Crystal structure and Hirshfeld surfaces analysis of Heterocyclic-and circulenes

The crystal structure of the new diazatrioxa[9]circulene and tetrahydro-diazatetraoxa[10]circulene which represent the first synthesized representatives of "higher" hetero[n]circulenes with n>8, was analyzed in details. Hirshfeld surface analyses, the dnorm surfaces and two-dimensional fingerprint plots were used to verify the contributions of the different intermolecular interactions within the crystal structure of diazatrioxa[9]circulene and tetrahydro-diazatetraoxa[10]circulene. The Hirshfeld surface analysis of the crystal structure clarifies that the most important contribution for crystal packing is from H∙∙∙H and C∙∙∙H intermolecular interactions for both circulenes. The shape-index surface shows that in the case of diazatrioxa[9]circulene two sides of the molecules are involved with the same contacts in neighbouring molecules and curvedness plots show flat surface patches that are characteristic of planar stacking. Such face-to-face structural organization provides the main charge transfer pathway in [9]circulene. In the case of [10]circulene, the area involved in the same contacts is much less, however, two types of intermolecular packing modes can form such flat surface patches at curvedness plots which is useful for more efficient charge transfer.

The structural feature of heterocirculene molecules is their planar or quasi-planar structure responsible for the specific photophysical properties and unique architecture of their molecular crystals [16][17][18][19]. Remarkably, that the electronic properties of heterocirculene molecules can be easily tuned via diverse chemical modifications. In particular, the replacement of one heteroatom by another or the introduction of side substituents into the heterocirculene molecule significantly affect their electronic molecular structure and structural organization in the solid-state. The relationship between crystal packing and transport properties for a series of synthesized hetero [8]circulenes is highlighted in our recent paper [20].
In this paper, we have systematically studied the crystal structure of the novel diazatrioxa [9]circulene and tetrahydro-diazatetraoxa [10]circulene. The application of Hirshfeld surface analysis approach [21] represent a convenient tool for the investigation of intermolecular interactions in crystals. The analysis of the Hirschfeld surface is especially suitable for visualizing the variations in intermolecular interactions in compounds and is becoming more common in crystalography. Plotting the corresponding fingerprint plots [22] provides a quantitative analysis of all types of intermolecular contacts that are present in the molecule and presents this information on a color plot.

Computational details
The Hirshfeld surface analysis represents a quantitative way to study the intermolecular interactions in crystalline systems. The Hirshfeld surfaces of the studied [9]-and [10]circulenes and its corresponding two-dimensional (2D) fingerprint plots were calculated with the aid of the Crystal Explorer 17.5 software at very high resolution [23][24][25]. The analysis is visualized by the normalized contact distance (dnorm), which is defined in terms of external and internal distances (de and di, respectively) and the van der Waals radii (vdW) of atoms and is computed with the following equation 1: are the van der Waals radii of the appropriate atoms internal and external to the surface, respectively. The contacts with distances equal to the sum of the vdW radii are indicated in white and the contacts with distances shorter or longer than vdW radii are represented in red and blue, respectively.

Structural analysis
The compound of diazatrioxa [9]circulene crystallizes in the orthorhombic system in space group Pbca; tetrahydro [10]circulene possesses triclinic crystalline structure of P-1 symmetry space group (Figure 2). In crystal the structure of [9]circulene macrocyclic core is perfectly planar, however, the presence of tert-butyl and propyl groups prevents the formation of a layered structure that is characteristic of planar heterocirculenes [20]. In the case of [9]circulene, the molecules form zigzag-type ribbons along ac plane (Figure 2a).
The tetrahydro [10]circulene adopts V-shaped cis form with unilaterally directed hydrogen atoms on four bridging sp 3 carbon atoms. The angle between the mean-plane of the two carbazoles is 73.7°. V-shaped molecules are stacked in a crystal in such a way that one molecule provides a free vacancy for the tert-butyl and propyl groups of another molecule ( Figure 2b). a b Fig. 2. Crystal structure of diazatrioxa [9]circulene and tetrahydro-diazatetraoxa [10]circulene.

Hirshfeld surface analysis
Hirshfeld surface analysis is performed in order to underpin the nature of the intermolecular contacts stabilizing the crystal structures of the diazatrioxa [9]circulene and tetrahydrodiazatetraoxa [10]circulene.

[9]Circulene
A Hirshfeld surface of diazatrioxa [9]circulene is illustrated in figure 3 and represents the interaction of the electron density of selected molecule with that of the surrounding crystal medium. Figure 3a, b shows that [9]circulene possesses two red spots, indicating contacts of hydrogen atom with each carbon atom of closely located benzene ring.
The shape-index map of the [9]circulene molecule was generated in the ranges −1 to 1 Å and is presented in figure 3c. From the shape-index map, it can be seen that the molecules are related to one another by π-π stacking interactions, which are indicated by adjacent red and blue triangles (highlighted by grey circles) on the shape-index surface. The blue triangles represent convex regions and symbolize the presence of ring carbon atoms of the molecule inside the surface, while the red triangles symbolize concave regions caused by carbon atoms of the π-stacked molecule above it. The presence of π-π stacking is clearly visible on the curvedness surface, generated in the ranges −4 to 4 Å (Figure 3d). The large green regions represent a relatively planar surface area, while the blue regions demonstrate areas of curvature. On the Hirshfeld surface the π-π stacking interactions is evident as planar green regions around the rings where the contact distances are similar (Figure 3d).
It should be noted that in the case of diazatrioxa [9]circulene the π-π stacks provides the main charge transfer pathway than other configurations with the hole and electron transfer integral for π-π stack modes are of 7.58 and 8.35 meV, respectively [15]. The final hole and electron mobility values in [9]circulene were reported to be 0.013 and 0.038 cm 2 V −1 s −1 , respectively [15].  Hirshfeld dnorm surface (a and b) of the intermolecular interactions, shape index map (c), and curvedness map (d) for diazatrioxa [9]circulene.
The two-dimensional (2D) fingerprint plot illustrates the contributions from the different intermolecular interaction types that overlap in the full fingerprint and then separated. The 2D fingerprint plot of [9]circulene is demonstrated in figure 4 and reveals that the main intermolecular interactions in [9]

[10]Circulene
The 3D representations of a Hirshfeld surface for tetrahydro [10]circulene is presented in figure 5. The distinct circular depressions (red spots) on the dnorm surface correspond to the short C•••H contacts (Figure 5a). The presence of the adjacent red and blue triangles (highlighted by grey circles) on the shape-index surface (Figure 5d) indicates the two sides of the [10]circulene molecules are associated with the same contacts in neighboring molecules, and the curvature plot (Figure 5e) show flat surface areas that are characteristic of planar overlap. It is interesting to note, that in the case of [10]circulene, the packing modes in figures 5a and 5b provide the main pathways for charge transfer with final hole and electron mobility calculated to be 0.163 and 0.009 cm 2 V −1 s −1 , respectively [15]. The relative contributions of the intermolecular contacts to the Hirshfeld surface area in tetrahydro [10]circulene crystal are shown in figure 6. For tetrahydro [10] Fig. 6. Fingerprint plots for tetrahydro [10]circulene (The de and di denote the external and internal distances, respectively). Below are presented the relative contributions of the intermolecular interactions to the Hirshfeld surface (%); interactions with contributions less than 1 % are not presented here.

Conclusion
In this study, we have performed a detailed analysis of the X-ray crystals for both [9]-and [10]circulenes based on the Hirshfeld surface analysis and their associated two-dimensional fingerprint plots to provide a complete understanding of the intermolecular interactions in a facile and immediate way. Molecular Hirshfeld surfaces revealed that the [9]-and [10]circulene compounds were supported mainly by H•••H and C•••H intermolecular interactions. It was also shown that the [9]circulene possesses π-π stacking interactions that follows from the shape index and curvedness maps. Moreover, fingerprint plots made it possible to identify all types of intermolecular interactions present in [9]-and [10]circulene crystals.
There are no conflicts to declare. This work was supported by the Ministry of Education and Science of Ukraine (Project 0121U107533).