On the origin of ‘synthetic metals’

Who coined the term ‘synthetic metals’?

Doped conjugated polymers, and other organic materials that exhibit metallic conductivities, are often referred to as synthetic metals [1], [2], [3], [4] and [5]. This is exemplified by Alan MacDiarmid's Nobel lecture “‘Synthetic Metals’: A Novel Role for Organic Polymers” [3]. The term can also be found in the titles of the Elsevier journal dedicated to these materials [4] and the International Conference on the Science and Technology of Synthetic Metals [5]. The term ‘synthetic metals’ has now been in use long enough that it is somewhat commonplace and few question its origins. As such, it is worthwhile to review the history behind the origin of this term as a part of continuing efforts towards documenting the history of conjugated materials [6], [7] and [8].

In reviewing previous discussions, it is found that credit is commonly given to Alfred Ubbelohde, who began using the term in 1969 [9] and [10]. A clear example of this can be found in Weinberg's biography of Ubbelohde [11]: “Ubbelohde coined the evocative expression “Synthetic Metals” to cover the creation of materials with metallic conduction but formed entirely of such non-metallic atoms as carbon, nitrogen, hydrogen, the halogens and oxygen.” In reality, the term predates Ubbelohde and can be found in the 1911 work of Herbert McCoy [12]. The current discussion will thus begin with McCoy before returning to Ubbelohde.

Herbert Newby McCoy (1870–1945) earned a Ph.D. at the University of Chicago in 1898 and held faculty positions at Utah and Chicago before moving to industry [13] and [14]. Although known primarily for his rare earth chemistry, McCoy has also been credited with preparing the first organic metal [13] via the electrolysis of (CH₃)₄N⁺ salts in 1911[12] and [15]. Expanding on reports of ammonium amalgam dating back to 1808, McCoy thought that reduced ammonium could allow metallic properties similar to sodium metal. As explained by McCoy [12]: “In this case…positive ions are attracted to the cathode, and…can gain electrons. If then the electron theory of the metallic state is as fundamental as it seems to be, the aggregate of such free “neutralized” radicals should be a body having metallic properties; in other words, a “synthetic metal.”

Utilizing a mercury electrode, electrolysis produced a solid of metallic luster which resembled sodium amalgam. Although not very stable, it was believed to be a mercury amalgam of ammonium radicals that exhibited metallic conductivity. McCoy concluded[12]: “The facts just reviewed, though few in number, seem to me…to lead to the conclusion that it is possible to prepare composite metallic substances, which may be termed synthetic metals, from constituent elements, some of which at least are nonmetallic.” In 1986, Bard and coworkers concluded that such products are actually Zintl ion salts resulting from mercury reduction to give NH₄⁺(Hg₄⁻) [16]. As such, these are not organic metals as originally believed, but do seem to be the origin of ‘synthetic metals’. The term was then not used in the literature again until Ubbelohde used it to describe intercalated graphites in 1969 [9] and [10].

Alfred Rene Ubbelohde (1907–1988) was awarded a D.Sc. from Oxford University in 1941 and held academic positions at Queen's University and Imperial College, spending his career on a range of subjects including graphite and intercalation compounds, hydrogen in metals, phase transitions, and ionic melts [11]. The intercalated graphites reported by Ubbelohde exhibited conductivities up to 2.5 × 10⁵ S cm⁻¹ and thus provided the first practical example of a metallic organic species [9], [10], [11] and [17]. He first reported these materials in 1951 [18], but did not describe them as synthetic metals until 1969 [9] and [10]. The 1969 papers reported significantly higher conductivities than his previous reports and this may be why he used the term to describe these later materials. Whatever the reason, the term then became a mainstay in his writings, which resulted in the belief that he originated the term.

The remaining question is whether Ubbelohde developed the term ‘synthetic metals’ independently or if he learned of it from McCoy's work and simply applied it to his own. This is not possible to answer conclusively, although Ubbelohde's language can provide clues. It is important to note that Ubbelohde never claims the term as his own, nor does he define the term and always uses it as if it is a known term that does not require explanation. For example, the first sentence of his first 1969 paper states [9]: “With the development of methods for producing near-ideal graphites…and with improved methods for controlled progressive formation of intercalation compounds…it becomes possible to study variations in charge carrier behaviour in these synthetic metals, in much greater detail than is usually feasible with natural metals.”

As seen, the term is not explained, nor does he provide a reference for it. While McCoy is never mentioned, it should be noted that Ubbelohde published two papers on ammonium amalgams in 1951 [18] and [17], the same topic of McCoy's original paper on synthetic metals and thus it seems plausible that he was familiar with McCoy's work. While this cannot be confirmed, this author believes that Ubbelohde learned of the term from McCoy and did not develop it independently. If correct, why Ubbelohde never referenced or acknowledged McCoy will remain a puzzling mystery.

Through the early 1970s, additional metallic materials were discovered, including organic charge-transfer salts, metal chain compounds, and poly(sulfurnitride). As this research spanned a range of scientific disciplines and geography, a workshop was organized in the summer of 1976 in Siofok, Hungary to bring these interdisciplinary researchers together [5]. This ultimately resulted in a long-standing international conference, the International Conference on the Science and Technology of Synthetic Metals, commonly known as ICSM. Held annually from 1976 to 1982, this conference has been held biennially ever since.

Following the November 1976 [7] discovery that polyacetylene films could be doped to give high conductivity materials, MacDiarmid, Heeger, and Shirakawa first reported this at the second ICSM conference in New York City [5]. These results then appeared in the literature in late 1977 [19], thus expanding the scope of synthetic metals to include doped polyacetylenes [20]. Although the term was not used in the initial polyacetylene papers, MacDiarmid defined synthetic metals as metallic compounds derived from poly(sulfurnitride), polyacetylene, and graphite in a 1979 review [21]. With the continued development of conducting polymers, the term was further expanded by 1991 to include doped polymers such as polyparaphenylene, poly(phenylene vinylene), polypyrrole, polythiophene, and polyaniline [1].

By October 1979, a new Elsevier journal was launched dedicated to these materials, aptly titled Synthetic Metals [4]. The Founding Editor was F. Lincoln Vogel, with Associate Editors Wayne Worrell and future Nobel laureate Alan Heeger. The initial Editorial Board also included Alfred Ubbelohde and future Nobel laureate Hideki Shirakawa. To date, this is still the only journal dedicated to organic conducting materials.

As illustrated above, the history of synthetic metals can be traced much further back than commonly thought. In addition, as our concept of conducting materials has changed over the last 50+ years, the materials represented by the term ‘synthetic metals’ have also changed since its first use. However, in all cases, these materials have always fit McCoy's original 1911 use to represent “composite metallic substances…from constituent elements, some of which at least are nonmetallic”.

On the origin of ‘synthetic metals’

Who coined the term ‘synthetic metals’?

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