Polymorphism of Helicenes studied by DSC and VT-XRD

V. Pokorný¹, J. Obuch¹, Š. Kocian², V. Štejfa²

¹Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovského nám. 2, 162 00, Prague 6, Czech Republic

² Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic

pokorny@imc.cas.cz

Helicenes are a class of inherently chiral, π-conjugated polycyclic aromatic hydrocarbons characterized by their helical structures and unique chiroptical properties [1]. Their rigid yet non-planar geometry makes them valuable model systems for investigating the relationship between molecular shape and crystal packing [2]. In the solid state, subtle variations in intermolecular interactions can lead to polymorphism, which can significantly influence the physical properties of these materials. Despite extensive crystallographic studies of helicenes [3], polymorphism in shorter homologues such as [4]- and [5]-helicene remains insufficiently explored.

New polymorphs of [4]-helicene and [5]-helicene were identified by differential scanning calorimetry (DSC, TA Q2000) and characterized by variable-temperature powder X-ray diffraction (VT-PXRD; Anton Paar XRDynamic 500 equipped with a TTK600 temperature chamber, rotating 0.5 mm capillary). Temperature-dependent PXRD measurements were used to monitor phase transitions and isolate the new forms. Crystal structures were determined from powder data by indexing and structure solution using FOX, followed by Rietveld refinement in Jana2020.

An overview of previously reported and newly identified polymorphs is given in Table 1. While the molecular conformation remains unchanged across all structures, significant differences are observed in crystal packing (see Figures 1 and 2). These packing variations correlate with differences in melting temperature, suggesting changes in lattice stability between polymorphs.

Table 1. Parameters of previously known and newly determined structures of [4]-helicene and [5]-helicene.^a

polymorph^b	a / Å	b / Å	c / Å	β / ˚	Z	Space group	Tmelt / ˚C
[4]-helicene
BZPHAN	14.660	14.157	5.785	90	4	P212121	66
New	14.497	10.400	8.025	90	4	Pbc21	45
[5]-helicene
DBPHEN02	5.815	14.178	17.498	94.41	4	P21/c	155
DBPHEN03	26.013	8.898	19.494	103.94	12	A2/a	178
DBPHEN05	20.307	8.977	7.756	90	4	Pbcn	n.d.^c
New	9.924	17.536	8.395	90	4	Pna21	162

^a parameters α and γ are 90˚ in all cases. β = 90˚ indicates an orthorhombic cell.

^b identified by the CSD refcode, if available.

^c n.d. stands for not determined.

Figure 1. Comparison of crystal packing in different polymorphs of [4]-helicene.	Figure 2. Comparison of crystal packing in different polymorphs of [5]-helicene.

These findings extend the known structural diversity of helicenes and support their use as model systems for packing-driven polymorphism in rigid π-conjugated molecules.

1. M. Cei, L. Di Bari, F. Zinna, Chirality, 35, (2003), pp. 192-210

2. Y. Shen, C. Shen, Chem. Rev., 112, (2012), pp. 1463-1535

3. J. A. Schmidt, E. H. Wolpert, G. M. Sparrow, E. R. Johnson, K. E. Jelfs, Cryst. Growth Des., 23, (2023), pp. 8909-8917

We acknowledge financial support from the Czech Science Foundation (project No. 24-15057L). We would like to thank Michal Šámal from UOCHB for the synthesis of Helicene samples.