Adv. Mater. http://doi.org/f3rfh7 (2016)

Helices are well-known secondary structures in biology, the canonical example being the DNA double helix, and have since been found in numerous organic and polymeric materials. LiP has been predicted computationally to adopt stable double helices, but although templated examples exist, stable inorganic helices have yet to be realized. Now, Tom Nilges and colleagues at the Technical University of Munich and other institutions in Germany have synthesized the first inorganic compound with an atomic-level double helical structure.

The researchers prepared crystalline SnIP in a single phase by a solid-state annealing reaction. The unit cell contains twisted P− and SnI+ chains wrapped around one another to form a double helix around 1 nm in diameter. Coordinative interactions of the Sn and P lone pairs stabilize the two helices and, along with van der Waals forces, these attractive interactions between the two interlocked helices are stronger than typical hydrogen bonding (for example in DNA). This leads to interesting mechanical properties; bulk crystals can be fully folded without breaking. The material has a direct bandgap of 1.86 eV and shows photoluminescence, leading the authors to suggest these semiconducting helices for flexible electronics applications. Bulk SnIP can also be delaminated to form nanorods ∼15 nm in diameter with a moderately larger calculated bandgap (2.28 eV).