A Two‐Dimensional Organic Metal Based on Fullerene

Abstract

We report the synthesis and studies of the physical properties of the first two-dimensional (2D) fullerene organic metal to have 2D layers with a honeycomb arrangement of C 60 C À , (MDABCO + )·TPC·(C 60 C À ) (1); we employed a multicomponent and molecular symmetry synthetic approach, starting from the N-methyldiazabicyclooctane cation (MDABCO + ) and triptycene (TPC). Compound 1 is a fascinating example of a material composed of only light elements (C, H, N) that exhibits a metallic state down to 1.9 K. Salts of fullerene C 60 with a number of different inorganic cations have previously shown metallic or superconducting properties. Among the fullerene metals, the best known families are MC 60 salts (M = K, Rb, Cs), which contain linearly polymerized C 60 C À , and superconducting M 3 C 60 salts (M = alkali metals), obtained by doping C 60 with alkali metals, which have transition temperatures (T c ) of up to 38 K. [1][2][3][4] As metal cations expand the three-dimensional (3D) lattice of the initial C 60 framework, M 3 C 60 salts exhibit 3D metallic conductivity, whereas MC 60 salts are either 3D (when M = K) or quasi-1D metals (when M = Rb or Cs). showed rather high conductivity, yet was a semiconductor. [5] By synthesizing organic molecular crystals from C 60 and organic electron donor molecules, it is possible to produce conductors with unique structures and properties.We have developed a multicomponent approach for synthesizing ionic fullerene compounds that enables the synthesis of ionic fullerene solids D I + ·D II ·C 60 C À of various structures. [6,7] D I + is a small, strong donor or cation that ionizes C 60 and determines its charged state, whereas D II is a large, neutral molecule that defines the crystal packing of the complex. In order to exhibit metallic properties, the fullerene sublattice should have a close-packed structure. However, C 60 C À radical anions have a strong tendency for dimerization, and when they are allowed to approach each other they normally form diamagnetic single-bonded (C 60 À ) 2 dimers. [7][8][9] In this study, by choosing D II molecules with suitable spatial geometry and size, we were able to synthesize a complex with a close-packed fullerene 2D sublattice in which the C 60 C À

Metallic picene/C60heterojunctions and the effect of potassium doping

“…Finally, a charge transfer from the metal to the hexaaza-triphenylenehexacarbonitrile layer was observed when it was deposited atop of a monolayer of tris(8-hydroxyquinolinato)aluminum on Ag(111) [22]. Our data indicate a similar trend, but in a multilayer structure of heterojunctions, which offers further flexibility, as also observed for a multilayer of multicomponent N-methyldiazabicyclooctane cation, triptycene, and C 60 [23].…”

“…Therefore, forming a continuous metallic state in bulk K x C 60 may allow following the possible dome-shaped behavior of T c as a function of doping. Moreover, it opens the way towards novel molecular heterojunctions that may create new electronic phases in strongly correlated materials [20][21][22][23].…”

Design of Spin-Frustrated Monomer-Type C60•− Mott Insulator

“…It should be emphasized that "localization" or "itinerancy" of spins in C 60 solids is strongly associated with the rotational disorder and ordering of C 60 The donor molecules that can reduce fullerene moieties are sparse because of the weak electron-accepting ability of fullerene moiety [140]. Simple use of very strong donor species, such as alkali metal has not led to 2D triangular or hexagonal packing of C 60 • [12,146]. Higher than threefold symmetry would be satisfactory.…”

“…TPC was dissolved in the obtained solution and n-hexane was layered. The diffusion was carried out over a period of two months to give black hexagonal prisms on the walls of the tube of sizes up to 0.5 × 2 × 2 mm 3 ( Figure 4c) [146]. Figure 5d) at 200 K. The neighboring TPC molecules are separated by 9.82 Å, so that it is able to prevent the bond-formation between C60 •− molecules when they pack on the template in a hexagonal lattice structure, where the intermolecular distance of MDABCO + molecules corresponds to N•••N (blue points in Figure 4(a-2)) with separation distance of 9.99 Å.…”

“…It exhibits an abrupt increase below 200 K reaching a maximum of 4.2 at 183 K, which coincides well with the rapid conductivity increase, then falls to 2.3-2.4 below 183 K. The A/B ratio slowly decreases below 100 K, but the Dysonian shape is observed even Figure 9. (a) Temperature dependent resistivity in the ab plane and the ratio σ //c /σ //ab of (TPC 0 ) (MDABCO + ) (C 60 •− ) by the four probe method (inset: single crystal with gold wires and paste) (from [146]); (b) Temperature dependence of conductivity by the four probe method, microwave and optical conductivity (from [12]); (c) Temperature dependent EPR peak ratio A/B for the Dysonian line from the single crystal (inset: the spectrum at 183 K) (from [146]) is compared with that of a superconductor β-(ET) 2 The temperature range of this anomaly between 200 K and 185 K, indicated by two red lines, coincides well with that of an ordering of C 60 •− in Layer B, showing that the ordered C 60 •− radical anions in Layer B start to participate in the metallic transport below 200 K. The resistivity could not be measured correctly below 70 K because of a large increase in the contact resistance, due to its air-sensitivity, even though it was measured in an inert atmosphere. The contactless microwave and optical measurements revealed that the conductivity increased down to 100-25 K and the metallic state is preserved down to 5 K (Figure 9b) [12].…”