Monomers and dimers of fullerenes in the radical anion salts with dyes, (astrophloxine+)2(C60˙−)2 and (astrophloxine+)2(C70−)2: suppression of the C60˙− dimerization by π-stacking

Abstract

Radical anion salts of fullerenes have been obtained through the reaction of Cs·C₆₀ and Cs·C₇₀ with cationic dye (astrophloxine⁺)(I⁻) yielding crystalline salts (astrophloxine⁺)₂(C₆₀˙⁻)₂·C₆H₄Cl₂·C₆H₁₄ (1) and (astrophloxine⁺)₂(C₇₀⁻)₂ (2). Salt 1 contains C₆₀˙⁻ radical anions arranged in 3D packing among which pairs of types I and II can be outlined. Strong antiferromagnetic coupling of spins is observed in the monomeric phase with a Weiss temperature of −28 K. Partial dimerization of C₆₀˙⁻ is observed in the pairs of type I at 210–150 K yielding a mixture of diamagnetic singly bonded (C₆₀⁻)₂ dimers and pairs of C₆₀˙⁻. The reaction is accompanied by a decrease in the magnetic moment. Pairs of C₆₀˙⁻ of both types show no dimerization due to π-stacking. Monomeric C₆₀˙⁻ radical anions in the pairs are also separated by cations which form short H(astrophloxine⁺)⋯C(C₆₀˙⁻) contacts and can hinder dimerization. Thus, despite close distances between C₆₀˙⁻ radical anions, they can dimerize only partially in 1. Strong antiferromagnetic coupling between C₆₀˙⁻ radical anions is observed below 140 K, and the estimated exchange interaction (J/k_B) is equal to −24 K. Below 40 K, antiparallel alignment of S = 1/2 spins in the pairs occurs. The phase containing a mixture of monomers and dimers demonstrates a lower Weiss temperature of −11 K. The C₇₀˙⁻ radical anions form singly bonded (C₇₀⁻)₂ dimers in salt 2. The astrophloxine dye absorbs light in the visible range with maxima at 516 and 553 nm, and these bands are red shifted to 530 and 556 nm. Low-energy absorption at about 3300 cm⁻¹ manifested by salt 1 is attributed to charge transfer within the π-stacking pairs.