A combined experimental and theoretical study of the charge-transfer compound between C₆₀Br₈ and tetrathiafulvalene

C₆₀Br₈, unlike C₆₀Br₆ and C₆₀Cl₆, forms a solid charge-transfer compound with tetrathiafulvalene (TTF), the composition being C₆₀Br₈ (TTF)₈. The unique complex-forming property of C₆₀Br₈ can be understood on the basis of the electronic structures of the halogenated derivatives of C₆₀. Molecular orbital calculations show that the low LUMO energy of C₆₀Br₈ compared with the other halogen derivatives renders the formation of the complex with TTF favourable, the four virtual LUMOs being able to accept 8 electrons. The Raman spectrum of C₆₀Br₈ (TTF)₈ shows a marked softening of the bands (–46 cm⁻¹ on average) with respect to C₆₀Br₈, suggesting that indeed 8 electrons are transferred per C₆₀Br₈ molecule, one from each TTF molecule. The complex is weakly paramagnetic and shows a magnetic transition around 80 K.