Reactivity of Solid Rubrene with Potassium: Competition between Intercalation and Molecular Decomposition

Abstract

We present the synthesis and characterization of the K + -intercalated rubrene (C 42 H 28 ) phase, K 2 Rubrene (K 2 R), 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, K 2 R, K(RR∗), and K x R′ (where R∗ and R′ are rubrene decomposition derivatives C 42 H 26 and C 30 H 20 , respectively). K 2 R 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 x R′ is obtained only as a secondary phase to the majority K 2 R phase. The crystal structure of K 2 R 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 η 6 , η 3 , and η 2 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-C phenyl bonds. For the crystalline decomposition derivative products K(RR∗) and K x R′, 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.