Halogen Bonding in Diaza-Triisopropylsilyl-Tetracene Crystals

Article pubs.acs.org/crystal

Halogen Bonding in Diaza-Triisopropylsilyl-Tetracene Crystals? Michael Porz,† Frank Rominger,† and Uwe H. F. Bunz*,†,‡ †

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany

‡

S Supporting Information *

ABSTRACT: Four halogenated triisopropylsilylethynyl (TIPS)-tetracene derivatives were synthesized, their single crystal structures and optical and electronic properties were examined. Both, diiodo-TIPS-diazatetracene and dibromo-TIPSdiazatetracene showed herringbone arrangements, in which the halogen atoms are directed toward the nitrogen atoms of a neighboring molecule with slightly lower distances than the van der Waals radii between halogen and nitrogen atoms. Computations of the electrostatic potentials showed the availability and the strength of σ holes, so that we assume a weak halogen−nitrogen bonding. Tribromo-TIPS-diazatetracene delivered a 2D-brick wall structure with no sign of a halogen bonding. Dibromo-TIPS-tetracene crystallized neither in a herringbone nor in a brick wall motif. These results show that for halogenated azaacenes at least weak halogen−nitrogen interactions exist.

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crystallizes in a nonoverlapping motif,8 whereas the corresponding heteroacene, the diaza-TIPS-tetracene 2a, develops a brick wall structure with overlapping acene core, due to its dipolar nature (Figure 1).9 For tetrafluorodiaza-TIPS-tetracene and tetrachlorodiaza-TIPS-tetracene 2b, brick wall structures were also found. The halogenated parts of two successive molecules

INTRODUCTION N-Heteroacenes, as a counterpart of acenes, are of interest in organic electronics since they bear, in contrast to acenes, lowlying lowest unoccupied molecular orbitals (LUMO), which facilitate the injection of electrons.1 Consequently, larger Nheteroacenes such as tetraazapentacene are used as efficient and stable n- or ambipolar semiconductors in organic field effect transistors (OFETs).2 Midsized N-tetracenes are interesting materials for organic light emitting diodes (OLEDs).3 For both applications, the intermolecular packing arrangement is one of the critical points: while OFETs require a strong intermolecular interaction for effective charge transport, for example, π-stacking of the aromatic core, OLEDs profit from amorphous packing with little intermolecular coupling to hinder nonradiative decay.4 It is therefore important, besides controlling electronic properties, to tune the solid state structure of a potential material toward its application. Possible targets for tuning the crystal packing can be π−π interactions, electrostatic forces, hydrogen bonds, and steric repulsion but also halogen bonding.5 Unsubstituted acenes typically crystallize in herringbone structures due to the repulsion of π-orbitals and CH/π interactions of neighboring molecules. 6 Anthony et al. introduced an efficient way to design a brick wall motif for pentacenes. Bulky substituents, triisopropylsilyl (TIPS)-alkynes, force pentacenes into brickwall packing.7 However, regarding the aromatic core of the smaller congener TIPS-tetracene 1, it © XXXX American Chemical Society

Figure 1. Crystal structures of literature-known tetracenes: (left) TIPStetracene 1 (CSD-number 962667); (middle) diaza-TIPS-tetracene 2a (CSD-number 696051); (right) tetrachlorodiaza-TIPS-tetracene 2b (CSD-number 846412). Received: August 1, 2014 Revised: August 30, 2014

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dx.doi.org/10.1021/cg5011538 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

kJ/mol) from Aakeröy et al.13 The strength of the σ-holes of 4, 5, and 6 can therefore be classified as medium. Besides the σ-hole, the potential electron donor in 4−6 is identifiable as red, appearing as an electron-rich area. The requirements for halogen−nitrogen interactions are present in halogenated diazatetracenes. Single crystals of compounds 3−6 were grown from methanol/dichloromethane solution (2:1) by slow evaporation of the solvents. Crystal structures for all four compounds could be obtained, so we could investigate the influence of possible halogen bonding on the crystal packing. The compounds 4 and 5 crystallized in very similar herringbone motifs with paired flipped molecules (Figure 3a,b). The average distances between two pairs in 4 and 5 are almost the same.

were stacked on top of the pyrazine unit of the other molecule (Figure 1).1a The commanding effect of flourine and chlorine in halogenated azaacenes raises the question whether we could establish halogen−nitrogen bonding in crystal structures of azaacenes. Potential azaacenes should include bromine or iodine for this purpose, since they form stronger halogen bridges than Cl and F.10 The halogenated diazatetracenes are good candidates for this study because of the combination of mildly electron donating nitrogens and electron accepting halogen atoms. Moreover, they are easily available through a condensation reaction of halogenated o-quinones and a suitable diaminonaphthalene (Scheme 1). Here we focus on the solid states of diiodo-, dibromo-, and tribromodiazatetracene, as well as that of nitrogen-free dibromotetracene 3 as comparison. Scheme 1. Exemplary Tetracenes and Synthesis of the Halodiazateracenes via Condensation Reaction

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Figure 3. (a) Herringbone motif and chain links with N−I distance of 4, (b) herringbone motif and chain links with N−Br distance of 5, (c) (left) brick wall motif of 6 and (right) view along the b-axis, and (d) (left) ABmotif and (right) packing of flipped pairs for 3. Hydrogen atoms and, for the chain links, the ethynyl substituents are omitted for clarity.

Figure 2. Electrostatic potential maps of 3−6, (iso value 0.002 au, Spartan 10, DFT, B3LYP, 6-311+ G**).

For halogen bonding, the σ-hole of the halogen must point toward the potential electron donor, which is in this case the free electron pair of a nitrogen atom. It is remarkable that for both 4 and 5, the halogen atoms of one molecule point to the free electrons of the nitrogen atoms of a molecule in the neighboring pair, creating a chain of diazatetracenes (Figure 3a,b). The halogen and nitrogen atoms do not form a linear arrangement, in which the free electrons and the σ-hole are in one axis. This geometrical disadvantage (angle of 122−124°) weakens the halogen bond, but it is still possible that an interaction occurs, shown by calculations of Tsuzuki et al.14 In addition, the distance between the halogen and nitrogen atoms must be less the sum of their van der Waals radii.12 Looking closely at the distances in the crystal structures of 4 and 5, we found that in each pair one of the gaps is somewhat smaller than the sum of the van der Waals radii, determined by Rowland, Tailor, and Alvarez (N−Br = 3.4 vs 3.5 Å and N−I = 3.6 vs 3.7 Å).15 Even though this is just a moderate effect, it critically influences the crystal packing, leading to a herringbone structure instead of a brick wall motif. Introducing a third bromine (6) changes the packing motif back to a 2D brick wall (Figure 3c). The brominated rings now stack on top of the pyrazine rings of the successive molecules forming centrosymmetric dimers. The distance between bromine and nitrogen atoms exceeds the sum of the van der Waals radii, and the σ-hole does not point to the nitrogen atoms

RESULTS AND DISCUSSION For the formation of a halogen bond, it is necessary that the halogen atoms display a significant σ-hole, which interacts with an electron donor.12 The molecules 3−6 were therefore first examined by DFT studies at the B3LYP/6-311+G** level. We minimized the molecular geometry and calculated the electrostatic potential maps of 3−6 (Figure 2). The σ-holes (blue) are visible, and the strength of the potentials vary from ∼80 to 120 kJ/mol, showing the deepest σ-hole for the diiodide 4. These results are consistent with a comparable study of the weak acceptors 1,4-diiodo- and 1,4-dibromobenzene (108 and 82

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Article

Table 1. Crystal Structure Data for Compounds 1−6 compound

space group

a [Å]

b [Å]

c [Å]

α [deg]

β [deg]

γ [deg]

V [Å3]

Z

1 2a 3 4 5 6

P212121 P1̅ P1̅ P1̅ P1̅ C2/c

14.2210(4) 7.3968(7) 14.599(1) 13.610(4) 13.244(4) 35.2753(13)

15.1330(4) 13.893(2) 18.371(1) 15.889(4) 15.750(5) 15.2226(5)

16.8450(5) 18.428(2) 29.645(2) 18.550(5) 18.711(6) 14.7459(5)

90.00 112.065(7) 75.894(2) 83.035(5) 96.878(7) 90.00

90.00 95.624(7) 82.961(2) 80.225(5) 100.070(7) 104.246(1)

90.00 90.530(8) 89.936(2) 90.002(5) 90.069(7) 90.00

3625.15 1744.31 7650(1) 3923(2) 3814(2) 7674.8(5)

4 2 8 4 4 8

at all; halogen bonding is absent. The average distance of the rows is 3.5 Å, which is 0.1 Å more than that of the unsubstituted 2a and an accommodation to the bulk of the bromine substituents. The packing motif of 6 is influenced by the need for space for the substituents and the dipolar character of 6. The nitrogen-free 3 forms two layers of different packing in an ABmotif (Figure 3d); one layer with an edge to edge alignment of two molecules and a second zigzag layer with flipped centrosymmetric pairs, interrupting a possible brick wall structure. Neither a strong dipolar interaction nor a halogen bond formation was observed for 3, since the essential criterion, the pyrazine moiety in the aromatic core, is absent. For comparison with unsubstituted 1 and 2a, the cell parameters are summarized in Table 1. Besides the single crystals, the new compounds were characterized by UV/vis spectroscopy and cyclic voltammetry. The optical spectra were recorded in micromolar DCM solutions (Figure 4a). Dibromo-TIPS-tetracene 3 features the characteristic fine structure of acenes at the low energy part of the spectra with a maximum at 541 nm. The absorption maxima of 4, 5, and 6 (599, 602, and 611 nm) are red-shifted compared with that of 3, caused by introduction of two nitrogen atoms into the aromatic core. The substitution of iodine by bromine produces only a small effect in the absorption of 4 and 5, attributed to the small coefficient of the HOMOs of 4 and 5 on the substituted ring (see Supporting Information for calculations). For 6, which has a third bromine attached, we observed a bathochromic shift about 10 nm. The emission behavior is the same for 4 and 5 (643 and 644 nm), while 6 has an emission once more about 10 nm redshifted (657 nm). The electrochemical properties of the acenes are summarized in Figure 4b. In contrast to the tetracene, which shows reversible oxidation and reduction within the used voltage range, the diazatetracenes exhibit two reversible reduction peaks. Comparing the diazatetracenes, the iodo-compound 4 has the highest LUMO energy of this series, closely followed by 5 and then 6 where the additional bromine facilitates both reduction steps. All three heteroacenes exhibit higher reduction potentials than the “naked” 2a (greater than −1.23 V vs ferrocenium/ferrocene).1d

Figure 4. (a) Absorption (solid lines) and emission (dashed lines) of tetracenes in DCM and (b) cyclic voltammograms of the tetracenes obtained in DCM vs ferrocene (FeCp2) and Bu4NPF6 as supporting electrolyte.

halogen bonds are sensitive to steric influences and the detected halogen−nitrogen bonding for the less halogenated relative 5 was moderate. The nitrogen-free 3, where strong electrostatic effects are absent, crystallizes neither in a brick wall nor in a herringbone motif. The heteroacenes 4, 5, and 6 have higher reduction potentials than 2a, facilitating the injection of electrons. Overall, we demonstrated that even weak halogen− nitrogen bonds influence the packing behavior of azaacenes. This effect should be considered when designing solid state structures of azaacenes with halogen substituents.

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CONCLUSION We synthesized four TIPS-tetracenes 3−6 and examined their single crystal structures and optical and electronic properties. For the dihalogenated diazatetracenes 4 and 5, weak halogen− nitrogen bonds were detected in the crystalline state, influencing the packing motif. The halogens of one molecule of 4 or 5 point to the free electron pairs of the nitrogen atoms of a neighboring molecule. This interaction creates, in contrast to the brick wall motif typical for TIPS-alkynylated diazatetracenes, a herringbone motif for 4 and 5. For 6, which features a third bromine substituent, a brick wall motif was observed. We attribute this “fall-back” to the steric effect of the additional substituent, since

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic details and 1H and 13C NMR spectra, and further crystallographic data (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. C



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Article

AUTHOR INFORMATION

Corresponding Author

*Prof. Uwe Bunz. E-mail: [email protected]. Funding

We thank the Deutsche Forschungsgemeinschaft for generous support (DFG-Bu 7771/7-1). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank T. Schwaebel for helpful discussions. ABBREVIATIONS TIPS, (triisopropylsilyl)acetylene; LUMO, lowest unoccupied molecular orbital; OFET, organic field effect transistor; OLED, organic light emitting diode; DCM, dichloromethane

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REFERENCES

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Halogen Bonding in Diaza-Triisopropylsilyl-Tetracene Crystals

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