure 5a). However, unlike the previous cocrystals, one of the symmetry independent bromine atoms forms a bifurcated C-H···Br (C6-H6···Br1, 2.92 Å, 125.1°; C5-H5···Br1, 2.91 Å, 125.9°) interaction with the adjacent DAN molecule, while the other bromine atom is involved in a type II halogen interaction with the bromine atom of an adjacent BA molecule (C1-Br1···Br2-C3, θ1 = 162.7°, θ2 = 121.6°) (Figure 5b). The θ1-θ2 of 41.1° indicating a Type II halogen bond is in agreement with a recent report on halogen substituted phenols, where the bromine atoms were shown to prefer type II halogen interactions.29



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(a)

(b)

Figure 5. a) N-H···O and C-H···O hydrogen bonds propagate the one-dimensional chain of DAN-BA cocrystal along the direction perpendicular to the π-stacking axis. b) C-H···Br and Br···Br secondary interactions extend the DAN-BA molecules into a two-dimensional architecture.

1,5-diaminonapthalene:tetraiodo-1,4-benzoquinone (DAN-IA, 4):

Unlike the previous cocrystals

where CT π-π interactions assisted by auxiliary N-H···O hydrogen bonds connect the D-A molecules, DAN-IA extends through auxiliary σ-hole directed halogen bonds instead of hydrogen bonds. The electrophilic character of iodine (positive electrostatic potential19,20 of iodine, Vs,max = 36.52 kcal/mol) takes precedence, and predictably forms a halogen bond with the –NH2 nitrogen of the adjacent DAN molecule. The preference for –NH2 nitrogen over the –C=O oxygen as halogen bond acceptor is due to the higher negative electrostatic potential19,20 (Vs,min) of the former functional group (Vs,min of nitrogen = 35.40 kcal/mol, Vs,min of oxygen = -30.09 kcal/mol). The I···N halogen bond (I1···N1, 2.90 Å, 173.7°) disposes IA and DAN molecules into a non-planar environment resembling a staircase-like arrangement (Figure 6a). The nearly linear C-I···N angle (173.7°) is in agreement with the n-σ* nature of the halogen


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interaction.32 This arrangement allows very large overlap of VDW (van der Waals) volumes, shortening the N···I distance by about 17 % with respect to the VDW radii of nitrogen (1.55 Å) and iodine atoms (1.98 Å). A crystal structural database (CSD) search based on I···N (NH2) distances resulted in 87 hits which revealed the I···N halogen bond exhibited in this cocrystal as the shortest in this category. Further, supported by C-H···O (C7-H7···O1, 2.45 Å, 137.3°) hydrogen bonds and type II (I1···I2, θ1 = 171°, θ2 = 85.7°, θ1- θ2 = 85.3°) halogen interactions, DAN-IA extend into a 2-dimensional architecture (Figure 6b). Remarkably, with I···N halogen bond taking precedence over the N-H···O hydrogen bond as the primary auxiliary interaction in guiding the DAN-IA cocrystal into a 3D architecture, DAN-IA crystallized in a segregated stack sequence.

(a)

(b)

Figure 6. a) Staircase-like disposition of IA and DAN molecules mediated by I···N halogen bond. b) Auxiliary C-H···O and type II halogen interactions extend the DAN-IA cocrystal into a 2-dimensional architecture.

The above analyses of DAN-XA cocrystals reveal supramolecular competition between hydrogen and halogen bonds.22 While the robust CT π-π interactions mediate the primary structural motif of all the cocrystals, the auxiliary hydrogen and halogen bonds significantly contribute to the overall packing of 11 ACS Paragon Plus Environment


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DAN-XA cocrystals. In DAN-FA, apart from the CT interactions, auxiliary N-H···O hydrogen bonds assisted by the isotropic and least polarizable halogen, fluorine participating in quasi type I/type II interactions extend the FA and DAN molecules into 3D space. While the N-H···O hydrogen bonds continue to assist the CT π-π interactions in guiding the D-A molecules in DAN-CA and DAN-BA, the electrophilic character and polarizability of the halogen atoms become increasingly prominent. As a consequence, the halogen interactions evolve from quasi type I/type II to type I and type II contacts in DAN-CA and pure type II interactions in DAN-BA. Numerous reports17,18 indicate that type I interactions are a result of VDW contacts and thus are not proper halogen bonds; on the other hand, type II interactions form through an electrostatic attraction between the electrophilic and nucleophilic regions of the participating groups, hence a ‘true’ halogen bond.30 Thus with increasing polarizability of the halogen atoms F < Cl < Br < I, the halogen interactions evolve from ‘quasi’ to ‘true’ halogen bonds in the DANXA cocrystals. As the polarizability reaches its maximum in IA, σ-hole mediated I···N halogen bonds take precedence as the primary auxiliary interaction in DAN-IA with hydrogen bonds demoted to play a secondary role in the form of C-H···O interactions. As a consequence of this supramolecular competition and cooperation, cocrystals 1, 2, and 3 exhibit mixed stack arrangement while DAN-IA assembles through a segregated stack arrangement (Figure 7).

(a)

(b)

Figure 7. a) Mixed stack arrangement in the DAN-FA, DAN-CA and DAN-BA cocrystals. b) Segregated stack arrangement in the DAN-IA cocrystal.


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In order to understand the generality of the observations drawn from this study (i.e. the influence of noncovalent interactions on the three dimensional structural arrangement), we have performed a CSD search to identify CT systems that have the potential for forming hydrogen and halogen bonds. Because molecular diversity could significantly alter the overall structural assembly, we have restricted our search to CT systems involving tetrahalobenzoquinones as one of the components. While a significant number of scattered CT complexes with a tetrahalobenzoquinone as acceptor are reported, to the best of our knowledge, tetrathiafulvalene (TTF)-tetrahalobenzoquinone (QX4) is the only other example where CT cocrystals of TTF are reported with all the halogen substitutions of tetrahalobenzoquinones (F to I), making it ideally suited for comparison with DAN-XA system. In the TTF-QX4 system, like the DAN-XA system, TTF-QF4, TTF-QCl4and TTF-QBr4 cocrystals, adopt a mixed stack arrangement.33,34,35 As a result it may be argued that the high degree of CT in these cocrystals, implying strong CT interactions, is the sole contributor to the mixed stack assembly and non-covalent interactions have negligible role in this regard. From this analysis, the segregated stack packing in DANIA cocrystal reported in this manuscript arise from the lower degree of CT (since iodo substituted halobenzoquinone is a poor acceptor) and not due to any significant contribution from I···N halogen bond in steering the overall packing into a segregated stack sequence. However, the three dimensional assembly in TTF-QI4, TTF-QBr2I2 and TTF-QBrI3 cocrystals argues against this interpretation.36 These cocrystals exhibit low ionicity values (ρ = 0.08, 0.11, 0.11 respectively) and yet stabilize in a mixed stack sequence.36,37 If strong CT interactions were to have a standalone influence on the preference for mixed stack arrangement, the lack of these (or rather weak based on the low ionicity) should have resulted in a segregated stack packing, through the influence of halogen bonding in TTF-QI4, TTF-QBr2I2 and TTFQBrI3 cocrystals. However, on contrary, the preference for mixed stack sequence in these cocrystals hints at the importance of other factors such as non-covalent interactions in influencing the structural arrangement. Based on the analysis of hydrogen and halogen bond distances (Table 1), the preference for



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mixed stack sequence in TTF-QX4 cocrystals may be reasoned to arise from the negligible influence of their weak hydrogen and halogen bonding interactions in comparison to the DAN-XA cocrystals. Table 1: Comparison of hydrogen and halogen bond lengths in DAN-XA and TTF-QX4 cocrystals Cocrystal DAN-FA

TTF-QF4 Refcode: TTFFAN DAN-CA

TTF-QCl4 (Refcode: TTFCAN02) DAN-BA

TTF-QBr4 (Refcode: ARIWAA) DAN-IA TTF-QI4 (Refcode: HUJNUX) TTF-QBr2I2 (Refcode: HUJPEJ) TTF-QBrI3 (Refcode: HUJPOT) EDOEDT-TTF (Refcode: VAFXIK)

Hydrogen Bonds (bond length (Å) and angle (°) N1-H1B···O1, 2.10, 156.9 C4-H4···O1, 2.34, 169.2 N1-H1A···F1, 2.38, 141.6 N1-H1A···F2, 2.58, 152.1 C3-H2···O1A, 2.66, 160.5 C1-H1···F1A, 2.46, 155.6 N1-H1B···O1, 2.02, 149.2 C15-H15A···O1, 2.48, 164.8 C9-H9A···Cl1, 2.86, 126.2 N1-H1A···Cl2, 2.77, 157.6 C1B-H1B···O1B, 2.27, 173.1 C3B-H2B···O1B, 2.41, 136.1 N1-H2···O1, 2.01, 157.4 C7-H7···O1, 2.32, 169.6 C6-H6···Br1, 2.92, 125.1 C5-H5···Br1, 2.91, 125.9 C8-H3···O1, 2.59, 165 C6-H1···Br1, 3.05, 153.8 C7-H7···O1, 2.45, 137.3 C6-H2···O1, 2.34, 123.4

Halogen Bonds (bond length (Å) and angle (°) Quasi Type I/Type II

Quasi Type I/ Type II Type I and Type II halogen bonds

Type I interactions Cl2B···S1B, 3.48 Type II halogen bonds

Br2-S4, 3.54 Type II halogen bonds I···N, 2.90 I1···S2A, 3.59

C6-H2···O1, 2.26, 126.9 C5-H1···Br1A, 3.03, 136.4 C5-H2···O1A, 2.36, 120.3

S1···I1A, 3.55 I1A···S2, 3.56

C10-H8···O3A, 2.66, 163.7

I2A···S3, 3.51

As shown in table 1, the dominating auxiliary interactions (hydrogen bond in case of DAN-FA, DAN-CA and DAN-BA and halogen bond in DAN-IA) in DAN-XA cocrystals are consistently shorter (stronger) as compared to TTF-QX4 cocrystals. Hence, the non-covalent interactions could be surmised to have a significant role in assisting the CT interactions in the three dimensional structural outcome in DAN-XA. Since the structural outcome of purely fluoro, chloro and bromo substituted cocrystals in DAN-XA and TTF-QX4 cocrystals is similar, the influence of non-covalent interactions may be best understood 14 ACS Paragon Plus Environment

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comparing DAN-IA cocrystal with TTF-QI4, TTF-QBr2I2 and TTF-QBrI3 cocrystals where, despite having similar ionicity values, the overall structural arrangement is strikingly different (segregated stack in DAN-IA and mixed stack in TTF-QI4, TTF-QBr2I2 and TTF-QBrI3). This contrasting structural preference may arise from the small negative electrostatic potential of sulfur (Vs,min = -19.16 kcal/mol) in TTF donor resulting in a weak σ-hole mediated I···S (3.55 Å -3.59 Å) halogen bond. In comparison, the negative electrostatic potential of –NH2 nitrogen of DAN (Vs,min of nitrogen = -35.40 kcal/mol) is much larger resulting in a strong I···N (2.90 Å) halogen bond. Hence, unlike the IA-DAN cocrystal, the weak halogen bonds in TTF-QI4, TTF-QBr2I2 and TTF-QBrI3 cocrystals have negligible contribution towards the three dimensional lattice arrangement and hence the CT interaction, despite weak, stabilize these cocrystals in a mixed stack sequence. For similar reasons, the only other known cocrystal of QI4 with ethylenedioxyethylenedithiothtrathiafulvalene (EDOEDT-TTF) also stabilizes (Vs,min of sulfur = -25.36 kcal/mol) in a mixed stack sequence.38

Degree of charge transfer: The degree of charge transfer or ionicity (ρ) is an important feature of a charge-transfer complex. It plays a central role in defining various properties such as optical absorption, electrical conductivity, Peierls transition, etc.28 In DAN-XA system, ionicity values are estimated for DAN-CA and DAN-BA cocrystals using Raman spectroscopy39 (Figure 8 and Table 2). In order to accomplish this measurement, anion radicals of CA and BA were synthesized using reported procedure (section 9, supporting information). The pure components (CA, BA), cocrystals (DAN-CA, DAN-BA) and the anion radicals40 (CA.-, BA.-) were characterized using Raman spectroscopy. The corresponding carbonyl peak in each spectrum (CA =1689.7 cm-1, CA.- = 1588.2 cm-1 and DAN-CA = 1663.3 cm-1 ; BA = 1681.3 cm-1, BA.- = 1582.3 cm-1 and DAN-BA = 1657.7 cm-1) was used to estimate ρ for both cocrystals41 (Figure 8) using the following equation:



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ρ = 2∆υ/υ0 (1- υ12 /υ02 )-1 Where ρ is ionicity or degree of charge transfer and ∆υ = υ0 - υcc. The υ0, υcc, and υ1 are the –C=O stretching frequency modes of pure components (CA or BA in cm-1), cocrystal (DAN-CA or DAN-BA in cm-1) and radical anions (CA.- or BA.- ) respectively. Ionicity (ρ) of a CT cocrystal is known to depend on various contributing parameters such as ionization potential of the donor, electron affinity of the acceptor, degree of tilt with respect to the crystallographic axes, electronegativity of the substituents etc.5 However, this dependence of ρ on any one of these parameters differs on a case to case basis and so far a universal factor guiding ρ in all the CT systems is lacking. In DAN-XA CT cocrystals, the experimentally derived ionicity values of DAN-CA and DANBA CT cocrystals were found to have a direct relationship to the electronegativity of the halogen substitution, i.e the more electronegative chlorine (3.0) substituted DAN-CA has an ionicity value of 0.27 e and the less electronegative bromine (2.8) substituted DAN-BA has an ionicity value of 0.23 e. This is in agreement with a recent report where ρ of the cocrystal had a direct relationship with the number of fluorine atoms attached to the acceptor.42


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(a)

(b) Figure 8. a) Stacked Raman spectra of the CA pure component, CA radical anion, and cocrystal. The carbonyl peak used to estimate ρ is differentiated (marked by the red ring) from the other parts of the 17 ACS Paragon Plus Environment


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spectra. b) Stacked Raman spectra of the BA pure component, BA radical anion, and DAN-BA cocrystal. The carbonyl peak used to estimate ρ is differentiated (marked by the red ring) from the other parts of the spectra.

Table 2: Degree of charge transfer (ρ) in DAN-CA and DAN-BA cocrystals. Cocrystal

(centroid- Electronegativity of dc-c centroid) (Ǻ) halogen atomsa

ρ(e)

DAN-CA

0.275±0.005

3.883, 3.958

3.0

DAN-BA

0.23±0.01

3.967

2.8

a

values were taken from ref. 43

CONCLUSION We have presented the structural properties of four cocrystals based on 1,5-diaminonapthalene as donor and 2,3,5,6-tetrahalo-1,4-benzoquinones (fluoranil, chloranil, bromanil, and iodanil) as acceptors. Structural analyses of these CT cocrystals reveal the importance of hydrogen and halogen bonding interactions in guiding the crystal packing. While strong CT interactions guide the primary structural assembly in all the cocrystals, N-H···O and C-H···O hydrogen bonds play a significant role in stabilizing the 3D architecture in the DAN-FA, DAN-CA, and DAN-BA cocrystals, leading to mixed stack arrangements. However, as I···N halogen bonds takes precedence over the hydrogen bonds in


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determining the 3D assembly of DAN-IA cocrystal, its crystal packing corresponds to a segregated stack sequence. Further, comparison with the reported tetrathiafulvalene (TTF)-halogen substituted benzoquinone (QX4) system revealed the importance of bond strength as an influential factor in controlling the three dimensional structural arrangement. Ionicity (ρ) of DAN-CA and DAN-BA cocrystals estimated using Raman spectroscopy is found to be 0.27 e and 0.23 e respectively. These values exhibit a direct relationship with the electronegativity of the substituted halogen atoms. This study underlines the importance of hydrogen and halogen bonding interactions on the 3D structural assembly of CT cocrystals. Competition and cooperation between the hydrogen and halogen bonding synthons assisted by the polarizability of halogen atoms, resulted in both mixed stack (DAN-FA, DANCA and DAN-BA) and segregated stack arrangements in these cocrystal series. With mixed stack and segregated stack sequences being an essential criterion for organic semiconducting and conducting materials respectively, the cocrystals presented here might have potential applications in organic electronics.

ASSOCIATED CONTENT Supporting Information The supporting information for this article is available free of charge on the ACS publication website. Optical microscopy images, detailed experimental preparation methods, Crystallographic parameters, hydrogen and halogen bonding distances, Raman spectra, Powder X-ray diffraction (PXRD) data, Differential scanning calorimetry (DSC) data, ORTEP diagrams, of all the cocrystals and PXRD patterns of radical anions of chloranil and bromanil compounds.



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Accession Codes CCDC numbers 1505948, 1505949, 1505953, 1505954 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the Army Research Office (ARO) in the form of a Multidisciplinary University Research Initiative (MURI) (W911NF-13-1-0387). We thank Dr. Jeff W. Kampf for single crystal X-ray analysis and acknowledge the funding from NSF Grant CHE-0840456 for the Rigaku AFC10K Saturn 944+ CCD based X-ray diffractometer. We would also like to thank Dr. Jean-Luc Bredas, Dr.Veaceslav Coropceanu and Dr. Rakesh K. Behera for helpful discussions.


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For Table of Contents Use Only The Impact of Hydrogen and Halogen Bonding Interactions on the Packing and Ionicity of Charge-Transfer Cocrystals N. Rajesh Goud, and Adam J. Matzger *

The supramolecular assembly of charge-transfer cocrystals involving 1,5-diaminonapthalene (donor) and tetrahalo-1,4-benzoquiones (acceptor) is influenced by the nature of dominating auxiliary interaction, i.e., when N-H···O hydrogen bond takes on the primary role in guiding the supramolecular assembly of DANFA, DAN-CA and DAN-BA cocrystals, they exhibit mixed stack arrangement. However as I···N halogen bond influences the molecular packing in IA-DAN cocrystal, it assembles in a segregated stack sequence (see figure).


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Impact of Hydrogen and Halogen Bonding Interactions on the Packing

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