Effect of [ n]-Helicene Length on Crystal Packing

Abstract

Chiral π-conjugated organic molecules hold potential for emerging technologies as they are capable of introducing novel functionalities into electronic devices owing to their strong chiroptical properties. However, capitalizing on chiral molecules for electronic devices is reliant on their molecular packing-a factor that impacts their charge-transport properties. The solid-state behavior of molecules is sensitive to subtle differences in molecular interactions, chirality, and shape, but these relationships are not fully understood. Here, we employ crystal structure prediction (CSP) as a tool to probe the lattice-energy landscape for a family of chiral organic molecules: [n]helicenes, where n ranges from 3 to 12. Our results show excellent agreement between the CSP landscapes and experimentally reported structures. By analyzing the packing motifs within the polymorph landscapes, we begin to develop an understanding of how helicene length affects the shape and π-π stacking interactions seen in the polymorphs. Furthermore, we propose how helicene length can be used as a tool to design new functional organic electronics.

Figure 1

Molecular structures of naphthalene and [n]helicenes of varying helicene length n, here n = 3–12. Hydrogens are omitted for clarity.

Figure 2

CSP landscapes for naphthalene and [3]–[12]helicenes for the polymorphs within 10 kJ mol^–1 of the global minimum. Each data point is colored by the chirality of the crystal structure, either in yellow (enantiopure) or teal (racemic). The intergrowth crystal structure of [6]helicene is shown in red. Black circles indicate the experimentally observed polymorphs. If the experimental structure had Z′ > 1, the structure is represented as a diamond rather than a circle. CSD codes are provided next to the corresponding structures when available.

Figure 3

Lowest energy enantiopure and racemic polymorphs for naphthalene and [3]–[12]helicenes as found in the CSP landscapes. Polymorphs that have been experimentally synthesized are underlined. For [3]helicene, there is no reported structure in the CSD, but the experimental structure was obtained by using the crystal structure data provided in ref (48) to generate a CIF file, which was then overlaid with predicted structures of similar space group and crystal parameters. For the enantiopure structures from n = 2, 4, 6, 7, 9, 10, and 11, the synthesized polymorphs correspond to crystal structures with the CSD codes: NAPHTA18, BZPHAN, HEXHEL, IMEJIW, QUJNEQ, THELIC, and UHELIC. For the racemic structures from n = 5–7, the synthesized polymorphs correspond to crystal structures with the CSD codes: DBPHEN04 and HPTHEL. The gray and blue helicenes in the racemic crystals indicate the two enantiomers.

Figure 4

Packing motifs of lowest energy racemic structures of [11] and [12]helicene, which correspond to a motif that has high hole mobility in [6]helicene. The gray and blue colors of the helicenes indicate the two enantiomers.

Figure 5

Number of polymorphs (red), the percentage of which are enantiopure (blue), and angle between terminal rings (green), for naphthalene and each helicene structure within 20 kJ mol^–1 of the global minimum from each CSP search. As naphthalene and [3]helicene are not chiral, the % of enantiopure crystals has been omitted.

Figure 6

CSP landscapes for naphthalene and [3]–[12]helicenes for the polymorphs within 10 kJ mol^–1 of the global minimum. Each data point is colored by the degree of π–π stacking of the single molecule in the asymmetric unit of the crystal. 100% (0%) denotes that every (no) benzene ring of the respective backbone is involved in π–π stacking. Black circles indicate experimentally observed polymorphs. If the experimental structure had Z′ > 1, the structure is represented as a diamond rather than a circle and colored orange as CRYSTACK is unable to calculate the extent of π–π stacking. CSD codes are provided next to the corresponding structure where available.