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Chemists prepare an inorganic double-helix structure for the first time

The well-defined semiconducting material features intertwined tin iodide and phosphide chains

by Stephen K. Ritter
September 29, 2016 | A version of this story appeared in Volume 94, Issue 39

A photo shows purple needlelike crystals of SnIP
Credit: Courtesy of Tom Nilges
Needlelike crystals of SnIP.

Double-helix molecules are frequently encountered in biological and synthetic organic systems, where they typically provide improved strength and better electrical properties relative to materials containing linear chains or single helices. DNA is the defining example. A purely inorganic double helix has been hard to come by, until now.

Doing the twist
Credit: Courtesy of Tom Nilges
This structural model of the inorganic double-helix compound SnIP depicts a helical tin iodide cation chain coiled with a helical phosphide anion chain.
A structural model of the SnIP double helix.
Credit: Courtesy of Tom Nilges
This structural model of the inorganic double-helix compound SnIP depicts a helical tin iodide cation chain coiled with a helical phosphide anion chain.

A team of some 20 researchers led by Tom Nilges of the Technical University of Munich has prepared the first completely inorganic substance, SnIP, featuring a well-defined double-helix structure. This semiconducting material consists of a twisted tin iodide (SnI+) chain intertwined with a twisted phosphide (P) chain. The team prepared gram amounts of SnIP by heating tin, red phosphorus, and tin tetraiodide together (Adv. Mater. 2016, DOI: 10.1002/adma.201603135).

Chemists have been seeking out inorganic double helices for decades. Researchers have reported X-ray crystal structures of bulk LiP and LiAs containing spiral and coaxial chains, but it remained unclear as to whether they should be called double-helix structures. More recently, researchers have attempted making metal or metal salt double-helix materials using nanotubes or DNA as templates. But a nontemplated, carbon-free example with a fully characterized double-helix structure had remained elusive.

“This is a truly remarkable result,” says theoretical chemist Alexander I. Boldyrev of Utah State University. In 2012, Boldyrev’s group confirmed the controversial idea that inorganic double helices should exist by showing computationally that LiP can form a structure similar to DNA. “In the end, we might find that double-helix structures are not as rare in inorganic chemistry as we previously thought,” Boldyrev adds.

Nilges and his coworkers determined that the SnIP double helix is held together by weak interactions stemming from lone pairs of electrons on tin and phosphorus. And each double helix is coordinated to neighboring ones by interactions that are stronger than hydrogen bonding in DNA and impart hearty mechanical properties to bulk SnIP.

The team found that the needlelike SnIP crystals can bend in half without damage and that they can be cut down to form nanorods. With the extraordinary flexibility, coupled with its photoluminescence properties, “we are optimistic that SnIP can be used in semiconductor-based applications such as optical devices, including flexible solar cells,” Nilges says.

SnIP in 3-D
This short animation depicts a three-dimensional model of the inorganic double-helix compound SnIP, which consists of a tin iodide helix coiled with a phosphide helix.
Credit: Courtesy of Tom Nilges


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