M.
Klues
and
G.
Witte
*
Fachbereich Physik, Philipps-Universität Marburg, 35032 Marburg, Germany. E-mail: gregor.witte@physik.uni-marburg.de
First published on 22nd November 2017
Since optoelectronic properties of organic semiconductors (OSCs) are largely affected by the molecular packing in the solid phase, further advances of such materials require comprehensive structure–property interrelations beyond single molecule considerations. While single molecular electronic properties can be tailored by synthetic means and their electronic properties can be reliably predicted by quantum chemical calculations, crystal structure predictions of such van der Waals bond solids remain challenging. Here we analyze correlations between the molecular structure and the resulting packing motifs adopted in the crystalline phases of the prototypical OSC pentacene as well as various differently substituted but similarly shaped π-conjugated molecules. Based on a Hirshfeld surface analysis and related fingerprint plots, specific contact points and their distribution are identified which allows classification of different structural groups. Comparing the fingerprint plots with corresponding molecular properties such as electrostatic contour plots as well as quadrupole and polarizability tensors, which were calculated by density functional theory, allows rationalizing structure determining specific intermolecular interactions. Our analysis shows in particular that molecules with uniform electrostatic potential at their periphery favor a herringbone packing, while the highly electronegative substituents (O, N and F) enable the formation of H-bonds and prefer slip-stacking or criss-cross packing motifs. The present correlations might be useful guidelines for future strategies to synthesis new OSCs.
Among the π-conjugated OSCs, pentacene (PEN) has become a prototypical model system, as it forms highly ordered crystalline phases, which exhibit large charge carrier mobility, and thus allows correlating optoelectronic properties with the packing motifs adopted in the individual polymorphs.11,35–38 Furthermore, acenes are versatile starting substances which allow rich chemical variations to tailor the molecular electronic properties.5,39 For example, due to its large electronegativity, fluorine forms highly polar C–F bonds, and perfluorination of acenes causes an inverted charge density distribution of the aromatic compounds yielding n-type SCs,40 while non-symmetric partial fluorination yields molecules with permanent dipole moments.40 Notably, oxidation of PEN affects not only the conjugation of the π-system but also changes the molecular arrangement from the characteristic herringbone packing into a planar slip stacking, as depicted in Fig. 1f.41,42 More recently, also N-substituted heteroacenes are considered as potential semiconductors since they are proposed as n-type SC.43–48 Interestingly, although being isoelectronic with PEN, diazapentacene (DAP) reveals a “criss cross” packing whereas dihydrodiazapentacene (DHDAP), which forms an anti-aromatic electronic system,49 adopts a herringbone packing like PEN.46,48 These examples show that seemingly small chemical variations can lead to distinctly different packing motifs in crystalline molecular solids.
A powerful method to analyze the prevalence of interactions and their spatial distribution in non-covalently bound molecular crystals is based on Hirshfeld surfaces and the related fingerprint plots.50–53 Such fingerprint plots provide a visualization containing implicit information of all intermolecular interactions as well as purely geometrical aspects like close contacts. This allows identification of prevailing interaction types such as, e.g. C⋯C or C⋯H interactions or hydrogen bonds.34,54,55 Moreover, the correlation analysis of the fingerprint plots of the various crystal structures enables a quantitative comparison of the respective distribution of all contact points. Since such contact point distributions are characteristic for the prevailing intermolecular forces, this allows detailed comparison of the various packing motifs.19
Here, we use such Hirshfeld surface and fingerprint plot analyses to compare intermolecular interactions occurring in crystalline phases of various pentacene-like molecules including the before mentioned azaacenes, fluorinated and oxidized pentacenes as well as hybrid forms of these species. Although not directly related to PEN, our analysis also covers dinaphthothienothiophene (DNTT), because of its exceptional charge carrier mobility56 as well as its geometrical resemblance. All molecules can be regarded as cuboids with dimensions similar to those of a PEN-molecule, which, in good approximation, also holds true for DNTT. The restriction to molecules without spatially extended or flexible functional side groups allows in particular excluding steric effects on the structure formation as well as entropic effects due to low energetic vibrations. Therefore, the molecules can be regarded as rigid and compact which allows dense packing. Thereby, the decisive intermolecular interactions of the partly quite different crystal structures emerge more clearly and may enable conclusions about the interrelation between the molecular structure and resulting packing motifs. In this direction, our study aims at finding correlations how chemical substitutions influence the molecular packing motifs. This complements current synthetic approaches that are usually driven by single molecular electronic considerations such as, e.g. influencing the charge distribution through mesomeric effects by substituents or functional groups as well as the addition of spacer groups.57–59 On the other hand, present developments of sophisticated ab initio based theoretical analyses are presently mostly limited to single molecules or dimers, while systematic studies are yet not available for OSC crystals.26,27 From this position, the present study provides a link between the various communities. In a long-term perspective, the presently analyzed molecular selection might even serve as meaningful benchmark collection for detailed theoretical analyses in order to quantify contributions of the different intermolecular interactions.
Fig. 2 Chemical structure of all molecules whose packing motifs are analyzed in this study. Besides their names, also the sum formula and abbreviations used throughout this work are given. |
Many organic compounds crystallize in different polymorphs, which occur also for some of the molecules examined here. As the Hirshfeld analysis requires an exact knowledge of the crystal structure with all atomic coordinates, we have also analyzed the packing motifs for the various polymorphs of PEN, PFP and QUA, since their precise structural data are available. For PEN, both bulk-structures, the Campbell-phase62 (denoted as PEN-C) and the Siegrist-phase63 (PEN-S) as well as the thin-film phase (PEN-TF)64 are considered, the latter of which is particularly relevant for thin-film device applications. For PFP, the bulk phase (PFP-bulk)40 and a substrate induced π-stacked phase (PFP-π)65 are taken into account, while for QUA, three well known phases (QUA-α, -β and -γ)66 are analyzed. Table 1 summarizes the symmetry and packing motifs of all crystalline phases considered in this study. Additional information on the unit cell parameters as well as visualizations of the corresponding crystal structures are provided in the ESI† (cf. Table ST1, Fig. S1).
Structure | Z | Space group | Packing motif | Ref. |
---|---|---|---|---|
PEN-C | 2 | P1 | HB | 62 |
PEN-S | 2 | P1 | HB | 63 |
PEN-TF | 2 | P1 | HB | 64 |
PFP-bulk | 2 | P21/c | HB | 40 |
PFP-π | 2 | P1 | SS | 65 |
DNTT | 2 | P21 | HB | 56 |
QUA-α | 1 | P1 | SS | 66 |
QUA-β | 2 | P21/c | SS/CC | 66 |
QUA-γ | 2 | P21/c | CC | 66 |
QUI | 2 | P21/b | SS/CC | 42 |
TET | 1 | P1 | SS | 41 |
DHDAP | 2 | P1 | HB | 45 |
DAP | 2 | P21/a | CC | 47/48 |
HFDP | 4 | P21/c | CC | 39 |
Generally, all molecules within the unit cell must be considered to fully represent the crystal structure by such fingerprint plots. In the case of QUA-α and TET, this is trivial as both structures possess primitive unit cells. Although the unit cells of some of the considered phases are not primitive, their crystal structure exhibits symmetry elements (such as e.g. a screw axis) which imply a uniform molecular environment for all molecules so that a fingerprint plot of only one molecule is sufficient to account for the full structure. However, this situation is not generally fulfilled. For example, the three PEN-phases as well as the DHDAP structure and the π-stacked polymorph of PFP have P1 symmetry with two molecules in the unit cell exhibiting a slightly different local environment. Since fingerprint plots are intended to serve as a measure to compare the interactions within the different structures, it would be useful also for these cases to have only one representative plot. To account for all intermolecular interactions within the unit cell, the distribution of the (di,de)-pairs of both molecules are averaged. This fingerprint plot is then compared with the fingerprint plots of other phases.
In addition, fingerprint plots also allow a quantitative comparison of the similarity of the appearance of interaction pairs using a method introduced by Parkin et al.55 For this purpose, the (di, de)-pairs are sorted in a grid of 15 × 15 bins with distances ranging from 0 to 3 Å and calculating correlation coefficients (average of Spearman and Pearson coefficients) of the contact point distributions for the various structures by utilizing standard routines implemented in Python.68 This yields quantitative values for the correlations while the graphical representation of the individual distributions in the form of the fingerprint plots enables only a visual comparison of the similarity. Moreover, the result of this correlation analysis is shown graphically as a dendrogram using the unweighted pair-group method with arithmetic mean (UPGMA).
In addition to the Hirshfeld analyses, also electronic properties of the molecules such as their quadrupole moments and polarizabilities are computed in the frame of density functional theory (DFT) using the US-GAMESS code.69,70 The DFT-calculations are performed using the B3LYP hybrid functional and a 6311G(d,p) basis set starting with structure optimization for each molecule. To visualize the electron density, molecular electrostatic potential (MEP) plots are generated for an isovalue of 0.002 au by using Molekel,71 which yields similar molecular volumina as the Hirshfeld volume and thus are comparable.72
The first objective of our study is to compare the fingerprint plots of the various crystal structures and divide them into groups according to their resemblance. This corresponds to sorting them by similar intermolecular interactions. Computing correlation (as described in the previous section) allows quantification of similarity that is beyond a qualitative comparison based on their visual appearance. The matrix with the pairwise correlations for the various structures is given in the ESI† (cf. Table ST3). These correlations are also visualized in Fig. 4 by a dendrogram, which is constructed by considering at first the largest pairwise similarities. This allows identifying three main groups each with similar distributions of contact pairs that are discussed in more detail below.
For QUI, TET and the three QUA phases, such features show up most clearly for the O⋯H contacts, while DAP and QUA-β also exhibit an acicular distribution of H⋯H contacts with distances smaller than 2.2 Å. Since DAP and HFDP do not contain any oxygen atoms, the needle-like extensions are related to N⋯H, C⋯H and F⋯H contacts. A comparison of their crystal structures shows that QUI, TET and QUA-α adopt a slip-stack packing motif, wherein QUI is slightly different and shows a small contortion of the molecules relative to each other. We note that also for QUI, a thin film phase (QUI-TF) has been identified recently.73 In this phase, the molecules adopt a similar structural packing motif to that in the bulk phase. Accordingly, also the corresponding fingerprint plot is rather similar (cf. Fig. S7, ESI†) and, therefore, has not been analyzed separately.
The QUA-β phase reveals a layered structure of uprightly oriented and slip-stacked molecules, while the slip-stacking direction in neighboring layers is alternately tilted (cf. Fig. S1, ESI†). This results in a needle-shape distribution of H⋯H contact points in the fingerprint plots (see Fig. 3n) which belong to distances between H-atoms at the short molecular side.
DAP, HFDP and QUA-γ display a third distinct packing motif, with molecules aligned in parallel stacks, with a twisting of adjacent stacks relative to each other. In such crystal structures, molecules adjoin on another at their long side yielding only few distinct point contacts of neighboring molecules as shown in Fig. 1g. Following the previous work of Paulus et al., this arrangement is denoted as the “criss-cross” motif.66 Interestingly, such a structure is also adopted by HFDP but additionally exhibits an alternating stacking of the molecular planes (cf. Fig. S1, ESI†). This feature is attributed to the static dipole moment oriented along the long molecular axis, which is introduced by the partial fluorination and will be discussed below.
Attractive interactions between π-conjugated organic molecules are generally ascribed to van der Waals forces which is actually a collective term for different electrostatic forces. These include direct interactions between permanent molecular charges or multipoles (so called Keesom interaction), Debye forces arising from interactions between permanent dipoles and induced dipoles in adjacent molecules, as well as dispersion forces (also denoted as London forces) due to interactions between fluctuating dipoles (formed by spontaneous polarization) and induced dipoles. Debye forces make up the smallest contribution to the total energy amount10,74 and seem to have only a subsidiary influence on the packing motif. In contrast, London dispersion forces constitute the main contribution whose strength scales with the dipolar polarizability of the involved molecules.26,27,75,76 Since the polarizability is a tensor, London forces generally depend not only on the static dipole polarizability, αtot, but also on the anisotropy of the tensor components. The presently considered ensemble reveals, however, rather similar relations (cf. Tab. ST4, ESI†). This results from the specific molecular selection, since the polarizability of such π-conjugated entities essentially depends on the number of valence electrons and the overall size of the molecule.26 Recent analyses of the benzene lattice energy have further shown that a precise description of dispersion forces in crystals of conjugated molecules also requires the consideration of nonadditive three-body interactions within sufficiently large distances.77,78
Although the presently considered molecules are of similar size, the oxygen containing molecules show somewhat lower polarizability, which is attributed to a loss of conjugation within the ring system. Since localization of charge at the CO bonds causes a permanent polarization, this reduces in turn the molecular π-polarizability. As the London dispersion is weaker in these cases, direct electrostatic interactions become more decisive for the packing motifs.
Commonly, such electrostatic interactions are described within a multipole expansion where typically only charges, dipoles and quadrupoles are considered. Except for HFDP, all molecules analyzed in this study exhibit no permanent dipole moment but differ in their quadrupole moments (cf. Tab. ST5, ESI†). Regarding multipolar electrostatic interactions, we would like to recall that in organic crystals, the molecular extension typically exceeds the intermolecular distances, so that a simple description by the far-field approximation is not valid anymore and instead other methods are required.79,80 Especially heteroatom substitutions can lead to notable charge redistribution within the π-conjugated molecular planes, resulting in strongly affected electrostatic moments. To illustrate this effect, the charge distributions within the molecules were calculated and are compared in Fig. 5 by the corresponding molecular electrostatic potential plots of the molecular planes. These MEP plots indicate characteristic differences and reveal in particular the before mentioned localized charge accumulations for the oxygen containing molecules QUI, TET and QUA. Although such MEP plots provide a vivid explanation for the occurrence of specific packing motifs, this simplified electrostatic description should be treated with some caution since polarization and screening effects due to mutual interactions are not taken into account. In contrast to fingerprint plots, which implicitly contain the information of the crystalline packing, all calculations concerning polarizability, quadrupole moments and MEPs are performed for isolated molecules without consideration of intermolecular interactions in the crystalline environment. In fact, previous studies have pointed out the importance of notable mutual polarization of π-conjugated systems at short distances. This charge penetration effect arises from a Coulomb interaction between the electron density of π-orbitals and positive core potentials of neighboring molecules due to a reduced screening of their nuclei and has to be taken into account when quantitatively analyzing the individual interactions.26,27,81,82
Besides the van der Waals forces, other attractive forces can occur which exhibit local dipolar interactions with a nearly covalent character due to hydrogen bonds. Such H-bonds appear when electron lone pairs of sufficiently small atoms (typically N, O or F) are in close contact with an electron deficient hydrogen atom of neighboring molecules.83
All previously considered forces are mainly of attractive nature and therefore counteract the Pauli exchange repulsion, which will dominate at sufficiently short intermolecular distances and hence limits the molecular packing density. Next, we analyze correlations between molecular characteristics and their packing motifs in the crystalline phases to find qualitative explanations for the various stacking patterns.
Herringbone packing motifs are commonly found for many unsubstituted polycyclic aromatic molecules such as acenes, thiophenes, and oligophenylenes. Interestingly, the same also applies to the case of DHDAP and DNTT. Although both species contain heteroatoms in their π-system, the electrostatic potential at the rim is rather uniform without sign change (cf.Fig. 5b and c). In the corresponding fingerprint plots, this packing motif manifests in a large fraction of C⋯H contacts distributed over a wide range of pair distances. This characteristic indicates that C⋯H contacts should not be seen as specific pair interactions but rather as π⋯H interactions mediated by the quadrupole moment. Within this approach, it becomes clear why C⋯H and N⋯H contacts are indistinguishable in the fingerprint plots of DHDAP because nitrogen is a part of the planar backbone and the π⋯H interaction does not distinguish between different ring atoms. In contrast, H⋯H interactions are repulsive which explains the generally large pair distances in the fingerprint plots (cf.Fig. 3f). The shortest H⋯H distance occurs only between the short sides of neighboring molecules. It results from the herringbone-packing motif, which appears together with a layered structure of uprightly oriented molecules and thus minimizes the region of H⋯H interaction. Notably, such crystallographic planes also have the lowest free surface energy87 and therefore form the preferred orientation of single crystals or crystalline molecular films grown on inert substrates (e.g. PEN(001) on SiO2).38
Interestingly, the fingerprint plots show some additional characteristics that do not appear for molecules of the first structural group. The most interesting feature is the distinct accumulation of F⋯F contacts (shown as green-red region in Fig. 3g and h) at an equilibrium distance of about 3.2 Å, while H⋯H contacts of PEN are homogeneously distributed (cf. inset in Fig. 3f). As the F⋯F interactions are not intrinsically attractive, this indicates a mediation of the Pauli exchange repulsion mainly via these contacts. The simple reason why this occurs for fluorine but not for hydrogen in the foregoing cases is their distinctly different atomic size. This becomes immediately evident when comparing the molecular packing motifs shown in Fig. 1b and d. The space-fill representation, using atomic van der Waals radii, shows fluorine atoms with about the same size as carbon atoms. This also yields an explanation for the orthogonal packing motif within the PFP-crystal: the PFP molecules can be considered as rectangular “bricks” (indicated by the enveloping rectangles in Fig. 1d). In contrast, molecules with a hydrogen rim appear slanted at the edge (indicated by dashed line in Fig. 1b). While this geometrical detail generally hampers a rectangular arrangement for PEN, it appears to be the best packing motif for the “PFP bricks”. This simple geometrical consideration is well supported by the corresponding molecular packing coefficients computed according to the definition of Kitaigorodskii.88 Using van der Waals radii provided by the CrystalExplorer package,89,90 typical packing coefficients of approximately 0.7 are obtained for all structures (see Table ST6, ESI†). Only the two PFP structures are more densely packed and yield values of about 0.8, while the packing coefficient of the partially fluorinated HFDP amounts to an intermediate value of 0.75.
Considering PFP molecules as cuboids also allows rationalizing the slip-stack packing motif occurring in the π-stacked phase (PFP-π) of PFP.65 As discussed in the previous section, electrostatic coupling of molecular quadrupoles favors either herringbone or slip-stack architectures. A detailed comparison of the crystal structure of both polymorphs of PFP shows that the latter arrangement is obtained from the bulk herringbone structure by turning every second molecule by 90°. This is possible since the molecules reveal already a quasi-one-dimensional slip-stack arrangement in the bulk structure (cf. Fig. S10, ESI†), hence demonstrating that both PFP packing motifs are actually very similar. Also, the C⋯F distances in both PFP phases are almost identical, again showing their intrinsic similarity. In addition, the perfluorination also results in a reduction of the π-orbital density above and below the ring plane, which reduces the mutual repulsion of π-stacked planes. Together with the attractive πδ+–Fδ− interaction due to slip stacking, this results in the smallest π-plane separation among the studied molecules of below 3.2 Å.
In contrast, such a slip-stack arrangement is not favorable for PEN. Due to its slanted molecular rim, a planar stacking would lead to the formation of voids and increased C⋯H distances that reduce the attractive intermolecular interaction. In fact, such a packing motif neither occurs in any PEN bulk phase, nor has it been observed in any substrate mediated PEN thin film.
The role of substituent effects on interaction and packing has been theoretically analyzed in great detail for small aromatic compounds such as thiophenes or substituted benzenes.86,92–95 Besides an energetically more favored slip stack-packing instead of a herringbone motif, also an alternately aligned stacking due to permanent dipoles is found. The quantitative analyses showed further that although electrostatic interaction is substantially weaker than dispersion interaction, it is highly orientational dependent and thus becomes a decisive parameter that controls the packing motif.
Among the presently studied molecules, such an alternating stacking is also found for HFDP (cf. Fig. S1, ESI†), which exhibits a permanent dipole moment of 5.2 D oriented along the long molecular axis due to partial fluorination (cf.Fig. 5i).
Remarkably, the crystalline structures of the third group exhibit an additional interaction. In all cases, atoms with lone electron pairs such as N, O or F are in close contact with hydrogen atoms of neighbor molecules, which allows the formation of hydrogen bonds. As such interactions are effective only at short “bond lengths”, their appearance manifests in the presence of characteristic needle like features in the element resolved fingerprint plots corresponding to hydrogen contacts to the N, O or F atoms.52,54 Good examples are QUI, TET and QUA (cf.Fig. 3j–o and S6, ESI†) where such needle-like features occur at O⋯H contact distances below 2.5 Å (or even <1.9 Å for QUA, cf. Tab. ST2, ESI†). In this context, it should be noted that because of the negligible X-ray cross section of H-atoms their position can only be determined approximately by means of X-ray diffraction, thus leaving some uncertainty in the exact strength of such H-bonds.96
Despite additional static electrostatic interactions, QUI shows surprisingly small lattice energy and, although having a larger mass than PEN, sublimates at lower temperature.41 This can be realized by reduced polarizability (caused by the polar CO bonds, cf. Tab. ST4, ESI†) and an increased distance between the molecular planes due to oxygen atoms acting as the spacer. Both effects reduce the attractive dispersion interaction, which shows that additional directed electrostatic interactions may not automatically enhance the lattice energy.
Within a theoretical study for various substituted benzenes, similar characteristics were reported. While in most cases substitution leads to a stabilization compared to benzene, some configurations show reduced interaction energies due to increased repulsion.22
This competition of interactions is also reflected in the crystal structure of QUI, which cannot be unambiguously assigned to one of the before mentioned packing motifs and instead shows a mixture of slip-stack and criss-cross (cf.Table 1 and Fig. S1, ESI†). Notably, by introducing an external boundary condition like a substrate interaction, a thin-film phase is formed for QUI, which features a truly parallel molecular arrangement.73
The situation is different for the other oxo-species of PEN, namely TET. Here, the O⋯H distance is 0.2 Å shorter, which yields stronger hydrogen bonds and leads to a uniform slip-stack motif. Even stronger H-bonds occur in all three QUA structures revealing H⋯O distances below 1.9 Å. Notably, the strong H-bonds even weaken the CO bond within the molecule, allowing the molecule to gain conjugation, which in turn affects the energy-levels as reported previously.60,97
The profound influence of H-bonds on the resulting packing motifs has also been demonstrated for larger aromatic systems, such as hexabenzocoronene (HBC) and its partially-fluorinated derivatives. Here, the symmetrically fluorinated hexafluoro-HBC, which has no permanent dipole moment, reveals pronounced F⋯H-bonds and a slip stack packing, whereas the non-substituted HBC adopts a herringbone packing.98
Commonly, short π–π distances are considered beneficial to achieve high charge carrier mobility. Remarkably, the best performing OSC among the assortment analyzed here, such as PEN, DHDAP and DNTT, exhibit, however, no π-stacking and instead adopt a herringbone packing motif. This indicates that also other contact points could be of great importance. To illustrate this situation, the Hirshfeld surfaces of PEN-C, DNTT and QUA-γ are compared in Fig. 6. Here, the color code denotes the comparison of di and de values relative to the van der Waals radii of the corresponding atoms. Red areas mark areas with the sum of d-values smaller than the sum of vdW-radii, while blue areas indicate the opposite. Hence, red indicates regions where neighboring molecules draw very close, thus yielding a maximal orbital overlap. A conclusive example constitutes QUA-γ where the O⋯HN contact points can be clearly seen as red regions. Comparison of the Hirshfeld surfaces of PEN and DNTT reveals particularly pronounced contact points only for DNTT. They are located at the middle of the long side of DNTT molecules and can be assigned to C⋯S contacts. Considering that overlap integrals depend on distances in a nonlinear fashion and that the sulfur atoms of DNTT are constituent of the conjugated system (in contrast to the H-atoms of PEN), this provides an efficient electronic coupling and might explain the superior charge transport properties of DNTT.
• A herringbone motif is favored for molecules with a uniform sign of charge for the electrostatic potential at the periphery of the molecule.
• Conversely, disturbances of the uniform potential at the periphery cause slip-stack or criss-cross packing via introduction of strong local electrostatic interactions or even hydrogen bonds.
• In most cases, the latter interactions tend to dominate structure formation.
• The van der Waals radius of atoms at the periphery of π-conjugated molecules determines the herringbone angles and allows explaining the similarity of the seemingly different polymorphs of PFP.
• Larger heteroatomic substituents within the aromatic ring systems like, e.g. sulfur in DNTT, provide contact points with efficient electronic coupling to neighboring molecules.
These observations might be helpful guidelines for future strategies to synthesize new OSCs aiming to achieve or avoid specific packing motifs. Moreover, the present selection of molecules and the analysis of their contact points and interactions in crystals may serve as a meaningful benchmark ensemble for refined theoretical descriptions of intermolecular forces. The recent progress in the computational-based description of non-covalent interactions in molecular solids has recently also allowed detailed analyses of the individual forces between larger molecules such as acenes.26,30 It would therefore be very important to extend these studies also to the crystalline phases of the presently considered molecular ensembles of geometrically similar but chemically modified molecules, in order to quantify the influence of substitution on the balance of acting intermolecular forces. This would provide an important step towards a rational approach to tailor molecular packing motifs. In the future, similar analyses might also be performed for other geometrical classes of molecules such as e.g. phthalocyanines or coronenes to study packing motifs in crystals of four-leaf clover-shaped or discoid molecules.
Footnote |
† Electronic supplementary information (ESI) available: Details on correlation coefficients, packing coefficients, polarizabilities, quadrupole moments and close contacts, as well as the visualization of packing motifs for all structures are given. See DOI: 10.1039/c7ce01700f |
This journal is © The Royal Society of Chemistry 2018 |