Crystal Structure and Stoichiometric Composition of Potassium-Intercalated Tetracene

Abstract

The products of the solid-state reactions between potassium metal and tetracene (K:Tetracene, 1:1, 1.5:1, and 2:1) are fully structurally characterized. Synchrotron X-ray powder diffraction shows that only K₂Tetracene forms under the reaction conditions studied, with unreacted tetracene always present for x < 2. Diffraction and ¹³C MAS NMR show that K₂Tetracene has a crystal structure that is analogous to that of K₂Pentacene, but with the cations ordered on two sites because of the influence of the length of the hydrocarbon on possible cation positions. K₂Tetracene is a nonmagnetic insulator, thus further questioning the nature of reported superconductivity in this class of materials.

Figure 1

(a) Results of a Rietveld refinement of K₂Tetracene structure to SXRD data (R_wp = 2.96%; R_exp = 0.47%, P2₁/c, a = 7.2599(4) Å, b = 7.2749(4) Å, c = 25.756(1) Å, β = 91.78(1)°). Inset shows the (002) diffraction peak; no unreacted tetracene or other secondary phases are present. (b) SXRD patterns from K_xTetracene (1≤ x ≤ 2) samples compared to pristine tetracene (λ = 0.82608(1) Å). Patterns offset by 0.5 arbitrary units for clarity. Asterisks (*) indicate positions of tetracene (“Type 1” polymorph) Bragg peaks observed in samples x ≤ 1.75. (c) Raman spectra of pristine tetracene and K₂Tetracene (excitation laser λ = 785 nm).

Figure 2

(a, b) Views of K₂Pentacene (a) and K₂Tetracene (b) along the a axis. Physical and unphysical K···K distances are shown in black and red, respectively. Tetracene molecules are shown in green, fully occupied K sites are shown in red, and ∼50% occupied sites are shown in blue. Unit cell edges are represented by bold black lines. (c) Herringbone reorientation from tetracene (left) to K₂Tetracene (right), with expansion of the layers to accommodate potassium ions.

Figure 3

Experimental (full line) and simulated (dotted line) ¹³C solid-state MAS NMR spectra of K₂Tetracene at 9.4 T. The simulated spectrum is based on the chemical shifts predicted from the DFT model with the signal intensity fixed to the number of corresponding carbons. The spectral assignment is given on a molecule of tetracene for simplicity. The similar carbons of the two independent tetracene molecules in K₂Tetracene are not resolved experimentally and differ by a maximum of 0.1 ppm within the predicted shifts (Table S3); these carbons are therefore not labeled for clarity. Quaternary carbons are in purple and CH carbon in light blue. Asterisks (*) denote spinning sidebands arising from ¹³C chemical shift anisotropy.

Figure 4

(a) Zero-field-cooled (ZFC, red data points) and field-cooled (FC, black data points) molar magnetic susceptibility of K₂Tetracene in a 20 Oe (0.002 T) field with a Curie–Weiss fit to the field-cooled data shown with a solid black line. Parameters for the Curie–Weiss fit are given in the Experimental Section. (b) Inverse X-band EPR susceptibility of the sample with nominal composition K_1.50Tetracene (XRD analysis shows this consists of K₂Tetracene and tetracene in 3:1 ratio).