Abstract
We present the synthesis and characterization of the K+-intercalated rubrene (C42H28) phase, K2Rubrene (K2R), and identify the coexistence of amorphous and crystalline materials in samples where the crystalline component is phase-pure. We suggest this is characteristic of many intercalated alkali metal-polyaromatic hydrocarbon (PAH) systems, including those for which superconductivity has been claimed. The systematic investigation of K-rubrene solid-state reactions using both K and KH sources reveals a complex competition between K intercalation and the decomposition of rubrene, producing three K-intercalated compounds, namely, K2R, K(RR*), and K xR' (where R* and R' are rubrene decomposition derivatives C42H26 and C30H20, respectively). K2R is obtained as the major phase over a wide composition range and is accompanied by the formation of amorphous byproducts from the decomposition of rubrene. K(RR*) is synthesized as a single phase, and K xR' is obtained only as a secondary phase to the majority K2R phase. The crystal structure of K2R was determined using high-resolution powder X-ray diffraction, revealing that the structural rearrangement from pristine rubrene creates two large voids per rubrene within the molecular layers in which K+ is incorporated. K+ cations accommodated within the large voids interact strongly with the neighboring rubrene via η6, η3, and η2 binding modes to the tetracene cores and the phenyl groups. This contrasts with other intercalated PAHs, where only a single void per PAH is created and the intercalated K+ weakly interacts with the host. The decomposition products of rubrene are also examined using solution NMR, highlighting the role of the breaking of C-Cphenyl bonds. For the crystalline decomposition derivative products K(RR*) and K xR', a lack of definitive structural information with regard to R* and R' prevents the crystal structures from being determined. The study illustrates the complexity in accessing solvent-free alkali metal salts of reduced PAH of the type claimed to afford superconductivity.
Highlights
Superconductivity has recently been claimed in alkali metal intercalated polyaromatic hydrocarbons (PAHs) synthesized by solid-state reactions
The claimed superconductivity seen in K3Picene or alkali metal salts of phenanthrene cannot be assigned to K2Picene, K2Pentacene, or (Cs/Cs2)Phenanthrene, suggesting that the small superconducting shielding fractions observed in K3Picene and (K/ Rb)3Phenanthrene can only be understood in terms of a more complex chemistry between PAHs and strong reducing agents at high temperature
In K2R, the rearrangement of pristine rubrene to incorporate K+ completely disrupts the dominating intermolecular interactions seen in pristine rubrene, creating two large voids per rubrene within the molecular layers, each large enough to accommodate a single K+
Summary
Superconductivity has recently been claimed in alkali metal intercalated polyaromatic hydrocarbons (PAHs) synthesized by solid-state reactions. Superconductivity was initially claimed in K3Picene, with a critical temperature of 18 K, and subsequently in phenanthrene-, dibenzopentacene-, and coronene-based materials, with the maximum superconducting temperature of 33 K claimed in potassium-doped 1,2:8,9dibenzopentacene.[1−4] the reproducibility of the claimed products is poor and a lack of detailed characterization inhibits the understanding of the properties of these materials.[5]. In most cases, these materials are stated to be synthesized by reacting K metal with the respective PAH in order to achieve intercalation. The high purity and crystallinity of the materials prepared from these routes allowed their crystal structures and electronic properties to be determined, but superconducitivty was not observed.[6,7] The claimed superconductivity seen in K3Picene or alkali metal salts of phenanthrene cannot be assigned to K2Picene, K2Pentacene, or (Cs/Cs2)Phenanthrene, suggesting that the small superconducting shielding fractions observed in K3Picene and (K/ Rb)3Phenanthrene can only be understood in terms of a more complex chemistry between PAHs and strong reducing agents at high temperature
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