Effect of the size of the substituents in triptycene (TPC-X) on the magnetic and conducting properties of the layered 2D organic metals (MDABCO+)(C60•−)(TPC-X), where X = H, Br, and I

Abstract

Modification of the neutral triptycene TPC-X molecule by increasing the size of the X substituent at the 9th position from X = H to Br and I affects the magnetic and conducting properties of the (MDABCO⁺)(C₆₀^•−)(TPC-X) salts (1–3, respectively), where MDABCO⁺ is N-methyldiazabicyclooctanium. These salts feature hexagonal fullerene layers composed of C₆₀^•−, which remain undimerized owing to spatial separation by TPC-X. Two distinct fullerene layers, A and B, differ in their surroundings. The calculated bandwidth of 0.133–0.141 eV for 3 indicates a narrow band nature and as a result both fullerene layers are close to a Mott insulating state. Only one type of fullerene layers demonstrate metallic conductivity in 2 and 3, while another type of fullerene layers have localized electronic state and shows strong antiferromagnetic spin coupling down to low temperatures. Magnetic coupling in 2 and 3 follows the Heisenberg model for 2D hexagonal layers, with J =−28 and −34 cm⁻¹, respectively. The 2D metallic conductivity enables the observation of a Dysonian-type electron paramagnetic resonance signal from oriented single crystals of 2 and 3, similar to 1. Heating 2 and 3 above 300 K decreases their A/B ratio, reaches unity above 350 K, signaling a crossover to a nonmetallic state. The interfullerene center-to-center (ctc) distances increase upon heating, essentially reducing overlap and transfer integrals in (MDABCO⁺)(C₆₀^•−)(TPC-X). Because these salts are narrow-band metals, such an increase in ctc distances may drive a transition of these metals to a Mott-insulating state.